Pressure and Temperature Concepts on Cooling Circuit
The temperature and pressure of refrigerant in the system in the cooling circuit system components and shows the fluid operating conditions. These concepts are described as follows.
High Pressure Side
By pressing the compressor discharge pipe of a cooling circuit, condenser, liquid tank, the portion from the liquid pipes and the expansion valve “high pressure side” it is called.
Low Pressure Side
Since the output of an expansion valve cooling circuit cooling units, suction pipe and a suction side of the compressor until the “low pressure side” it is called.
Condensing PressureThe temperature and pressure of refrigerant in the system in the cooling circuit system components and shows the fluid operating conditions.
Refrigerant superheated steam condenser, saturated steam, matching the fluid temperature and fluid pressure in the case of wet steam is condensing pressure. Condensing pressure is also high side pressure of the refrigerant circuit.
When the compressor off in the cooling system refrigerant temperature in the high pressure side after a certain period is equal to the air temperature surrounding the high pressure side. In this case, the high side pressure of the refrigeration cycle is determined as an appropriate pressure on the temperature of ambient air. When activated again compressor remaining inactive for a certain period the temperature of the condensation medium with refrigerant temperature (if used as the ambient air condensing environments) between not due to condensation is a very short time temperature is the difference. high side temperature condensation environment in a very short time (rise above the ambient air temperature occurs a temperature difference. This difference in heat transfer to an adequate value reaches the refrigerant in the condenser condensing environment coming sufficient condition would have started a stable condensation process.
In case of refrigerant in the condenser condensing the saturated steam is wet and fluid temperature in the case. Condensation temperature of the condenser is determined by the condensation temperature of the environment with the heat transfer surface. Condensation temperature of the condenser for economic reasons and on the temperature of the ambient atmosphere of the set temperature of the higher temperature is selected to avoid possible over time. However, this difference in height is desired to be selected as small as possible.
Discharge Line Temperature
Discharge line temperature is different from the condensation temperature. The compressor of the refrigerant vapor is sent to the discharge line, in case of saturated steam, wet steam can be considered equal to the discharge line temperature condensation temperature. In practice, the fluid cooler is the case of superheated steam discharge line. The temperature of the superheated steam, a higher temperature than saturated steam or wet steam at the same pressure. the discharge line temperature condensation temperature are separately temperature concept.
The pressure of the evaporating refrigerant in the cooling unit is called vapor pressure. Vapor pressure depends on the temperature of the cooled air cooler unit volume heat transfer surface. for a given value of the heat transfer surface cooling unit, upon receipt of vapor pressure values lower than the temperature of the cooled volume decreases. increasing the temperature of the cooled volume increases the vapor pressure.
Each cooling fluid, depending on the vapor pressure has evaporation temperature varies depending on the pressure at this temperature. Evaporation temperature decreases, the vapor pressure will decrease. Evaporation temperature varies depending on the temperature of the cooled volume of air.
Units of volumes being cooled refrigerant in the weight of the entire amount of heat it absorbs, it is fluid cooler’s cooling effect. For example, consider one kg of ice at 0 ° C for 1 kg weight. The ambient air until all of the ice melts into water absorbs heat 335 kJ. 335 kJ at 0 ° C and 1 kg of ice weighing ‘is un latent heat is cooling effect to the ice.
Let’s look at the example of refrigerants such as ice. 1 kg of the refrigerant evaporates in the air volume of the cooled cooling unit is the amount of heat absorbed by the cooling effect, the amount is equal to the latent heat of evaporation of the refrigerant. However, in the case of the comparison with the temperature of the refrigerant fluid, the temperature in the case of evaporation was conducted according to the state of being equal. In practice, the temperature in the case is always higher than the evaporation temperature of the liquid refrigerant in the condensing unit. Refrigerant before heat absorption of the air volume of the fluid being cooled evaporator temperature is lowered to evaporation temperatures. Therefore, the refrigerant evaporates only in a particular portion of refrigerant absorbs heat from the air volume unit and being cooled. Means that the cooling effect of the liquid refrigerant temperature in the case is determined by the evaporating temperature in the refrigeration unit, this value is smaller than the cooling fluid of the total latent heat. Total latent heat of vaporization is an ideal value that can be attained due to the cooling effect.
cooling effect of different refrigerant by evaporation latent heat of refrigerant because each different specific liquid and the evaporation temperature also becomes different values. cooling effect of the cooling system with a high refrigerant are preferred because less refrigerant in the system to be circulated in the system will be less outside the business. Cooling effects when given operating temperature or thermodynamic tables are available at the pH diagrams.
Sample Problem 7.1
+30 ° C condensing temperature, the evaporation temperature of -10 ° C to find the R-134a refrigerant fluid cooling coefficient of the system will work? (Table R-134a)
Solution: -10 ° C from thermodynamic tables wherein the enthalpy of the refrigerant fluid in the saturated vapor, +30 ° C the liquid refrigerant enthalpy availability saturated steam fluid refrigerant liquid with the refrigerant enthalpy at the actual enthalpy difference of 1kg of refrigerant returns the fluid to the cooling effect.
R-134A heat retention from thermodynamic tables;
+30 ° C in case of saturated liquid R-134a refrigerant heat retention = 142 kJ / kg,
-10 ° C and saturated steam in the case of R-134a refrigerant heat attitude = 294 kJ / kg,
S. Influence = 294-142 = 152 kJ / kg
Sample Problem 7.2
Example cooling system in working conditions at 7-1, find the change in the R-12 refrigerant fluid running if the cooling effect. (Table R-12)
in the case of R-12 to +30 ° C saturated liquid enthalpy hf = 227 kJ / kg
-10 R-12 case of saturated steam enthalpy hg = 346 kJ / kg
q0 = 346-227 = 119 kJ / kg
Source: Mec. High Eng., Orhan KISA