Refrigeration and heat pump techology - seconda parte

rolling piston, rotary vane and scroll compressors

By reducing the volume we are increasing the pressure - the rolling piston compressor is a positive displacement.

Rotary vane compressor - there are several vanes that are moving inside the rotor.

They are mainly hermetic compressors. Many rolling piston compressor has an accumulator.

Scroll compressors are largely used, there is a rotor and a stator, is a positive displacement compressor.

only one orbit is moving

The discharge valve is not necessary, but many compressors have it to avoid the back flow and to control better the pressure. In the scroll we can have as many inlet as we want. It can also have a lot of pocket to reduce the leakages.

There are also reed-valves that can be called bypass valve - help to manage the liquid droplets and at the beginning, when I am giving a lot of power. They are very silent, have less vibration.

capacity control in scroll compressor - varying the velocity (inverter - also the inverter has efficiency).

Permanent magnet brushless motor has a better efficiency than the asynchronous one.

Linear compressor - are a particular type of compressor.

carbon dioxide : cycles and main component

Lower the evaporating temperature, lower the pressure ; if the isentropic efficiency is the same ->the discharge temperature at the compressor is higher ; in a transcritical cycle it is also higher. For technological reasons in a transcritical cycle the discharge temperature has not to be higher than 140°C ->there could be problem with the lubrication.

For evaporation temperature lower than -20°C a two stage compressor is needed. Two stage compressor can't be used with high evaporation pressure.

Single stage cycle -> we have only one stage compression ; at point 1 we have an internal heat exchanger, after that the throttling device, the evaporator, the internal heat exchanger, compressor, gas cooler. Thanks to the internal heat exchanger there are higher superheating, higher efficiency, avoid the presence of liquid at the inlet of compressor.

We can also have two-stage compression, one stage throttling cycle with inter-stage cooling. We are using this process to decrease the desuperheating losses. The compressors must work above minimum compressions ratios.

Split cycle: two-stage compression, two-stage throttling cycle(the expansion is divided into two devices)with inter-stage cooling and intermediate cooling heat exchanger.

is at an intermediate pressure we can cool down the CO2 before it reaches the evaporator

We want to reduce the throttling losses -> to do that we have to reduce the temperature at the throttling device and the evaporator.

Open flash tank cycle : two-stage compressors, two-stage throttling cycle with inter-stage cooling and open flash tank. we have 3 pressure level: discharge pressure, evaporation pressure, intermediate pressure. Liquid -> is sent to evaporator and the vapor is send at the intermediate pressure -> this is referred to the open flash tank.

If the pressure ratio is not high enough to have two-stage compression as before we can have an auxiliary compression (parallel compression) cycle. -> it has, also two stages throttling cycle.

COMPRESSORS

All the compressors we have seen are good for CO2. The most used is the reciprocating piston compressor.

smaller compressors

The cylinder is smaller, because with CO2 we have higher volumetric refrigerating effect. Also the swept volume for the CO2 is lower than R134a. For CO2 we have an higher pressure difference between the inlet and outlet, but the pressure ratio is lower. For the compressors is nice to have CO2, even if from a technological point of view CO2 compressor design is challenging. Leakage mass flow rate can be relevant. The pressure losses can impact on the compression efficiency - those through the valve have a lower relative impact.

Twin rotary compressor -> two stages in the same hermetic compressor -> we have two rolling piston to compress in two stage the CO2, but, here, we are heating the fluid.

LUBRICANTS

Tribology = characteristics of lubrication of the compressor. The viscosity is related to the mixture oil + refrigerant.

Miscibility => if my refrigerant is helping me to make the oil back to the compressor, if it is present in other part of the circuits.

Solubility => how much refrigerant is present in the oil

VALVES

Expansion valves have different function in a subcritical and in a transcritical cycle

maintain the pressure difference and control the mass flow rate

control high pressure, and they have to maintain the pressure difference, the mass flow rate

the valve cannot do all this thing and so it control the pressure difference and the high pressure

The valve that control the high pressure is called back pressure valve -> by closing the valve the pressure before the valve will be higher. It operates to control the pressure at the gas cooler. Lao's equation is important in this field.

Instead of the expansion device we can have the ejectors. They can have an adjustable or multiple suction.

we are not able to control the evaporating temperature.

No all the transcritical cycle has a back pressure valve ->