- About the principle of recuperation
- Technological solutions
- Definition of performance
- Recuperator control
- Place and method of installation
- Recuperator installation
Energy-saving technologies are of interest not only among those who are puzzled by the state of the ecosystem, but also those who simply want to save people. We will tell you what benefits the recuperation units promise, how they function and what is the peculiarity of installing such a system in your own home.
About the principle of recuperation
The term recuperation comes from the Latin word meaning exchange, transfer of something. In the context of ventilation, this concept means the transfer of heat from the extract air to the supply air without mixing the two streams. Initially, the interest in recuperation devices was dictated mainly by innovation trends and the prospects for environmental safety. Later it became clear that this is a truly effective way to optimize the energy efficiency of a building..
Air recuperator working principle
The principle of return heat transfer has a quantitative expression. Heat transfer efficiency increases with increasing temperature differences. Also, due to the lack of mixing of flows, it is obvious that the full operation of the device is possible only with a sufficiently high ratio of the thermal contact area to the mass of the air passing through the recuperator..
In essence and in principle of operation, each recuperator is an economizer that collects waste of low-grade energy and directs it for useful work. Heat recovery is not characterized by high efficiency, but in well-insulated buildings, heat leaks through ventilation are among the main losses, therefore their reduction is the most important task to ensure the lowest possible heat balance..
Heat recuperators have many technical implementations, among which there are both local air handling units and equipment for installation in centralized systems. In any particular model, developers strive to think over every little thing, because for such devices, an increase in one of the indicators inevitably causes a deterioration in other parameters..
For example, in order to have time to give off maximum heat, the exhaust air must travel along as long a path as possible, which inevitably increases the total aerodynamic resistance of the ventilation system. It turns out that for the correct operation of a highly efficient recuperator, either an accelerating section of a very long length is required, or forced air movement with the resulting dependence on power supply.
In accordance with the device and the principle of operation, plate, tubular and rotary recuperators are distinguished – these are the three most popular types that are suitable for use in the civilian sphere due to their simple design.
Plate recuperators are containers with a complex labyrinth of partitions, along which two air streams move in opposite directions. This is the simplest type of construction, which is most widely used in domestic recuperators. The main disadvantage is the increase in aerodynamic resistance at the installation point.
Tubular recuperators are more complex, in fact, they represent one large channel in which several tubes of a smaller diameter are laid. To achieve a thermal contact area comparable to a plate structure, an increase in the length of the channels is required, which leads to an increase in material consumption, and has a negative effect on the dimensions and cost of the device. But there is also a positive aspect: the turbulence of the air moving through the tube system promotes more efficient heat transfer without slowing down the exhaust flow..
Rotary recuperators use a working fluid for heat exchange – a set of thin rotating discs that heat up when passing through a warm channel and cool down in a cold one. The disadvantage of such recuperators is the technological gaps between the discs, which, although insignificant, still lead to partial mixing of flows.
In general, all designs have a primitive device, which affects the efficiency, therefore many manufacturers supplement the classical device circuit with some interesting solutions. A lot of work is being done to find materials that can be processed well and transfer heat as best as possible. In plate recuperators, the walls are made corrugated or fins are installed on them, tubular heat exchangers are made of thin-walled non-ferrous metals.
One of the most interesting solutions is the installation of Peltier elements, and due to the positive COP, their number is literally unlimited. The same principle is used in recuperators combined with an air heating system: heat pumps in such installations have a much wider operating temperature range and an increased power gain..
The most advanced recuperators have a dual flow reversal system. Warm exhaust air is initially supplied to the colder part of the heat exchanger, where, due to the large temperature difference, a significant increase in heat transfer efficiency is observed. Condensate is also formed in the process, which is heated and transferred to the evaporator inside the supply chamber. This helps to level the dehumidification of air during heating, in addition, water as a carrier of latent heat contributes to an even more intensive transfer of energy. Some points are thought out to the smallest detail: for example, the motors are specially placed at the beginning of the exhaust and the end of the supply duct, and they are also supplied with high-quality fins for a complete return of parasitic heat.
Definition of performance
For a recuperator as a part of ventilation, three parameters are most important: reduced aerodynamic resistance, permissible flow rate and efficiency expressed in terms of the heat recovered to the total amount of energy contained in the air at the effective temperature delta. This ratio is not constant: the colder the supply air, the more efficient the recuperator generally works, and the dependence of these changes is not linear. Therefore, it is so important to pay attention to the diagrams of changes in the main characteristics depending on other conditions..
