Ventilation with heat recovery: why it is needed and how to use it. Energy-efficient building ventilation systems with heat recovery Air handling units with recuperator
Due to the increase in tariffs for primary energy resources, recovery has become more relevant than ever. In air handling units with recovery, the following types of recuperators are usually used:
- plate or cross-flow recuperator;
- rotary recuperator;
- recuperators with intermediate coolant;
- Heat pump;
- recuperator chamber type;
- recuperator with heat pipes.
Principle of operation
The operating principle of any recuperator in air handling units is as follows. It provides heat exchange (in some models - both cold exchange and moisture exchange) between the supply and exhaust air flows. The heat exchange process can occur continuously - through the walls of the heat exchanger, using freon or an intermediate coolant. Heat exchange can also be periodic, as in a rotary and chamber recuperator. As a result, the exhaust air is cooled, thereby heating the fresh air supply air. The cold exchange process in certain models of recuperators takes place during the warm season and makes it possible to reduce energy costs for air conditioning systems due to some cooling of the supply air supplied to the room. Moisture exchange occurs between the exhaust and supply air flows, allowing you to maintain comfortable humidity in the room all year round, without the use of any additional devices– humidifiers and others.
Plate or cross-flow recuperator.
The heat-conducting plates of the recuperative surface are made of thin metal (material - aluminum, copper, stainless steel) foil or ultra-thin cardboard, plastic, hygroscopic cellulose. The supply and exhaust air flows move through many small channels formed by these heat-conducting plates in a counterflow pattern. Contact and mixing of flows and their contamination are practically excluded. There are no moving parts in the recuperator design. Efficiency rate 50-80%. In a metal foil recuperator, due to the difference in air flow temperatures, moisture may condense on the surface of the plates. In the warm season, it must be drained into the building's sewerage system through a specially equipped drainage pipeline. In cold weather, there is a danger of this moisture freezing in the recuperator and its mechanical damage(defrosting). In addition, the formed ice greatly reduces the efficiency of the recuperator. Therefore, when operating in the cold season, heat exchangers with metal heat-conducting plates require periodic defrosting with a flow of warm exhaust air or the use of an additional water or electric air heater. In this case, supply air is either not supplied at all, or is supplied to the room bypassing the recuperator through an additional valve (bypass). Defrost time averages from 5 to 25 minutes. A heat exchanger with heat-conducting plates made of ultra-thin cardboard and plastic is not subject to freezing, since moisture exchange occurs through these materials, but it has another drawback - it cannot be used for ventilation of rooms with high humidity for the purpose of drying them. The plate heat exchanger can be installed in the supply and exhaust system in both vertical and horizontal positions, depending on the requirements for the size of the ventilation chamber. Plate recuperators are the most common due to their relative simplicity of design and low cost.
Rotary recuperator.
This type is the second most widespread after the lamellar type. Heat from one air flow to another is transferred through a cylindrical hollow drum, called a rotor, rotating between the exhaust and supply sections. The internal volume of the rotor is filled with tightly packed metal foil or wire, which plays the role of a rotating heat transfer surface. The material of the foil or wire is the same as that of the plate recuperator - copper, aluminum or stainless steel. The rotor has a horizontal axis of rotation of the drive shaft, rotated by an electric motor with stepper or inverter control. The engine can be used to control the recovery process. Efficiency rate 75-90%. The efficiency of the recuperator depends on the flow temperatures, their speed and rotor speed. By changing the rotor speed, you can change the operating efficiency. Freezing of moisture in the rotor is excluded, but mixing of flows, their mutual contamination and transfer of odors cannot be completely excluded, since the flows are in direct contact with each other. Mixing up to 3% is possible. Rotary heat exchangers do not require large amounts of electricity and allow you to dry air in rooms with high humidity. The design of rotary recuperators is more complex than plate recuperators, and their cost and operating costs are higher. However, air handling units with rotary heat exchangers are very popular due to their high efficiency.
Recuperators with intermediate coolant.
The coolant is most often water or aqueous solutions of glycols. Such a recuperator consists of two heat exchangers connected by pipelines with a circulation pump and fittings. One of the heat exchangers is placed in a channel with the exhaust air flow and receives heat from it. The heat is transferred through the coolant using a pump and pipes to another heat exchanger located in the supply air channel. The supply air receives this heat and heats up. Mixing of flows in this case is completely excluded, but due to the presence of an intermediate coolant, the efficiency coefficient of this type of recuperator is relatively low and amounts to 45-55%. Efficiency can be influenced using a pump by influencing the speed of coolant movement. The main advantage and difference between a recuperator with an intermediate coolant and a recuperator with a heat pipe is that the heat exchangers in the exhaust and supply units can be located at a distance from each other. The installation position for heat exchangers, pumps and pipelines can be either vertical or horizontal.
Heat pump.
Appeared relatively recently interesting variety recuperator with intermediate coolant - so-called. a thermodynamic recuperator in which the role of liquid heat exchangers, pipes and a pump is played by a refrigeration machine operating in heat pump mode. This is a kind of combination of a recuperator and a heat pump. It consists of two refrigerant heat exchangers - an evaporator-air cooler and a condenser, pipelines, a thermostatic valve, a compressor and a 4-way valve. Heat exchangers are located in the supply and exhaust air ducts, a compressor is necessary to ensure circulation of the refrigerant, and the valve switches the refrigerant flows depending on the season and allows heat to be transferred from the exhaust air to the supply air and vice versa. Wherein supply and exhaust system may consist of several supply units and one higher-capacity exhaust unit, united by one refrigeration circuit. At the same time, the capabilities of the system allow several air handling units to operate in different modes (heating/cooling) simultaneously. The conversion coefficient of the COP heat pump can reach values of 4.5-6.5.
