1, vapor compression refrigeration
Principle: In the vapor compression refrigeration cycle system, the compressor from the evaporator inhalation of low-temperature and low-pressure refrigerant vapor, adiabatic compression by the compressor to become a high temperature and high pressure superheated vapors, and then pressed into the condenser in the fixed-pressure cooling and cooling medium heat release, and then cooled as a Subcooled liquid refrigerant, liquid refrigerant through the expansion valve (or capillary tube) adiabatic throttling into a low-pressure liquid refrigerant, evaporated in the evaporator to absorb the heat in the air conditioning circulating water (air), thereby cooling the air conditioning circulating water (air) to achieve refrigeration purposes, outflow of low-pressure refrigerant is sucked into the compressor, so the cycle of work.
System cycle
Enthalpy: a state function of the energy of a material system.
Equal to the internal energy of the mass plus the product of its volume and absolute pressure.
That is, H=U+pV
Entropy: a physical quantity (denoted S) of the state of a material system that indicates the extent to which that state may occur. In thermodynamics, a more abstract physical quantity used to describe the irreversibility of thermal processes. A process that actually occurs in an isolated system must necessarily increase its entropy.
Isentropic process: refrigerant compression in the compressor is an isentropic process;
Isopressurized process: refrigerant in the cooling and condensing process is an isopressurized process;
Isenthalpic process: refrigerant throttled through the expansion valve, the enthalpy before and after throttling is equal;
Isothermal process: refrigerant in the evaporator and the condenser in the no pressure loss.
Refrigeration four parts: compressor, condenser, evaporator, throttle.
Compressor
Function: Compress the refrigerant vapor from the low-pressure state to the high-pressure state, creating a refrigerant in the condenser at room temperature liquefaction conditions. Known as the whole device "heart".
Compressor classification:
Condenser
Function: the compressor discharged refrigerant superheated vapors cooled, and condensed into refrigerant liquid, in the condenser refrigerant heat discharge to the cooling medium.
Categories: water-cooled condenser, air-cooled condenser, evaporative condenser.
Air-cooled condenser: easy to use and install, does not require cooling water, heat by the extension will be brought into the atmosphere. But the same heat transfer coefficient is low, relative to other types of weight is large, fin surface will accumulate ash is the ability to reduce heat dissipation, must be cleaned up in a timely manner.
Evaporator
Function: rely on the evaporation of refrigerant liquid to absorb the heat of the cooling medium heat transfer equipment, it is in the refrigeration system is the task of the external output of cold.
Classification: full liquid (immersion) evaporator, dry evaporator.
Dry evaporator: immersed serpentine tube, shell and tube, plate, spray type and so on.
Throttling device
Function
1, cut off the pressure: high-pressure, room temperature refrigerant liquid flows through the expansion valve, it becomes low-pressure, low-temperature refrigerant liquid.
2, control refrigerant flow: the expansion valve through the temperature-sensitive package to feel the changes in the refrigerant superheat at the outlet of the evaporator to control the opening of the valve to regulate the flow of refrigerant into the evaporator, so that the flow rate of the evaporator to match the heat load.
3, control superheat: the expansion valve has the function of controlling the superheat of the refrigerant at the outlet of the evaporator, i.e., to maintain the full utilization of the heat transfer area of the evaporator, and to prevent the compressor from flushing the cylinder accident.
Categories: manual throttle valve, thermal expansion valve, capillary tube, electronic expansion valve, float plate, fixed orifice plate, variable orifice plate.
2, steam absorption refrigeration
Working fluid
The refrigerant-absorbent as the working fluid, known as the absorption of the working fluid pair.
Commonly used pairs of substances: lithium bromide - water (refrigerant is water), ammonia - water (refrigerant is ammonia) - low-boiling point of the work is a refrigerant.
Device: absorption refrigeration device by the generator, condenser, evaporator, absorber, circulating pumps, throttle valve and other components, the working medium including the production of cold refrigerant and absorption, desorption of refrigerant absorber, the two components of the work of the pair.
Advantages:
1, summer need to supply cold air, winter need to supply warm air year-round air-conditioned areas, the most suitable for the use of absorption system. At present, the United States, Japan's central air-conditioning system, absorption system accounts for more than 80%.
2, quiet operation, can reduce wear and tear to a minimum (in addition to liquid pump operation), fewer failures, simple maintenance.
3, does not rely on electricity.
4, capacity control is easy, only need to control the generator's heat source.
5, high system safety, no explosion.
