Pressure and Temperature Concepts in Cooling Circuits
In the cooling circuits, the temperature of the refrigerant in the system and the parts of the pressurized system and the operating conditions of the fluid. These concepts are explained as follows.
High Pressure Side
The part of a refrigeration cycle from the compression of the compressor up to the pressure vessel, condenser , liquid tank, liquid pipe and expansion valve is called the “high pressure side”.
Low Pressure Side
The portion of a cooling circuit from the outlet of the expansion valve to the inlet of the cooling unit, the suction pipe and the compressor is called the “low pressure side”.
The pressure of the refrigerant in the condenser corresponding to the superheated steam, saturated steam, wet steam and fluid temperature in the liquid state is the condensation pressure.The condensation pressure is also the high side pressure of the cooling circuit.
After a certain period of time when the compressor is deactivated in the cooling system, the refrigerant temperature on the high pressure side equals the air temperature surrounding the high pressure side. In this case, the high side pressure of the refrigerant is determined as a pressure suitable for the temperature of the ambient air.
When the compressor is switched off again after a certain period of inactivity, no condensation occurs because there is no temperature difference between the refrigerant temperature and the temperature of the condensation medium (if the ambient air is used as the condensation medium) for a very short time. In a very short period of time, the high side temperature condensation medium (a temperature difference above the ambient air temperature, which causes a temperature difference to be reached), the heat transfer to the condensation medium from the upstream side to the condensation side of the condenser becomes adequate and a stable cooling process is started.
The saturated vapor of the refrigerant in the case of condensation in the condenser is the temperature in the wet and liquid state.
The coagulation temperature is determined by the condenser heat transfer surface and the temperature of the condensation medium. The coagulation temperature is chosen from the temperature of the condensation medium due to economic reasons and the atmospheric temperature is chosen to be higher against the possibility of leaving over time at the determined ambient temperature. However, it is desirable that this height difference be selected as small as possible.
Patch Line Temperature
The discharge line temperature is different from the condensation temperature. The refrigerant vapor sent to the compressor discharge line can be considered equal to the discharge line temperature if the saturated vapor is in the form of wet steam. In practice, the refrigerant is in the state of superheated steam in the discharge line. The hot steam temperature is the saturated steam at the same pressure or the higher temperature than the wet steam. Compression line temperature and coagulation temperature are separate temperature concepts.
The pressure of the refrigerant evaporating in the refrigerating unit is called evaporation pressure.The evaporation pressure depends on the temperature of the space air cooled by the cooling unit heat transfer surface. For a given value of the heat transfer surface of the refrigerating unit, the evaporation pressure drops if the cooled volumetric temperature is lower. The increase in the volume temperature of the cooling increases the evaporation pressure.
Each refrigerant has a vapor temperature depending on the vapor pressure, which varies with the pressure. The evaporation pressure will decrease as the evaporation temperature decreases. The evaporation temperature also changes with the temperature of the air that is cooled.
The amount of heat absorbed by the entire volume of refrigerant from the volume being cooled is the cooling effect of that refrigerant. For example, take 1 kg bouquet at 1 ° C at 0 ° C. It absorbs 335 kJ of heat from the ambient air until all of the ice mass melts and becomes watery. 335 kJ is the latent heat of ice at 0 ° C and weight of 1 kg and is the cooling effect for ice.
Let’s also examine refrigerants such as ice. The amount of heat that 1 kg of a refrigerant absorbs in the refrigerated volume when it vaporizes in the refrigerating unit is the cooling effect, which is equal to the latent heat of evaporation of that refrigerant. However, this comparison is based on the assumption that the temperature of the refrigerant in the liquid state is equal to the temperature in the evaporation state. In practice, the liquid temperature of the refrigerant is always higher than the evaporation temperature of the refrigerant. The temperature of the refrigerant in the evaporator is reduced to the evaporation temperature before the heat is absorbed through the volume of air being cooled. For this reason, only a certain portion of the refrigerant vaporizes in the refrigerating unit and absorbs heat from the volume air being cooled.This means that the temperature of the refrigerant is determined by the liquid temperature of the refrigerant and the evaporation temperature of the refrigerant unit, which is smaller than the total evaporation latent heat of the refrigerant. The total evaporation is an ideal value that can be achieved due to cooling the latent heat.
Since the evaporation latent heat of each refrigerant differs, the cooling effects of different refrigerants vary according to the specific liquid and vaporization temperatures.
Cooling systems with higher cooling efficiency are preferred because less work will be given to the system in order to circulate less cooling fluid in the system.
The cooling effects can be found in the thermodynamic table or in the pH diagram when operating temperatures are given.
Do you have the cooling effect coefficient of the system to work with R-134a refrigerant between +30 ° C cooldown temperature and -10 ° C evaporation temperatures? (Table R-134a)
Solution: The enthalpy of the saturated vapor of the refrigerant at -10 ° C, the enthalpy of the liquid refrigerant at +30 ° C, and the enthalpy difference of the liquid refrigerant with the enthalpy of the saturated refrigerant gives the cooling effect of the refrigerant at -10 ° C.
Heat attitudes from thermodynamic table R-134a;
Heat attenuation of R-134a refrigerant at +30 ° C saturated liquid condition = 142 kJ / kg,
Heat attenuation of R-134a refrigerant with saturated steam at -10 ° C = 294 kJ / kg,
Cooling power = 294 – 142 = 152 kJ / kg
If the cooling system is operated with refrigerant R-12 in the operating conditions in Example 7-1, we have to change the cooling effect. (Table R-12)
The enthalpy of R-12 at +30 ° C saturated liquid hf = 227 kJ / kg
The enthalpy of R-12 in the case of saturated steam at -10 ° C hg = 346 kJ / kg
q0 = 346 – 227 = 119 kJ / kg
Source: Air Conditioning and Refrigeration Technologies, Orhan KISA, High Mechanical Engineer