Supply and exhaust ventilation with heat recovery: operating principle, overview of the advantages and disadvantages

The intake of fresh air in the cold period of time leads to the need for heating to ensure the correct indoor climate. For minimizing the cost of electricity can be used supply and exhaust ventilation with heat recovery.

Understanding the principles of its operation will allow the most efficient reduction of heat loss while maintaining a sufficient volume of replaced air. Let's try to figure this out.

Energy saving in ventilation systems

In the autumn-spring period when ventilation is a major problem is the large temperature difference between the incoming and inside air. The cold stream rushes down and creates an unfavorable microclimate in homes, offices and in the workplace or an unacceptable vertical temperature gradient in the warehouse.

A common solution to the problem is integration into the supply ventilation air heaterby which the flow is heated. Such a system requires energy consumption, while a significant amount of outgoing warm air leads to significant heat loss.

Exit to the outside with intense steam serves as an indicator of significant heat loss, which can be used to heat the incoming stream

If the air supply and exhaust channels are located nearby, then it is possible to partially transfer the heat of the outgoing stream to the incoming. This will reduce the energy consumption of the heater or completely abandon it. A device for providing heat exchange between different temperature gas flows is called a recuperator.

In the warm season, when the outside temperature is much higher than room temperature, a recuperator can be used to cool the incoming stream.

Unit unit with recuperator

The internal structure of the ventilation system with integrated recuperator simple enough, therefore their independent element-wise purchase and installation is possible. In the event that the assembly or self-assembly is difficult, you can purchase ready-made solutions in the form of typical monoblock or individual prefabricated structures on order.

A typical design of a supply and exhaust ventilation system device with a recuperator located in a single housing can be supplemented by other nodes at the user's discretion

The main elements and their parameters

The case with heat and noise insulation is usually made of sheet steel. In the case of wall mounting, it must withstand the pressure that occurs when foaming slots around the unit, and also prevent vibration from the fans.

In the case of a distributed intake and air flow over various rooms, they are connected to the housing duct system. It is equipped with valves and dampers for flow distribution.

In the absence of air ducts, a grille or diffuser is installed on the supply air outlet from the side of the room to distribute the air flow. An outdoor air intake grille is mounted on the inlet opening from the street to prevent birds, large insects and litter from entering the ventilation system.

Air movement is provided by two axial or centrifugal fans. In the presence of a recuperator, natural air circulation in a sufficient volume is impossible due to the aerodynamic drag created by this unit.

The presence of a recuperator involves the installation of fine filters at the inlet of both flows. This is necessary to reduce the clogging of dust and fat deposits in the thin channels of the heat exchanger. Otherwise, for the full functioning of the system will have to increase the frequency of preventive maintenance.

Fine filters must be changed or cleaned periodically. Otherwise, increased air flow resistance will cause the fans to break.

One or more recuperators occupy the bulk of the supply and exhaust device. They are mounted in the center of the structure.

In the case of severe frosts typical for the territory and insufficient efficiency of the heat exchanger for heating outdoor air, an air heater can be additionally installed. Also, if necessary, a humidifier, an ionizer and other devices are mounted to create a favorable microclimate in the room.

Modern models include an electronic control unit. Sophisticated modifications have functions for programming operating modes depending on the physical parameters of the air. External panels have an attractive appearance, due to which they can be well integrated into any room interior.

Solving the problem of condensation

The cooling of the air coming from the room creates the prerequisites for the discharge of moisture and the formation of condensate. In the case of a high flow rate, most of it does not have time to accumulate in the recuperator and goes outside. With slow air movement, a significant portion of the water remains inside the device. Therefore, it is necessary to ensure the collection of moisture and its removal outside the housing supply and exhaust system.

An elementary device for the collection and removal of condensate is a pan located under the recuperator with a slope towards the drain hole

Conclusion of moisture produced in a closed container. It is placed only indoors to avoid freezing of the outflow channels at sub-zero temperatures.There is no reliable algorithm for calculating the volume of water obtained when using systems with a recuperator, so it is determined experimentally.

The reuse of condensate to humidify the air is undesirable, since water absorbs many pollutants, such as human sweat, odors, etc.

