What is recuperation?
What is recuperation?
Economical management of energy resources is becoming a necessity and a duty in the modern world. For many years, professors of the best universities, all over the world, they are working on better and better devices, which will rationally use energy resources, as well as recovering previously produced energy in later processes. At the same time, the use of energy-saving solutions in private housing has led to a dramatic deterioration in indoor air quality. Tight window joinery eliminates free infiltration of outside air. The effect of limited ventilation is the concentration of pollutants emitted during the process of using the rooms and an excessive amount of water vapor that may condense on the cold structural elements of the building., which leads to the growth of mold and fungus. All these factors adversely affect our health. So there was a need for a ventilation system, which would ensure air exchange, and thus the basic living comfort, and at the same time, to the maximum extent save expensive thermal energy. So important, at the same time, it has become very economical to recover ventilation heat. Nowadays it is especially easy because more and more buildings are equipped with mechanical ventilation systems with heat recovery.
The easiest, and at the same time the cheapest way of recovering ventilation heat is recirculation of ventilation air, consisting in introducing the removed air to the supply systems. However, this solution can only be used in a few cases. Presence of cigarette smoke in the exhausted air, kitchen smells, bacteria and dust exclude the possibility of recirculation. The most advantageous solution is the use of recuperators, which are the heart of a hygienically correct mechanical ventilation system. The main advantage of the recuperation system is the fact that the air stream is removed, from which we recover the ventilation heat, it does not come into direct contact with the fresh air blown in.
Types of recuperator construction
Compiled, a wide range of various types of recuperators has been constructed and connected to the supply and exhaust installation, which recover very valuable heat. The following devices are most often used in ventilation systems for heat recovery:
• recuperators with an intermediate medium (glycol or freon), including heat tube, and heat pumps,
• recuperators with movable filling (rotary exchangers),
• plate recuperators.
It should be noted here that the efficiency of using all types of recuperators is limited by the irreversibility of heat exchange processes and the flow resistance of the air flows supplied and removed from the premises.. The efficiency of recuperators is lowered, because heat exchange takes place due to temperature differences between the supplied air, and removed from the premises. It is related to the heat transfer area, i.e.. In order to obtain greater efficiency of the process of collecting heat from the exhaust air, a larger heat exchange surface should be used, which in turn leads to an increase in flow resistance and increases the cost of the device, because we increase the surface of the exchanger.
• Recuperators with an intermediate medium – they are structurally the most complex, which translates into the widest possible application. The air streams are completely separated from each other, and the distances between them reach up to several dozen meters. In this case, the position of the main supply and exhaust ducts is practically optional. Heat pumps – the most commonly used is the steam compressor system. They are used where there is a need for both heating and cooling rooms (mostly large objects). Heat pipe – it is a finned tube, filled with low-boiling refrigerant. One part is in the supply air duct, the second in the exhaust. The limitation of the application of this solution is the necessity to locate the supply duct above the exhaust duct, if the tube works in a gravity system.
• Recuperators with movable filling (rotary) – they consist of a rotor with an accumulative filling driven by an electric motor, as well as a sluice and a housing. The heat transfer process takes place due to the rotational movement of the exchanger rotor. Despite the fact that these devices have many advantages, they are reluctantly installed due to the necessity to use electric motors (also with great power) for rotating the rotor with accumulative filling. In addition, there are problems with the sealing between the moving filler, and the fixed housing. This leads to undesirable air flow through these leaks.
• Plate recuperators – they have slightly worse technical parameters, but are most often used (especially in single-family housing) due to its simple structure, and at the same time a competitive price. The lack of moving parts and an intermediate medium simplifies the operation and significantly reduces its costs. The following constructions are used in this group: -cross-flow plate heat exchanger, -counter-flow heat exchanger made of flat plates, -spiral heat exchanger. Among plate recuperators, the most commonly used are cuboid-shaped cross-flow exchangers, and often even a cube, which fits very well in the segments of the air handling unit. The flow of cooled and heated air takes place in perpendicular directions. In order to prevent the condensed moisture from freezing on the recuperator exchanger, a defrosting system is used here. During severe frosts, the outside air bypasses the recuperator, which is virtually out of operation or the fan supplying external air to the system is temporarily disabled. The spiral heat exchanger is created by winding metal sheets with a constant distance between them, which results in a cylinder-shaped structure. Solutions of this type can be used in the case of small ventilation air streams, due to the small cross-sections of the channels. The most promising and promising solutions seem to be countercurrent exchangers, due to the fact that they achieve the highest efficiency of heat recovery. Additionally, thanks to the uniform temperature distribution in the individual cross sections of the cooled air stream, they are more resistant to the phenomenon of moisture condensation, and frost is practically non-existent. In these types of recuperators there is an efficient removal of the condensed water vapor, therefore these devices should be mounted almost vertically like this, flow of cooled air downwards, and the condensate was draining into a drainpipe, installed near the ventilation stub in the lower part of the recuperator. The condensate should be discharged into the sewage system.