Recuperator with heat pipes.
According to the principle of operation, a recuperator with heat pipes is similar to a recuperator with an intermediate coolant. The only difference is that not heat exchangers are placed in the air flows, but so-called heat pipes or more precisely thermosyphons. Structurally, these are hermetically sealed sections of copper finned pipe, filled inside with a specially selected low-boiling freon. One end of the pipe in the exhaust flow heats up, the freon boils in this place and transfers the heat received from the air to the other end of the pipe, blown by the flow of supply air. Here the freon inside the pipe condenses and transfers heat to the air, which heats up. Mutual mixing of flows, their pollution and transfer of odors are completely excluded. There are no moving elements; pipes are placed in flows only vertically or at a slight slope so that the freon moves inside the pipes from the cold end to the hot end due to gravity. Efficiency rate 50-70%. Important condition to ensure its operation: the air ducts in which the thermosiphons are installed must be located vertically one above the other.
Chamber type recuperator.
The internal volume (chamber) of such a recuperator is divided into two halves by a damper. The damper moves from time to time, thereby changing the direction of movement of the exhaust and supply air flows. The exhaust air heats one half of the chamber, then the damper directs the flow of supply air here and it is heated by the heated walls of the chamber. This process is repeated periodically. The efficiency ratio reaches 70-80%. But the design has moving parts, and therefore there is a high probability of mutual mixing, contamination of flows and transfer of odors.
Calculation of recuperator efficiency.
IN technical specifications For recuperative ventilation units, many manufacturers usually provide two values of the recovery coefficient - based on air temperature and its enthalpy. The efficiency of a recuperator can be calculated based on temperature or air enthalpy. Calculation by temperature takes into account the sensible heat content of the air, and by enthalpy, the moisture content of the air (its relative humidity) is also taken into account. Calculation based on enthalpy is considered more accurate. For the calculation, initial data is required. They are obtained by measuring the temperature and humidity of the air in three places: indoors (where the ventilation unit provides air exchange), outdoors, and in the cross section of the supply air distribution grille (from where treated outdoor air enters the room). The formula for calculating the recovery efficiency by temperature is as follows:
Kt = (T4 – T1) / (T2 – T1), Where
- Kt– recuperator efficiency coefficient by temperature;
- T1– outside air temperature, oC;
- T2– temperature of the exhaust air (i.e. indoor air), °C;
- T4– supply air temperature, oC.
Enthalpy of air is the heat content of air, i.e. the amount of heat contained in it per 1 kg of dry air. Enthalpy is determined with using i-d state diagrams humid air, putting on it points corresponding to the measured temperature and humidity in the room, outside and supply air. The formula for calculating the recovery efficiency based on enthalpy is as follows:
Kh = (H4 – H1) / (H2 – H1), Where
- Kh– recuperator efficiency coefficient in terms of enthalpy;
- H1– enthalpy of outside air, kJ/kg;
- H2– enthalpy of exhaust air (i.e. indoor air), kJ/kg;
- H4– enthalpy of supply air, kJ/kg.
Economic feasibility of using air handling units with recovery.
As an example, let’s take a feasibility study for the use of ventilation units with recovery in the supply and exhaust ventilation systems of a car dealership.
Initial data:
- object – car showroom with total area 2000 m2;
- the average height of the premises is 3-6 m, consists of two exhibition halls, an office area and a station Maintenance(ONE HUNDRED);
- for supply and exhaust ventilation of these premises were selected ventilation units duct type: 1 unit with an air flow rate of 650 m3/hour and a power consumption of 0.4 kW and 5 units with an air flow rate of 1500 m3/hour and a power consumption of 0.83 kW.
- guaranteed range of external air temperatures for duct installations is (-15…+40) оС.
To compare energy consumption, we will calculate the power of a duct electric air heater, which is necessary to heat the outside air in the cold season in a traditional type air-handling unit (consisting of check valve, duct filter, fan and electric air heater) with an air flow of 650 and 1500 m3/hour, respectively. At the same time, the cost of electricity is 5 rubles per 1 kW*hour.
The outside air must be heated from -15 to +20°C.
The power of the electric air heater was calculated using the heat balance equation:
Qн = G*Cp*T, W, Where:
- Qн– air heater power, W;
- G- mass air flow through the air heater, kg/sec;
- Wed– specific isobaric heat capacity of air. Ср = 1000kJ/kg*K;
- T– difference in air temperature at the outlet of the air heater and the inlet.
T = 20 – (-15) = 35 oC.
1. 650 / 3600 = 0.181 m3/sec
p = 1.2 kg/m3 – air density.
G = 0.181*1.2 = 0.217 kg/sec
Qn = 0.217*1000*35 = 7600 W.
2. 1500 / 3600 = 0.417 m3/sec
G = 0.417*1.2 = 0.5 kg/sec
Qn = 0.5*1000*35 = 17500 W.
Thus, the use of ducted units with heat recovery in the cold season instead of traditional ones using electric air heaters makes it possible to reduce energy costs with the same amount of supplied air by more than 20 times and thereby reduce costs and accordingly increase the profit of a car dealership. In addition, the use of recovery units makes it possible to reduce the consumer's financial costs for energy resources for heating premises in the cold season and for air conditioning in the warm season by approximately 50%.
For greater clarity, we will carry out a comparative financial analysis of the energy consumption of supply and exhaust ventilation systems for car dealership premises, equipped with duct-type heat recovery units and traditional installations with electric air heaters.
Initial data:
System 1.
Installations with heat recovery with a flow rate of 650 m3/hour – 1 unit. and 1500 m3/hour – 5 units.
The total electrical power consumption will be: 0.4 + 5*0.83 = 4.55 kW*hour.