Significantly reduce the amount of condensate and avoid the problems associated with its appearance 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.

In the case of cooling the outgoing air flow to negative temperatures inside the recuperator, condensate transitions to ice, which causes a reduction in the living cross section of the flow and, as a result, a decrease in volume or complete cessation of ventilation.

For periodic or one-time defrosting of the recuperator, a bypass is installed - a bypass channel for the supply air movement. When the flow passes bypassing the device, the heat transfer stops, the heat exchanger heats up and the ice becomes liquid. Water flows into the condensate collection tank or evaporates outward.

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 the heat recovery of the heat exchanger in other ways is complicated and long

When the flow passes through the bypass, there is no heating of the supply air through the recuperator. Therefore, when this mode is activated, it is necessary to turn on the air heater automatically.

Features of various types of recuperators

There are several structurally different options for the implementation of heat transfer between cold and heated air flows. Each of them has its own distinctive features that determine the main purpose for each type of recuperator.

Plate cross flow heat exchanger

The design of the plate heat exchanger is based on thin-walled panels connected alternately in such a way as to alternate the passage between them of different temperature flows 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 compounds with a polyester resin

Heat transfer panels can be made of various materials:

• copper, brass and aluminum-based alloys have good thermal conductivity and are not susceptible to rust;
• a plastic made of a polymer hydrophobic material with a high coefficient of thermal conductivity is lightweight;
• absorbent cellulose allows condensate to penetrate through the plate and return to the room.

The disadvantage is the possibility of condensation at low temperatures. Due to the small distance between the plates, moisture or ice significantly increase aerodynamic drag. In case of freezing, it is necessary to shut off the incoming air flow to warm the plates.

The advantages of plate recuperators are as follows:

• low cost;
• long service life;
• a long period between preventative maintenance and its simplicity;
• 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 operation of furnaces.

Drum or rotary type

The principle of operation of a rotary heat exchanger is based on the rotation of the heat exchanger, inside which are layers of corrugated metal with high heat capacity.As a result of interaction with the effluent, the drum sector is heated, which subsequently gives off heat to the incoming air.

The fine-mesh heat exchanger of the rotary heat exchanger is prone to clogging, therefore, it is especially important to pay attention to the quality work of fine filters

The advantages of rotary recuperators are as follows:

• rather high efficiency in comparison with competing types;
• the return of a large amount of moisture, which in the form of condensate remains on the drum and evaporates upon contact with incoming dry air.

This type of recuperator is less commonly used for residential buildings with apartment or cottage ventilation. Often it is used in large boiler rooms to return heat to furnaces or for large industrial or retail facilities.

However, this type of device has significant disadvantages:

• a relatively complex structure with moving parts, including an electric motor, a drum and a belt drive, which requires constant maintenance;
• increased noise level.

Sometimes for devices of this type, the term “regenerative heat exchanger” can be found, which is more correct than a “recuperator”. The fact is that a small part of the exhaust air flows back due to the loose fit of the drum to the body of the structure.

This imposes additional restrictions on the possibility of using devices of this type. For example, contaminated air from heating stoves cannot be used as a heat carrier.

Tube and Casing System

The tubular type recuperator consists of thin-walled tubes of small diameter located in the insulated casing of the system, through which external air flows. On the casing produce warm air mass output from the room, which heats the incoming stream.

The output of warm air must be carried out precisely 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, thanks to the countercurrent principle of movement of the coolant and the incoming air;
• simplicity of design and the absence of moving parts provides a low noise level and rarely arising need for maintenance;
• long service life;
• smallest cross-section among all types of recovery devices.

Tubes for devices of this type use either light-alloy metal or, less commonly, polymer. These materials are not hygroscopic, therefore, with a significant difference in the temperature of the flows, the formation of intense condensate in the casing is possible, which requires a constructive solution for its removal. Another disadvantage is that the metal filling has considerable weight, despite its small dimensions.

The simplicity of the design of the tubular recuperator makes this type of device popular for self-manufacturing. As the outer casing, plastic pipes for air ducts, insulated with polyurethane shells, are usually used.

Intermediate heat transfer device

Sometimes the supply and exhaust 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, use an intermediate coolant circulating between the ducts through an insulated pipe. As a medium for the transfer of thermal energy using water or a water-glycol solution, the circulation of which is provided by heat pump.