Q = S v 3600
- Q – ventilation duct throughput, m3/ h;
- S – channel cross-sectional area, in m2;
- v – flow velocity, m / s.
Kt = (T3 – T1) / (T2 – T1)
- Kt – coefficient of efficiency of the recuperator in terms of temperature;
- T1 – outdoor temperature, ° C;
- T2 – air temperature in the room, ° С;
- T3 – supply air temperature, ° С.
The initial criterion – the permissible flow rate – is determined by the parameters of the ventilation system. Of course, air exchange cannot be lower than the norms established by SNiP: 3 m3/ h m2 or 30 m3/ h for each person with a space provision rate of less than 20 m3/ person In this case, the total frequency of air exchange per hour should be at least 0.35. If the parameters of the ventilation system at the moment do not correspond to the norm, the recuperator is selected according to regulatory requirements, and the ventilation system is subsequently refined..
If the capacity of the recuperator with forced air movement exceeds the capacity of the ventilation system by more than 50%, the excess noise is eliminated by installing a silencer. It should also be remembered that the fan performance at the supply channel is higher than at the extract channel; the difference must be selected in accordance with the number of additional points of natural air removal.
There are no specific requirements for the energy efficiency of the installation; in general, this parameter is important for determining the profitability of the purchase. You can estimate the conditional efficiency of the device using online calculators and data from the manufacturer, the difference in supply air temperatures is taken as a reference point. Additionally, you need to pay attention to the restrictions on air humidity and temperature differences, due to the discrepancy between these indicators, the recuperator may freeze up in winter.
As a rule, recuperators serve as an active element of a forced ventilation system or at least imply the ability to regulate the intensity of air exchange. There are several ways to establish interaction between the recuperator and the rest of the components..
In the simplest case, the recuperator does not have flow forcing devices, but it is equipped with an adjustable damper. It is necessary to ensure the correct ratio between the heat exchanger capacity and the current fan power, depending on the location of the latter. In one case, the control unit built into the recuperator regulates the fan rotation speed, but it is also possible that a PLC with a built-in proportional controller is used, the setting of which is carried out empirically.
In another case, the recuperator serves as the only device for forcing the flow and, accordingly, only the speed of its fans determines the intensity of air exchange. For such devices, manual mode switching is provided, as well as internal control algorithms that optimize heat exchange depending on the current temperature difference. The most advanced units in terms of ergonomics are connected to the general house automation system and independently adjust the performance depending on the number of people or based on the data of room gas analyzers.
Place and method of installation
Recuperators are available in floor or suspended ceiling installations. There is also a third option – point wall recuperators, which are mounted in each room adjacent to the street, and do not require the laying of additional communications.
Ceiling installation options are interesting for the ability to hide the technical equipment of the house in the cavity of suspended or stretch ceilings. Such devices are slightly more expensive due to the requirements for compactness, at the same time, additional bypass channels are not required for their connection. An obvious disadvantage of this type of placement is the increased noise due to the small distance of the running motors from the ventilation grilles..
Floor-standing (and wall-mounted) recuperators are designed for installation in technical rooms. Their performance is not limited by dimensions, but a high-quality strapping system is required. As a rule, devices in this category are used in combination with air heating and air conditioning systems..
The installation and connection of the recuperator itself is limited by its mechanical fastening to the capital surface and joining with the common exhaust and supply channels. After this, the joints are sealed, and the recuperator itself is put into a special case, which simultaneously performs the function of heat protection and sound absorption.
Much more complicated is the situation with the design of ventilation systems, if they provide for the installation of a recuperator. For duct recuperators, it is required to lay two air ducts in each living room for air intake and supply. At the same time, it is important to calculate the free cross-section of the ventilation grilles and select the correct sockets in order to avoid additional noise..
In the structure of general ventilation in the house, recuperators provide air exchange only between living quarters. Exhaust ducts from the kitchen and bathrooms are usually arranged bypassing the heat exchanger due to the latter’s sensitivity to dirty air and high humidity. In such a case, it is possible to recommend the installation of an additional air filtration unit with grease and dispersed filters. You can also make a choice towards multi-channel recuperators, the design of which provides for the connection of an auxiliary ventilation circuit for technical rooms.