System 2.
Traditional ducted supply and exhaust ventilation units - 1 unit. with a flow rate of 650m3/hour and 5 units. with a flow rate of 1500m3/hour.
The total electrical power of the installation at 650 m3/hour will be:
- fans – 2*0.155 = 0.31 kW*hour;
- automation and valve drives – 0.1 kW*hour;
- electric air heater – 7.6 kW*hour;
Total: 8.01 kW*hour.
The total electrical power of the installation at 1500 m3/hour will be:
- fans – 2*0.32 = 0.64 kW*hour;
- automation and valve drives – 0.1 kW*hour;
- electric air heater – 17.5 kW*hour.
Total: (18.24 kW*hour)*5 = 91.2 kW*hour.
Total: 91.2 + 8.01 = 99.21 kW*hour.
We assume the period of use of heating in ventilation systems is 150 working days per year for 9 hours. We get 150*9 =1350 hours.
Energy consumption of installations with recovery will be: 4.55 * 1350 = 6142.5 kW
Operating costs will be: 5 rubles * 6142.5 kW = 30712.5 rubles. or in relative terms (to the total area of the car dealership of 2000 m2) 30172.5 / 2000 = 15.1 rub./m2.
Energy consumption of traditional systems will be: 99.21 * 1350 = 133933.5 kW Operating costs will be: 5 rubles * 133933.5 kW = 669667.5 rubles. or in relative terms (to the total area of the car dealership of 2000 m2) 669667.5 / 2000 = 334.8 rubles/m2.
The supply of fresh air during the cold period leads to the need to heat it to ensure the correct indoor microclimate. To minimize energy costs can be used supply and exhaust ventilation with heat recovery.
Understanding the principles of its operation will allow you to most effectively reduce heat loss while maintaining a sufficient volume of replaced air. Let's try to understand this issue.
In the autumn-spring period, when ventilating rooms, a serious problem is the large temperature difference between the incoming air and the air inside. The cold stream rushes down and creates an unfavorable microclimate in residential buildings, offices and production, or an unacceptable vertical temperature gradient in a warehouse.
A common solution to the problem is integration into supply ventilation, through which the flow is heated. Such a system requires energy consumption, while a significant volume of warm air escaping outside leads to significant heat loss.
The exit of air to the outside with intense steam serves as an indicator of significant heat loss, which can be used to heat the incoming flow
If the air inlet and outlet channels are located nearby, then it is possible to partially transfer the heat of the outgoing flow to the incoming one. This will reduce the energy consumption of the heater or eliminate it altogether. A device for ensuring heat exchange between gas flows of different temperatures is called a recuperator.
In the warm season, when the outside air temperature is significantly higher than room temperature, a recuperator can be used to cool the incoming flow.
Design of a unit with a recuperator
The internal structure of supply and exhaust ventilation systems is quite simple, so it is possible to independently purchase and install them element by element. In the event that the assembly or self-installation causes difficulties can be purchased ready-made solutions in the form of standard monoblock or individual prefabricated structures to order.
An elementary device for collecting and discharging condensate is a tray located under the heat exchanger with a slope towards the drain hole
Moisture is removed into a closed container. It is placed only indoors to avoid freezing of the outflow channels at sub-zero temperatures. There is no algorithm for reliable calculation of the volume of water received when using systems with a recuperator, so it is determined experimentally.
Reusing condensate for air humidification is undesirable, since water absorbs many pollutants such as human sweat, odors, etc.
You can significantly reduce the volume of condensate and avoid problems associated with its occurrence by organizing a separate exhaust system from the bathroom and kitchen. It is in these rooms that the air has the highest humidity. If there are several exhaust systems, the air exchange between the technical and residential areas must be limited by installing check valves.
If the exhaust air flow is cooled to negative temperatures inside the recuperator, condensate turns into ice, which causes a reduction in the open cross-section of the flow and, as a consequence, a decrease in volume or a complete cessation of ventilation.
For periodic or one-time defrosting of the recuperator, a bypass is installed - a bypass channel for the movement of supply air. When the flow bypasses the device, heat transfer stops, the heat exchanger heats up and ice passes into liquid state. The water flows into the condensate collection tank or evaporates outside.
The principle of the bypass device is simple, therefore, if there is a risk of ice formation, it is advisable to provide such a solution, since heating the recuperator by other means is complex and time-consuming
When the flow passes through the bypass, there is no heating of the supply air through the recuperator. Therefore, when this mode is activated, the heater must automatically turn on.
Features of various types of recuperators
There are several structurally different options for implementing heat exchange between cold and heated air flows. Each of them has its own distinctive features, which determine the main purpose for each type of recuperator.
The design of the plate recuperator is based on thin-walled panels, connected alternately in such a way as to alternate the passage of flows of different temperatures between them at an angle of 90 degrees. One of the modifications of this model is a device with finned channels for air passage. It has a higher heat transfer coefficient.
Alternate passage of warm and cold air flow through the plates is realized by bending the edges of the plates and sealing the joints with polyester resin
Heat exchange panels can be made of various materials:
- copper, brass and aluminum-based alloys have good thermal conductivity and are not susceptible to rust;
- plastic made from a hydrophobic polymer material with a high thermal conductivity coefficient and low weight;
- hygroscopic cellulose allows condensation to penetrate through the plate and back into the room.
The disadvantage is the possibility of condensation forming when low temperatures Oh. Due to the small distance between the plates, moisture or ice significantly increases aerodynamic drag. In case of freezing, it is necessary to block the incoming air flow to warm the plates.
The advantages of plate recuperators are as follows:
- low cost;
- long service life;
- long period between preventive maintenance and ease of its implementation;
- small dimensions and weight.
