In today's world, when efficient use of energy is becoming a priority, heat recovery units are becoming increasingly popular as an effective tool for improving the energy efficiency of buildings. The key element of these advanced ventilation systems are heat exchangers, commonly called recuperators. What are the types of exchangers used in heat recovery units and how do they differ from each other? Find out in this article!
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How was the heat recovery system used in single-family homes?
In everyday language, the word " recuperator " has been commonly used for many years. What does this term actually mean? A recuperator is a ventilation unit with heat recovery. Initially, heat recovery was used in large ventilation units with an air capacity of several dozen thousand, then several thousand m3/h. Legal regulations have gradually reduced this value. According to the current regulation of the Minister of Infrastructure "On the technical conditions that buildings and their location should meet", devices for recovering heat from exhaust air must be used in mechanical ventilation with an air capacity of 500 m3/h and more.
The thermal efficiency of the heat recovery device must be at least 50%. Nevertheless, it is not only the current legal regulations that have contributed to the growing interest in the use of heat recovery in residential homes over the last few years. Often, the efficiency of the central unit in a home does not exceed 500 m3/h. People are increasingly aware of the importance of the quality of the air we breathe every day, which has consequently contributed to the popularization of mechanical ventilation in single-family buildings.
What role does the heat exchanger play in a recuperator?
The recuperator (heat recovery unit) consists of the following elements:
- Heat exchanger
- Supply fan
- Exhaust fan
- Air filter
- Exhaust filter
- Sensors
- Electric heater
- Bypass
- Housing
We will focus on the most important component located in the recuperation unit , namely the heat exchanger. It can be safely said that it is the heart of the device. The exchanger is where the heat is exchanged from the exhaust air (from the rooms) to the supply air (from outside). Thanks to this, we do not lose all the heat with the exhaust air (which happens in typical gravity ventilation), and a large part of it can be recovered. This allows us to save energy that we would have to spend on heating the air needed for ventilation.
An important topic accompanying the heat recovery process is moisture recovery or, in fact, its lack. The issue of the level of air humidity in a room often does not have a high priority when choosing the right device. Air humidity determines a number of aspects of life and seems to be one of the key elements influencing everyday comfort. Air humidity directly affects our well-being, health problems, the development of fungi and bacteria, the transmission of viruses or the durability of building materials. Therefore, the types of exchangers discussed in detail below will also be described in terms of this aspect.
Division and types of heat exchangers used in heat recovery units
In heat recovery ventilation units, heat exchangers occupy a central place, playing a key role in the efficient functioning of the system. They are responsible for transferring heat from exhaust air to supply air, which helps to minimize energy losses and increase the efficiency of the entire ventilation process. There are many types of heat exchangers used in heat recovery ventilation units, each of which has unique features and applications. What makes them stand out?
Plate exchangers
Plate exchangers are made of thin plates made of steel, aluminum, plastic or a special membrane in the case of enthalpy exchangers (recovering moisture). The aforementioned plates create supply and exhaust channels isolated from each other. The air streams do not contact each other as they flow through the exchanger. Then the phenomenon of recuperation , or heat recovery, occurs. The stream of warm air (exhausted from the rooms) heats the exchanger plates, while the stream of cold air (supplied from the outside) absorbs heat from the heated plates. In winter, moisture condenses on the surface of the exchanger due to the difference in the temperatures of the supply and exhaust air.
The condensed condensate is drained to a drip tray, and its excess partially freezes, blocking the spaces through which air flows. This process is popularly called exchanger frosting. At outside temperatures lower than -4 °C, anti-freeze protection must be used. Ventilation units with plate exchangers are equipped with protection systems in the form of an empty chamber with a damper, located next to the exchanger (commonly called a by-pass), an electric pre-heater or algorithms regulating the operation of fans. Depending on the air flow method, plate exchangers are divided into cross-flow or counter-flow.
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Cross flow exchanger
The cross-flow exchanger is constructed in such a way that the air channels of the supply and exhaust streams are perpendicular to each other. It looks as if the air streams were crossing each other (hence the name). In reality, they run in separate channels. Due to the relatively short contact time of the air streams, the efficiency of cross-flow exchangers is lower than that of counter-flow exchangers and ranges from 50 to 75%. Cross-flow exchangers are the most susceptible to frosting of all. Their advantage is undoubtedly their simplicity, which translates into a favorable price.
Counterflow exchanger
The counterflow exchanger is more developed than the cross-flow exchanger. It is its improved version. The basic difference is that in the middle part of the exchanger, the supply and exhaust channels run parallel to each other. The streams flow in opposite directions. Thanks to this, the cold air passing through the last section of the exchanger is heated by the warmest exhaust air. Recuperators with a counterflow exchanger are characterized by higher efficiency than those with a cross-flow exchanger. Their efficiency reaches up to 90%. They are less susceptible to frosting, but the unit still has a number of safeguards aimed at defrosting the exchanger in the winter, where the phenomenon of water vapor condensation occurs continuously. Due to their construction, they are more expensive than cross-flow exchangers.