The recuperator with an intermediate coolant is a volumetric and expensive device, whose use is economically justified for rooms with large areas

In the event that it is possible to use a different type of recuperator, 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, for small rooms with a low air flow, such devices are not used;
• significant volume and weight of the entire system;
• the need for an additional electric pump to circulate the fluid;
• 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 circuit is possible in a natural way, but only if the supply air duct is located above the exhaust duct.

Such a system does not require additional energy costs, but works for heating only at a significant temperature difference. In addition, it is necessary to fine-tune the point of change in the state of aggregation of the heat-transfer fluid, which can be implemented by creating the desired pressure or a specific chemical composition.

Main technical parameters

Knowing the required performance of the ventilation system and the heat exchange efficiency of the heat exchanger, it is easy to calculate the savings on heating the air for a room under specific climatic conditions. By comparing the potential benefits with the cost of buying and maintaining the system, you can reasonably make a choice in favor of a recuperator or a standard air heater.

Often, equipment manufacturers offer a model line in which ventilation units with similar functionality differ in the amount of air exchange. For residential premises, this parameter must be calculated according to table 9.1. SP 54.13330.2016

Efficiency

The efficiency of the recuperator is understood as the heat transfer efficiency, which is calculated by the following formula:

K = (T_P - T_n) / (T_at - T_n)

Wherein:

• T_P - temperature of the incoming air into the room;
• T_n - outdoor temperature;
• T_at - air temperature in the room.

The maximum value of efficiency with standard air flow rates and a certain temperature regime indicated in the technical documentation of the device. Its real rate will be slightly less.

In the case of independent manufacture of a plate or tubular heat exchanger, to achieve maximum heat transfer efficiency, the following rules must be adhered to:

• The best heat exchange is ensured by countercurrent devices, then cross-flow devices, and the smallest - with the unidirectional movement of both flows.
• The heat transfer rate depends on the material and the thickness of the walls separating the streams, as well as on the duration of the air inside the device.

Knowing the efficiency of the recuperator, it is possible to calculate its energy efficiency at various temperatures of the external and internal air:

E (W) = 0.36 x P x K x (T_at - T_n)

where P (m³/ hour) - air consumption.

Calculation of the efficiency of the recuperator in monetary terms and comparison with the costs of its purchase and installation for a 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 a complex structure and considerable 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 flow is determined by static pressure, which depends on the power of the fan and the main components that create aerodynamic drag.As a rule, its exact calculation is impossible due to the complexity of the mathematical model, therefore experimental studies are carried out for typical monoblock designs, and components are selected for individual devices.

The fan power must be selected taking into account the throughput of the installed recuperators of any type, which is indicated in the technical documentation as the recommended flow rate or the volume of air passed by the device per unit time. As a rule, the permissible air velocity inside the device does not exceed 2 m / s.

Otherwise, at high speeds in the narrow elements of the recuperator there is a sharp increase in aerodynamic drag. This leads to unnecessary energy costs, inefficient heating of the outdoor air and shorten the life of the fans.

The graph of pressure loss versus air flow rate for several models of high-performance heat exchangers shows a non-linear increase in resistance, therefore, it is necessary to adhere to the requirements for the recommended air exchange volume indicated in the technical documentation of the device

Changing the direction of air flow creates additional aerodynamic drag. Therefore, when modeling the geometry of the indoor duct, it is desirable to minimize the number of pipe turns by 90 degrees. Diffusers for air dispersion also increase resistance, so it is advisable not to use elements with a complex pattern.

Contaminated filters and grilles create significant interference with the flow, so they must be cleaned or replaced periodically. One of the effective ways to assess clogging is to install sensors that monitor the pressure drop in the areas before and after the filter.

Conclusions and useful video on the topic

The principle of operation of the rotary and plate recuperator:

Measuring the efficiency of a plate-type recuperator:

Domestic and industrial ventilation systems with an integrated recuperator have proven their energy efficiency in maintaining heat indoors. Now there are many offers for the sale and installation of such devices in the form of ready-made and tested models, as well as for individual orders. You can calculate the necessary parameters and perform the installation yourself.

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