This type of recuperator is most common for residential and office premises. It is also used in some technological processes, for example, to optimize fuel combustion during the operation of furnaces.
Drum or rotary type
The operating principle of a rotary recuperator is based on the rotation of a heat exchanger, inside of which there are layers of corrugated metal with high heat capacity. As a result of interaction with the outgoing flow, the drum sector is heated, which subsequently gives off heat to the incoming air.
The fine-mesh heat exchanger of a rotary recuperator is susceptible to clogging, so you need to pay special attention to quality work fine filters
The advantages of rotary recuperators are as follows:
- quite high efficiency compared to competing types;
- return large quantity moisture, which remains in the form of condensation on the drum and evaporates upon contact with incoming dry air.
This type of recuperator is less often used for residential buildings for apartment or cottage ventilation. It is often used in large boiler houses to return heat to furnaces or for large industrial or commercial premises.
However, this type of device has significant disadvantages:
- a relatively complex design with moving parts, including an electric motor, drum and belt drive, which requires constant maintenance;
- increased noise level.
Sometimes for devices of this type you can come across the term “regenerative heat exchanger”, which is more correct than “recuperator”. The fact is that a small part of the exhaust air gets back due to the loose fit of the drum to the body of the structure.
This imposes additional restrictions on the ability to use devices of this type. For example, polluted air from heating stoves cannot be used as a coolant.
Tube and casing system
A tubular type recuperator consists of a system of thin-walled tubes of small diameter located in an insulated casing, through which there is an influx of outside air. The casing removes warm air from the room, which heats the incoming flow.
Warm air must be discharged through the casing, and not through a system of tubes, since it is impossible to remove condensate from them
The main advantages of tubular recuperators are as follows:
- high efficiency due to the countercurrent principle of movement of the coolant and incoming air;
- simplicity of design and absence of moving parts ensures low noise levels and rarely requires maintenance;
- long service life;
- the smallest cross-section among all types of recovery devices.
Tubes for this type of device use either light-alloy metal or, less commonly, polymer. These materials are not hygroscopic, therefore, with a significant difference in flow temperatures, intense condensation may form in the casing, which requires constructive solution on its removal. Another disadvantage is that the metal filling has significant weight, despite its small dimensions.
The simplicity of the tubular recuperator design makes this type of device popular for self-made. Typically used as an outer casing plastic pipes for air ducts, insulated with polyurethane foam shell.
Device with intermediate coolant
Sometimes the supply and exhaust air ducts are located at some distance from each other. This situation may arise due to the technological features of the building or sanitary requirements for reliable separation of air flows.
In this case, an intermediate coolant is used, circulating between the air ducts through an insulated pipeline. Water or a water-glycol solution is used as a medium for transferring thermal energy, the circulation of which is ensured by operation.
A recuperator with an intermediate coolant is a voluminous and expensive device, the use of which is economically justified for premises with large areas
If it is possible to use another type of recuperator, then it is better not to use a system with an intermediate coolant, since it has the following significant disadvantages:
- low efficiency compared to other types of devices, therefore such devices are not used for small rooms with low air flow;
- significant volume and weight of the entire system;
- the need for an additional electric pump to circulate the liquid;
- increased noise from the pump.
There is a modification of this system when, instead of forced circulation of the heat exchange fluid, a medium with a low boiling point, such as freon, is used. In this case, movement along the contour is possible naturally, but only if the supply air duct is located above the exhaust air duct.
Such a system does not require additional energy costs, but only works for heating when there is a significant temperature difference. In addition, fine adjustment of the change point is necessary state of aggregation heat exchange fluid, which can be realized by creating the required pressure or a certain chemical composition.
Main technical parameters
Knowing the required performance of the ventilation system and the heat exchange efficiency of the recuperator, it is easy to calculate savings on air heating for a room under specific climatic conditions. By comparing the potential benefits with the costs of purchasing and maintaining the system, you can reasonably make a choice in favor of a recuperator or a standard air heater.
Equipment manufacturers often offer a model line in which ventilation units with similar functionality differ in air exchange volume. For residential premises, this parameter must be calculated according to Table 9.1. SP 54.13330.2016
Efficiency
Under the coefficient useful action recuperator understand the heat transfer efficiency, which is calculated using the following formula:
K = (T p – T n) / (T v – T n)
Wherein:
- T p – temperature of the air entering the room;
- Tn – outside air temperature;
- T in – room air temperature.
Maximum efficiency value at standard and certain temperature conditions indicated in the technical documentation of the device. Its actual figure will be slightly less.
In the case of self-manufacturing of a plate or tubular recuperator, in order to achieve maximum heat transfer efficiency, you must adhere to the following rules:
- The best heat transfer is provided by counter-flow devices, then cross-flow devices, and the least by unidirectional movement of both flows.
- The intensity of heat transfer depends on the material and thickness of the walls separating the flows, as well as on the duration of the air inside the device.
E (W) = 0.36 x P x K x (T in - T n)
where P (m 3 / hour) – air flow.
Calculation of the efficiency of the recuperator in monetary terms and comparison with the costs of its acquisition and installation for two-story cottage with a total area of 270 m2 shows the feasibility of installing such a system
The cost of recuperators with high efficiency is quite high, they have complex design and significant size. Sometimes you can get around these problems by installing several simpler devices so that the incoming air passes through them sequentially.
Ventilation system performance
The volume of air passed through is determined by static pressure, which depends on the power of the fan and the main components that create aerodynamic resistance. As a rule, its exact calculation is impossible due to the complexity of the mathematical model, therefore experimental studies are carried out for standard monoblock structures, and components are selected for individual devices.