Condensing counterflow heat exchanger
A counterflow heat exchanger that irreversibly removes condensate outside the air circulation is called a condensing, counterflow heat exchanger. This can be both its disadvantage and advantage. The water vapor gains generated by household members are usually not enough to cover the losses caused by the continuous operation of mechanical ventilation. Therefore, in the winter the air becomes dry, which has a significant impact on the comfort of use. In turn, during rainy and transitional periods, the presence of such an exchanger allows the air in the building to be dried.
This is also useful at the beginning of the use of a new building, when moisture is still accumulated in the plasters and screeds. At this point, it would be reasonable to consider using a supply air humidifier. However, their popularity on the Polish market is low. Manufacturers who offer a humidifier very often adapt it to work only with a dedicated central unit of their production. And from a purely commercial point of view, it is hard to blame them for this.
Enthalpy exchanger
The difference between the enthalpy and condensing counterflow heat exchanger is the material used to build the plates. In the case of the enthalpy counterflow heat exchanger, the plates are made of a polymer membrane, which is like a filter that allows the transfer of water vapor. Heat recovery and partial moisture recovery occur through such an exchanger.
An additional advantage of enthalpy exchangers is a wider operating range, without the need for additional anti-freeze protection (operation down to -8°C). Nevertheless, units with these exchangers are equipped with the protection systems mentioned above. Moisture recovery in winter is advisable and solves the problem of dry air. However, it should be remembered that the aforementioned recovery can also occur in summer. The humidity of the internal air must then be higher than the humidity of the external air.
Spiral counterflow exchangers
It is worth mentioning that there are counter-flow spiral heat exchangers. The design of such a heat exchanger is protected by a patent claim. This is a unique solution, implemented by one of the Polish companies. The exchanger is made of aluminum foil and galvanized sheet rolled spirally and shaped in the form of a cylinder. The exchanger is structurally divided into two independent spaces: supply air and exhaust air.
Both spaces are separated by a seal, ensuring precise separation, which allows heat recovery from the exhaust air without mixing it with the supply air. The spiral design of the exchanger makes the air flow in a circle, counter-currently and partially cross-currently. The large heat exchange surface and long air flow path make the exchanger frost-resistant, and its efficiency exceeds 85% with the amount of supply air to exhaust air in a ratio of 1:1.
High frost resistance distinguishes heat recovery units with this solution and allows them to be used without protective heaters at the fresh air inlet and without defrosting systems required in other devices of this type. The lack of need to use a pre-heater to protect the exchanger from frosting at low temperatures gives measurable savings throughout the year.
Rotary condensing heat exchanger
The condensing rotary heat exchanger is made of alternating corrugated and flat aluminium sheets wound on a special cylinder (drum). The sheet creates channels through which the air streams supplied and extracted from the rooms flow alternately. A drive belt is wound around the exchanger, which, driven by the engine, sets the cylinder in motion (rotation around its own axis). During rotation, energy recovery takes place based on the regeneration process.
The regeneration process is characterized by the fact that the internal and external air streams contact the same surface of the exchanger (mix), which enables heat recovery and partial moisture recovery. In winter conditions, when the external air temperature is low, the air extracted from the rooms cools to a temperature below the dew point, which leads to condensation of the condensate. The water settles in the channels and then, as a result of the exchanger rotation, is in the cold environment of the external air.
The outside air heats up and causes some of the condensate to evaporate and then recirculate, thus partially humidifying the fresh air. The process is continuous and the condensate does not have enough time to freeze. This is where the advantage of rotary heat exchangers emerges, as they can operate at very low outside temperatures (-30°C) without the need for anti-freeze protection. The disadvantage, however, is the dependence on mechanical components (engine, drive belt, etc.) and additional power consumption (necessity of additional power supply).
Sorption rotary heat exchanger
A sorption rotary heat exchanger is nothing more than a condensing exchanger equipped with additional hygroscopic material, which allows for the absorption of water vapor from the air. Such an exchanger is able to recover up to 90% of moisture from exhaust air. The absorption of water vapor from the air occurs both in winter and summer, and is not based on the phenomenon of condensation. The use of this type of exchanger will work well in rooms with low air humidity and the introduction of another exchanger (condensing rotary or enthalpy countercurrent) would not bring satisfactory results due to too low moisture recovery.
What to consider when choosing a recuperator?
The choice of a recuperator should be preceded by a basic analysis of the needs of people using ventilated rooms. We will choose a device differently for office rooms than for a residential house. When it comes to single-family and multi-family buildings, it is always necessary to take into account that each family is different, has different habits and ways of functioning, which is closely related to the generation of moisture gains. Moisture gains should be of key importance when choosing the right energy recovery solution.