Control Principle of Rotary Heat Recovery Air Conditioning Unit

With the growth of my country's economic strength and the continuous improvement of people's material and cultural living standards; the rapid development of high-rise buildings, high airtightness and high thermal insulation have affected people's working and living environment, and people's requirements for indoor air quality have become higher and higher. They all desire to have a healthy and comfortable indoor environment. Especially after the attack of SARS, people pay more and more attention to indoor air quality and put forward higher requirements for introducing outdoor fresh air for ventilation. However, ventilation will inevitably bring energy loss, and introducing fresh air consumes more energy. Therefore, it is necessary to consider an effective energy-saving method to exchange heat between fresh air and exhaust air through a heat recovery device. The heat exchanger is a key device for air conditioning and waste heat recovery.
The main types of heat recovery units include rotary full heat recovery, rotary sensible heat recovery, plate full heat recovery, plate sensible heat recovery, heat pipe heat recovery, and ethylene glycol heat recovery.
Heat recovery principle and characteristics
Ⅰ) Rotary heat recovery device, which has a rotor that rotates at a constant speed/variable speed inside. It is made of aluminum foil or aluminum foil and high-efficiency desiccant using high-tech technology, or made of special composite materials. It is assembled in a box that allows the airflow to pass in the opposite direction without interfering with each other, and is driven by a transmission drive belt.
In winter, the enthalpy of indoor exhaust air is higher than that of outdoor fresh air. When the exhaust air passes through the rotor, the enthalpy of the rotor increases due to energy exchange. When it runs to the fresh air side, it releases energy to the fresh air with low enthalpy, and the fresh air is heated up. In summer, on the contrary, the temperature of the fresh air decreases. Due to the continuous rotation of the rotor, the energy of the air on the high temperature side is continuously exchanged with the air on the low temperature side.
In the full-heat type rotor, humidity transfer also takes place. When there is a pressure difference between the water vapor partial pressure of the air on both sides, moisture will be absorbed from the high side through the rotor and released on the low side after rotation, thus achieving latent heat exchange.
The characteristics of the rotary heat recovery device are: compact structure, small footprint, space saving; high heat recovery efficiency; large windward area of a single rotary wheel, small resistance. It is widely used in heat recovery of large air volume air conditioning systems.
Ⅱ) Plate heat recovery device recovers sensible heat or total heat when there is a temperature difference or water vapor partial pressure difference between the two airflows on both sides of its partition. In the plate heat recovery device, the two airflows cross the heat exchanger. The partition of the sensible heat exchanger is a non-permeable material with good thermal conductivity, generally aluminum. The total heat exchanger is a permeable air-to-air heat exchanger, and its partition is made of treated materials with good heat transfer and moisture permeability. The temperature (sensible heat) exchange mechanism is that when air of different temperatures flows through the two sides of the medium, heat is exchanged by conduction.
The exchange of humidity (latent heat) in a total heat exchanger occurs through the following two mechanisms:
① Humidity exchange is carried out through the difference in water vapor partial pressure on both sides of the medium.
②The water vapor on the high humidity side is absorbed by the desiccant and released to the low humidity side through the capillary action of the paper fibers.
Plate heat recovery unit features: good sealing, low air mixing rate; high heat recovery efficiency; no moving parts, stable and reliable operation, low maintenance cost. It is most widely used in heat recovery of air conditioning systems.
III) Heat pipe heat recovery device, first evacuates the sealed pipe, fills in an appropriate amount of working fluid in this state, heats the lower end of the heat pipe, and the working fluid absorbs heat and vaporizes into steam. Under a small pressure difference, it rises to the upper end of the heat pipe, releases heat to the outside, and condenses into liquid. Under the action of gravity, the condensate returns to the heated section along the inner wall of the heat pipe, and is heated and vaporized again, and the cycle repeats, continuously transferring heat from one end to the other. Because it is phase change heat transfer, the thermal resistance in the heat pipe is very small.
A heat pipe is a heat transfer element that achieves heat transfer through the phase change of its internal liquid. It has the following characteristics:
⑴ Each heat pipe is permanently sealed, there is no additional energy loss during heat transfer, no moving parts, and high operating reliability.
⑵The structure of the heat pipe heat exchanger determines that it is a typical countercurrent heat exchange, and the heat pipe operates almost isothermally, so the heat pipe heat exchanger has a very high efficiency.
⑶ Since the heat exchange of cold and hot gases takes place on the outer surface of the heat pipe, it is easy to expand the heating area.
⑷The hot and cold gases are separated by a partition, so there is no leakage and therefore no cross-contamination problem.
⑸ Because the fluid flow channel is spacious, the resistance loss is small.
⑹Each heat pipe is completely independent and easy to maintain.
⑺From the perspective of environmental adaptability, waste heat recovery efficiency, pressure loss, anti-clogging, cleaning, life and other comprehensive indicators, heat pipe heat exchangers have unique advantages.
V) Ethylene glycol heat recovery device, using heat exchanger and ethylene glycol solution as heat exchange medium, transfers the cold (heat) in the exhaust air to the ethylene glycol solution through the heat exchanger on the exhaust side, reduces (increases) the temperature of the ethylene glycol solution, and then transports the cooled (heated) ethylene glycol solution to the heat exchanger on the fresh air side through the circulation pump, reduces (increases) the fresh air temperature, reduces the system load and the operating cost of the entire air conditioning system.
Features of ethylene glycol heat recovery device: no cross contamination, easy installation, small space restriction, suitable for places with long air supply and exhaust distance.