The fan power must be selected taking into account the throughput of installed heat exchangers of any type, which is indicated in the technical documentation as the recommended flow rate or volume of air passed by the device per unit of time. As a rule, the permissible air speed inside the device does not exceed 2 m/s.
Otherwise on high speeds in the narrow elements of the recuperator there is a sharp increase in aerodynamic resistance. This leads to unnecessary energy costs, ineffective heating of the outside air and reduced fan life.
The graph of pressure loss versus air flow rate for several models of high-performance recuperators shows a nonlinear increase in resistance, so it is necessary to adhere to the requirements for the recommended air exchange volume specified in the technical documentation of the device
Changing the direction of air flow creates additional aerodynamic drag. Therefore, when modeling the geometry of an indoor air duct, it is desirable to minimize the number of pipe turns by 90 degrees. Air diffusers also increase resistance, so it is advisable not to use elements with complex patterns.
Dirty filters and grilles create significant interference with flow, so they must be periodically cleaned or replaced. One effective way to assess clogging is to install sensors that monitor the pressure drop in areas before and after the filter.
Conclusions and useful video on the topic
Operating principle of rotary and plate recuperator:
Measuring the efficiency of a plate-type recuperator:
Household and industrial systems ventilation systems with an integrated recuperator have proven their energy efficiency in retaining heat indoors. Now there are many offers for the sale and installation of such devices, both in the form of ready-made and tested models, and on individual orders. You can calculate the necessary parameters and perform installation yourself.
If you have any questions while reading the information or find any inaccuracies in our material, please leave your comments in the block below.
During the ventilation process, not only exhaust air is recycled from the room, but also part of the thermal energy. In winter, this leads to higher energy bills.
Heat recovery in centralized and ventilation systems will help reduce unjustified costs without compromising air exchange. local type. To recover thermal energy, different types of heat exchangers are used - recuperators.
The article describes in detail the models of units, their design features, operating principles, advantages and disadvantages. The information provided will help you choose optimal option for arrangement ventilation system.
Translated from Latin, recuperation means compensation or return. With regard to heat exchange reactions, recovery is characterized as a partial return of energy expended on a technological action for the purpose of application in the same process.
Local recuperators are equipped with a fan and a plate heat exchanger. The inlet “sleeve” is insulated with sound-absorbing material. The control unit of compact ventilation units is located on the internal wall
Features of decentralized ventilation systems with recovery:
- Efficiency – 60-96%;
- low productivity– the devices are designed to provide air exchange in rooms up to 20-35 sq.m;
- affordable price and a wide selection of units, ranging from conventional wall valves to automated models with a multi-stage filtration system and the ability to adjust humidity;
- ease of installation– for commissioning, no installation of air ducts is required; you can do it yourself.
Important criteria for choosing a wall inlet: permissible wall thickness, performance, efficiency of the recuperator, diameter of the air channel and temperature of the pumped medium
Conclusions and useful video on the topic
Comparison of natural ventilation and compulsory system with recovery:
The principle of operation of a centralized recuperator, calculation of efficiency:
Design and operating procedure of a decentralized heat exchanger using the Prana wall valve as an example:
About 25-35% of the heat leaves the room through the ventilation system. Recuperators are used to reduce losses and effectively recover heat. Climatic equipment allows you to use the energy of waste masses to heat the incoming air.
Do you have anything to add, or do you have questions about the operation of different ventilation recuperators? Please leave comments on the publication and share your experience in operating such installations. The contact form is located in the lower block.
Creating an energy-efficient administrative building that will be as close as possible to the “PASSIVE HOUSE” standard is impossible without a modern air handling unit (AHU) with heat recovery.
Under recovery means the process of recycling heat from internal exhaust air with temperature t in, emitted during the cold period with high temperature to the street, to heat the supply of outside air. The process of heat recovery occurs in special heat recuperators: plate recuperators, rotating regenerators, as well as in heat exchangers installed separately in air currents with different temperatures (in exhaust and supply units) and connected by an intermediate coolant (glycol, ethylene glycol).
The last option is most relevant in the case when the supply and exhaust are spaced along the height of the building, for example, the supply unit is in the basement, and the exhaust unit is in attic, however, the recovery efficiency of such systems will be significantly less (from 30 to 50% compared to PES in one building
Plate recuperators They are a cassette in which the supply and exhaust air channels are separated by aluminum sheets. Heat exchange occurs between the supply and exhaust air through aluminum sheets. The internal exhaust air through the heat exchanger plates heats the external supply air. In this case, the air mixing process does not occur.
IN rotary recuperators heat transfer from the exhaust air to the supply air is carried out through a rotating cylindrical rotor consisting of a package of thin metal plates. During operation of a rotary heat exchanger, the exhaust air heats the plates, and then these plates move into the flow of cold outside air and heat it. However, in the flow separation units, due to their leakage, the exhaust air flows into the supply air. The percentage of overflow can be from 5 to 20% depending on the quality of the equipment.
In order to achieve the set goal - to bring the building of the Federal State Institution "Research Institute CEPP" closer to passive, during long discussions and calculations, it was decided to install supply and exhaust ventilation units with a recuperator from a Russian manufacturer of energy-saving climate systems– companies TURKOV.
Company TURKOV produces PES for the following regions:
- For the Central region (equipment with two-stage recovery ZENIT series, which works stably down to -25 O C, and is excellent for the climate of the Central region of Russia, efficiency 65-75%);
- For Siberia (equipment with three-stage recovery Zenit HECO series works stably down to -35 O C, and is excellent for the climate of Siberia, but is often used in the central region, efficiency 80-85%);
- For the Far North (equipment with four-stage recovery CrioVent series works stably down to -45 O C, excellent for extremely cold climates and used in the harshest regions of Russia, efficiency up to 90%).