6, the system full load and light load effect is the same, when the load change, only need to adjust the generator heat source and water circulation.
7, when the evaporation temperature and pressure is reduced, the absorption capacity is reduced only to a limited extent, stable operation.
(2) disadvantages:
1, water as refrigerant, can not get low temperature (water freezing point of 0 ℃).
2, when operating improperly, lithium bromide is prone to crystallization.
3, steam injection refrigeration
Principle: the boiler supplied by the higher pressure of water vapor (known as working steam) into the main injector, in the Laval nozzle adiabatic expansion, the use of this high-speed steam flow from the evaporator constantly pumping steam, in which to maintain a higher vacuum, i.e., lower evaporative pressure. From the refrigeration unit to the cold water, after throttling and depressurization into the evaporator, part of which evaporates and absorbs the heat of the rest of the water to reduce the temperature.
The cooled down cold water is output by the pump, supplying cold after repeated use. Working steam in the ejector, together with the steam from the evaporator, flow through the diffuser tube to raise the pressure to the condensing pressure (still vacuum), into the condenser and the cooling water in direct contact with the cooling water and condensation in the cooling water. The non-condensable gases in the condenser are removed by one or two-stage auxiliary jets to maintain a certain vacuum in the condenser. The condenser shown is called a hybrid condenser. Steam injection refrigeration can also be used shell and tube condenser, this time into the condenser in the water vapor through the heat transfer tube is cooled and condensed into water, condensate can be injected into the boiler with a cooling water pump, repeated use.
4, adsorption refrigeration
Principle: a certain solid adsorbent on a refrigerant gas adsorption, and adsorption capacity with the adsorbent temperature changes. By periodically cooling and heating the adsorbent, so that it is alternately adsorbed and desorbed. In desorption, the refrigerant gas is released and condensed into a liquid; in adsorption, the refrigerant liquid is evaporated, producing refrigeration.
Categorized by adsorption mechanism: physical adsorption refrigeration, chemical adsorption refrigeration.
Principle: the basic structure of adsorption refrigeration by the solar collector, condenser, liquid storage, evaporator and valve five modules. Adsorption refrigeration system operating mechanism is: during the day, the collector temperature increases with the temperature, refrigerant evaporation collector in the pressure increases, the gas into the condenser and condensation, made of liquid; at night, the temperature decreases, the adsorbent will absorb the refrigerant vapors, the pressure in the evaporator decreases, so there will be more liquids to gasify, and evaporation of the heat absorbed to cool down.
5, thermoelectric refrigeration
Thermoelectric refrigeration is the use of thermoelectric effect (that is, the Peltier effect) a refrigeration method - also known as the temperature difference between the electric refrigeration, semiconductor refrigeration.
In 1834, the French physicist Peltier in the copper wire on each end of a bismuth filament, in the two bismuth filaments were connected to the positive and negative electrodes of the DC power supply, power, found that a connector to become hot, the other connector to become cold, i.e., the two joints occurred at the phenomenon of absorption and exothermic respectively.
The size of the thermoelectric effect depends on the thermal potential of the two materials, pure metal materials, the thermal potential is very small, commonly used semiconductor materials with high thermal potential to do small thermoelectric cooler.
Ceramic plate type first-class semiconductor refrigeration components
A pair of N, P thermocouples produce a very small amount of refrigeration, the actual thermoelectric refrigeration device is a number of thermocouples to form a thermoelectric stack used.
Principle: charge carriers are at different energy levels in different materials, and under the action of an external electric field, excess energy is released when the charge carriers move from a material at a higher energy level to a material at a lower energy level. On the contrary, the charge carriers need to absorb energy from the outside world when they move from the material of low energy level to the material of high energy level. The energy is given off or absorbed as heat at the interface of the different materials.
6, magnetic refrigeration, acoustic refrigeration
Magnetic refrigeration: magnetic refrigeration based on the "magneto-thermal effect" (MCE) is a promising alternative to the traditional vapor cycle refrigeration technology. In materials with this effect, the alignment and randomization of magnetic momentum when an applied magnetic field is applied and removed causes a change in temperature in the material, which can be transferred to the ambient air.
Acoustic refrigeration: based on the so-called thermoacoustic effect, the mechanism of thermoacoustic effect can be simply described as adding heat to the sound wave when it is dense, and discharging heat when it is sparse, the sound wave is strengthened; and vice versa, discharging heat when it is dense, and inhaling heat when it is sparse, the sound wave is weakened. Of course, the actual theory of thermoacoustics is much more complex than this.