However, the TURKOV PVU uses enthalpy plate recuperator, in which, along with the transfer of implicit heat from the exhaust air, moisture is also transferred to the supply air.
The working area of the enthalpy recuperator is made of a polymer membrane, which passes water vapor molecules from the exhaust (humidified) air and transfers them to the supply (dry) air. There is no mixing of the exhaust and supply flows in the recuperator, since moisture is passed through the membrane through diffusion due to the difference in vapor concentration on both sides of the membrane.
The dimensions of the membrane cells are such that only water vapor can pass through it; for dust, pollutants, water droplets, bacteria, viruses and odors, the membrane is an insurmountable barrier (due to the ratio of the sizes of the membrane “cells” and other substances).
Enthalpy recuperator essentially a plate recuperator, where a polymer membrane is used instead of aluminum. Since the thermal conductivity of the membrane plate is less than that of aluminum, the required area of the enthalpy recuperator is significantly larger than the area of a similar aluminum recuperator. On the one hand, this increases the dimensions of the equipment, on the other hand, it allows the transfer of a large volume of moisture, and it is thanks to this that it is possible to achieve high frost resistance of the recuperator and stable operation of the equipment at ultra-low temperatures.
IN winter time(outdoor temperature below -5C), if the humidity of the exhaust air exceeds 30% (at an exhaust air temperature of 22...24 o C), in the recuperator, along with the process of transferring moisture to the supply air, the process of moisture accumulation on the recuperator plate occurs. Therefore, it is necessary to periodically turn off the supply fan and dry the hygroscopic layer of the recuperator with exhaust air. The duration, frequency and temperature below which the drying process is required depends on the staging of the recuperator, the temperature and humidity inside the room. The most commonly used recuperator drying settings are shown in Table 1.
Table 1. Most commonly used heat exchanger drying settings
| Recuperator stages | Temperature/Humidity | ||||
|
|
<20% | 20%-30% | 30%-35% | 35%-45% | |
| 2 steps | not required | 3/45 min | 3/30 min | 4/30 min | |
| 3 steps | not required | 3/50 min | 3/40 min | 3/30 min | |
| 4 steps | not required | 3/50 min | 3/40 min | ||
Drying the recuperator is required only when installing air humidification systems, or when operating equipment with large, systematic moisture inflows.
- With standard indoor air parameters, the drying mode is not required.
In this article, as an example of an administrative building, we consider a typical five-story building of the Federal State Institution “Research Institute TsEPP” after the planned reconstruction.
For this building, the flow of supply and exhaust air was determined in accordance with air exchange standards in administrative premises for each room of the building.
The total values of supply and exhaust air flow rates by building floors are given in Table 2.
Table 2. Estimated flow rates of supply/exhaust air by building floors
| Floor | Supply air flow, m 3/h | Extract air flow, m 3/h | PVU TURKOV |
| Basement | 1987 | 1987 | Zenit 2400 HECO SW |
| 1st floor | 6517 | 6517 |
Zenit 1600 HECO SW Zenit 2400 HECO SW Zenit 3400 HECO SW |
| 2nd floor | 5010 | 5010 | Zenit 5000 HECO SW |
| 3rd floor | 6208 | 6208 |
Zenit 6000 HECO SW Zenit 350 HECO MW - 2 pcs. |
| 4th floor | 6957 | 6957 | Zenit 6000 HECO SW Zenit 350 HECO MW |
| 5th floor | 4274 | 4274 |
Zenit 6000 HECO SW Zenit 350 HECO MW |
In laboratories, PVUs operate according to a special algorithm with compensation for exhaust from fume hoods, i.e., when any fume hood is turned on, the hood exhaust is automatically reduced by the amount of the hood exhaust. Based on the estimated costs, Turkov air handling units were selected. Each floor will be served by its own Zenit HECO SW and Zenit HECO MW PVU with three-stage recovery up to 85%.
Ventilation of the first floor is carried out by PVU, which are installed in the basement and on the second floor. Ventilation of the remaining floors (except for laboratories on the fourth and third floors) is provided by PVU installed on the technical floor.
The appearance of the Zenit Heco SW installation PES is shown in Figure 6. Table 3 shows the technical data for each installation PES.
Installation Zenit Heco SW includes: - Housing with heat and noise insulation;
- Supply fan;
- Exhaust fan;
- Supply filter;
- Exhaust filter;
- 3-stage recuperator;
- Water heater;
- Mixing unit;
- Automation with a set of sensors;
- Wired remote control.
An important advantage is the possibility of installing equipment both vertically and horizontally under the ceiling, which is used in the building in question. As well as the ability to place equipment in cold areas (attics, garages, technical rooms, etc.) and on the street, which is very important during restoration and reconstruction of buildings.
Zenit HECO MW PVU is a small PVU with heat and moisture recovery with a water heater and a mixing unit in a lightweight and versatile polypropylene foam housing, designed to maintain the climate in small rooms, apartments, and houses.
Company TURKOVhas independently developed and produces Monocontroller automation for ventilation equipment in Russia. This automation is used in the Zenit Heco SW PVU
- The controller controls electronically commutated fans via MODBUS, which allows you to monitor the operation of each fan.
- Controls water heaters and coolers to accurately maintain supply air temperature in both winter and summer.
- For CO control 2 in the conference room and meeting rooms the automation is equipped with special CO sensors 2 . The equipment will monitor the CO concentration 2 and automatically change the air flow, adjusting to the number of people in the room, to maintain the required air quality, thereby reducing the heat consumption of the equipment.
- A complete dispatch system allows you to organize a dispatch center as simply as possible. A remote monitoring system will allow you to monitor equipment from anywhere in the world.
Control panel capabilities:
- Clock, date;
- Three fan speeds;
- Real-time filter status display;
- Weekly timer;
- Setting the supply air temperature;
- Display of faults on the display.
Efficiency mark
To assess the efficiency of the installation of Zenit Heco SW air handling units with recuperation in the building under consideration, we will determine the calculated, average and annual loads on the ventilation system, as well as costs in rubles for the cold period, warm period and for the entire year for three PVU options:
- PVU with recovery Zenit Heco SW (recuperator efficiency 85%);
- Direct-flow PVU (i.e. without a recuperator);
- PVU with heat recovery efficiency of 50%.
The load on the ventilation system is the load on the air heater, which heats (during the cold period) or cools (during the warm period) the supply air after the recuperator. In a direct-flow PVU, the air in the heater is heated from the initial parameters corresponding to the parameters of the outside air during the cold period, and is cooled during the warm period. The results of calculating the design load on the ventilation system in the cold period by floor of the building are shown in Table 3. The results of calculating the design load on the ventilation system in the warm period for the entire building are shown in Table 4.
Table 3. Estimated load on the ventilation system during the cold period by floor, kW
| Floor | PVU Zenit HECO SW/MW | Direct-flow PVU | PES with recovery 50% |
| Basement | 3,5 | 28,9 | 14,0 |
| 1st floor | 11,5 | 94,8 | 45,8 |
| 2nd floor | 8,8 | 72,9 | 35,2 |
| 3rd floor | 10,9 | 90,4 | 43,6 |
| 4th floor | 12,2 | 101,3 | 48,9 |
| 5th floor | 7,5 | 62,2 | 30,0 |
| 54,4 | 450,6 | 217,5 |
Table 4. Estimated load on the ventilation system during the warm period by floor, kW
| Floor | PVU Zenit HECO SW/MW | Direct-flow PVU | PES with recovery 50% |
| 20,2 | 33,1 | 31,1 |
Since the calculated outdoor air temperatures in the cold and warm periods are not constant during the heating and cooling periods, it is necessary to determine the average ventilation load at the average outdoor temperature:
The results of calculating the annual load on the ventilation system during the warm period and cold period for the entire building are shown in Tables 5 and 6.
Table 5. Annual load on the ventilation system during the cold period by floor, kW
| Floor | PVU Zenit HECO SW/MW | Direct-flow PVU | PES with recovery 50% |
| 66105 | 655733 | 264421 | |
| 66,1 | 655,7 | 264,4 |
Table 6. Annual load on the ventilation system during the warm period by floor, kW
| Floor | PVU Zenit HECO SW/MW | Direct-flow PVU | PES with recovery 50% |
| 12362 | 20287 | 19019 | |
| 12,4 | 20,3 | 19,0 |
Let us determine the costs in rubles per year for additional heating, cooling and fan operation.
The consumption in rubles for reheating is obtained by multiplying the annual values of ventilation loads (in Gcal) during the cold period by the cost of 1 Gcal/hour of thermal energy from the network and by the operating time of the PVU in heating mode. The cost of 1 Gcal/h of thermal energy from the network is taken to be 2169 rubles.
The costs in rubles for operating fans are obtained by multiplying their power, operating time and the cost of 1 kW of electricity. The cost of 1 kWh of electricity is taken to be 5.57 rubles.
The results of calculations of costs in rubles for the operation of the PES in the cold period are shown in Table 7, and in the warm period in Table 8. Table 9 shows a comparison of all options for the PES for the entire building of the Federal State Institution "Research Institute TsEPP".
Table 7. Expenses in rubles per year for the operation of the PES during the cold period
| Floor | PVU Zenit HECO SW/MW | Direct-flow PVU | PES with recovery 50% | |||
| For reheating | For fans | For reheating | For fans | For reheating | For fans | |
| Total costs | 368 206 | 337 568 | 3 652 433 | 337 568 | 1 472 827 | 337 568 |
Table 8. Expenses in rubles per year for the operation of the PES during the warm period
| Floor | PVU Zenit HECO SW/MW | Direct-flow PVU | PES with recovery 50% | |||
| For cooling | For fans | For cooling | For fans | For cooling | For fans | |
| Total costs | 68 858 | 141 968 | 112 998 | 141 968 | 105 936 | 141 968 |
Table 9. Comparison of all PES
| Magnitude | PVU Zenit HECO SW/MW | Direct-flow PVU | PES with recovery 50% |
| , kW | 54,4 | 450,6 | 217,5 |
| 20,2 | 33,1 | 31,1 | |
| 25,7 | 255,3 | 103,0 | |
| 11,4 | 18,8 | 17,6 | |
| 66 105 | 655 733 | 264 421 | |
| 12 362 | 20 287 | 19 019 | |
| 78 468 | 676 020 | 283 440 | |
| Reheating costs, rub | 122 539 | 1 223 178 | 493 240 |
| Cooling costs, rub | 68 858 | 112 998 | 105 936 |
| Costs of fans in winter, rub. | 337 568 | ||
| Costs of fans in summer, rub. | 141 968 | ||
| Total annual costs, rub | 670 933 | 1 815 712 | 1 078 712 |
An analysis of Table 9 allows us to draw an unambiguous conclusion - the air handling units Zenit HECO SW and Zenit HECO MW with heat and moisture recovery from Turkov are very energy efficient.
The total annual ventilation load of the TURKOV PVU is less than the load in the PVU with an efficiency of 50% by 72%, and in comparison with the direct-flow PVU by 88%. Turkov PVU will allow you to save 1 million 145 thousand rubles - in comparison with direct-flow PVU or 408 thousand rubles - in comparison with PVU, the efficiency of which is 50%.
Where else are the savings...
The main reason for failures in the use of systems with recovery is the relatively high initial investment, however, with a more complete look at the costs of development, such systems not only quickly pay for themselves, but also make it possible to reduce the overall investment during development. As an example, let’s take the most widespread “standard” development with use of residential, office buildings and shops.
Average heat loss value of finished buildings: 50 W/m2.
- Included: Heat loss through walls, windows, roofing, foundation, etc.
Included:
- Ventilation of apartments based on the purpose of the premises and multiplicity.
- Ventilation of offices based on the number of people and CO2 compensation.
- Ventilation of shops, corridors, warehouses, etc.
- The ratio of areas was chosen based on several existing complexes
Included:
- Compensation for toilets, bathrooms, kitchens, etc. Since it is impossible to organize an exhaust system from these rooms into the recuperation system, an influx is organized into this room, and the exhaust goes through separate fans past the recuperator.
The difference between the amount of supply air and the amount of compensation air.
It is this volume of exhaust air that transfers heat to the supply air.
So, it is necessary to develop the area with standard buildings with a total area of 40,000 m2 with the specified heat loss characteristics. Let's see what savings can be achieved by using ventilation systems with recovery.
Operating costs
The main purpose of choosing recuperation systems is to reduce the cost of operating equipment by significantly reducing the required thermal power to heat the supply air.
With the use of supply and exhaust ventilation units without recovery, we will obtain a heat consumption of the ventilation system of one building of 2410 kWh.
- Let's take the cost of operating such a system as 100%. There are no savings at all - 0%.
Using stacked supply and exhaust ventilation units with heat recovery and an average efficiency of 50%, we will obtain a heat consumption of the ventilation system of one building of 1457 kWh.
- Operating cost 60%. Saving with typesetting equipment 40%
Using monoblock highly efficient TURKOV supply and exhaust ventilation units with heat and moisture recovery and an average efficiency of 85%, we will obtain a heat consumption of the ventilation system of one building of 790 kWh.
- Operating cost 33%. Savings with TURKOV equipment 67%
As you can see, ventilation systems with highly efficient equipment have lower heat consumption, which allows us to talk about the payback of the equipment in a period of 3-7 years when using water heaters and 1-2 years when using electric heaters.
Construction costs
If construction is carried out in the city, it is necessary to extract a significant amount of thermal energy from the existing heating network, which always requires significant financial costs. The more heat required, the more expensive the supply cost will be.
Construction “in the field” often does not involve the supply of heat; gas is usually supplied and the construction of your own boiler house or thermal power plant is carried out. The cost of this structure is proportional to the required thermal power: the more, the more expensive.
As an example, assume that a boiler house with a capacity of 50 MW of thermal energy has been built.
In addition to ventilation, heating costs for a typical building with an area of 40,000 m2 and heat loss of 50 W/m2 will be about 2000 kWh.
Using supply and exhaust ventilation units without recovery, it will be possible to build 11 buildings.
With the use of stacked supply and exhaust ventilation units with heat recovery and an average efficiency of 50%, it will be possible to build 14 buildings.
Using monoblock highly efficient TURKOV supply and exhaust ventilation units with heat and moisture recovery and an average efficiency of 85%, it will be possible to construct 18 buildings.
The final estimate for supplying more thermal energy or building a high-capacity boiler house is significantly more expensive than the cost of more energy-efficient ventilation equipment. With the use of additional means of reducing the heat loss of a building, it is possible to increase the building size without increasing the required heating output. For example, by reducing heat loss by only 20%, to 40 W/m2, you can build 21 buildings.
Features of equipment operation in northern latitudes
As a rule, equipment with recovery has restrictions on the minimum outdoor air temperature. This is due to the capabilities of the recuperator and the limit is -25...-30 o C. If the temperature drops, the condensate from the exhaust air will freeze on the recuperator, therefore at ultra-low temperatures an electric preheater or a water preheater with non-freezing liquid is used. For example, in Yakutia the estimated street air temperature is -48 o C. Then classical systems with recovery work as follows:
- o With preheater heated to -25 o C (Thermal energy consumed).
- C -25 o The air is heated in the recuperator to -2.5 o C (at 50% efficiency).
- C -2.5 o The air is heated by the main heater to the required temperature (thermal energy is consumed).
When using a special series of equipment for the Far North with 4-stage recovery TURKOV CrioVent, preheating is not required, since 4 stages, a large recovery area and moisture return prevent the recuperator from freezing. The equipment operates in a graying manner:
- Street air with a temperature of -48 o C heats up in the recuperator to 11.5 o C (efficiency 85%).
- From 11.5 o The air is heated by the main heater to the required temperature. (Thermal energy is consumed).
The absence of preheating and high efficiency of the equipment will significantly reduce heat consumption and simplify the design of the equipment.
The use of highly efficient recovery systems in northern latitudes is most relevant, since low outside air temperatures make the use of classical recovery systems difficult, and equipment without recovery requires too much thermal energy. Turkov equipment successfully operates in cities with the most difficult climatic conditions, such as: Ulan-Ude, Irkutsk, Yeniseisk, Yakutsk, Anadyr, Murmansk, as well as in many other cities with a milder climate in comparison with these cities.
Conclusion
- The use of ventilation systems with recovery allows not only to reduce operating costs, but in the case of large-scale reconstruction or capital development, to reduce the initial investment.
- Maximum savings can be achieved in middle and northern latitudes, where equipment operates in difficult conditions with prolonged negative outdoor temperatures.
- Using the example of the building of the Federal State Institution "Research Institute TsEPP", a ventilation system with a highly efficient recuperator will save 3 million 33 thousand rubles per year - in comparison with a direct-flow PVU and 1 million 40 thousand rubles per year - in comparison with a stacked PVU, the efficiency of which is 50%.