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Abstract
In this article, an all fresh air-handling unit with high sub-cooling degree is presented. In this unit, refrigerant flows through the high-pressure liquid receiver before it goes through the sub-cooler so as to ensure sufficient sub-cooling degree. Based on the experimental comparison between this unit and conventional unit, coupling relationships between condensing temperatures and sub-cooling degrees of these two units are worked out and analyzed. Experimental results and exergy analysis show that, sub-cooling degree drops with the decrease of condensing temperature, and sub-cooling degree of the designed unit is kept over 7°C when the sub-cooling degree of the conventional unit is only close to 0°C. Furthermore, a method of year-round exergy calculation is presented and applied in calculating and analyzing the year-round exergy of the all fresh air-handling unit. Calculation and analysis show that the all fresh air-handling unit designed and investigated in this article has a year-round exergy efficiency of 28.38%, which is 3.17% higher than that of the conventional unit without high sub-cooling degree.
Nomenclature
| Di,cool | = | operation time cooling, days |
| Di,heat | = | operation time heating, days |
| Di,annual | = | operation time year-round, days |
| Eae | = | exergy received in year-round operation, kW |
| Esys,i | = | exergy received in specific working condition in a year, kW |
| Ecool | = | exergy in cooling condition, kW |
| Eheat | = | exergy in heat pump condition, kW |
| Ex | = | exergy, kW |
| Ex1 | = | exergy at compressor inlet, kW |
| Ex2 | = | exergy at condenser inlet, kW |
| Ex3 | = | exergy at expansion valve inlet, kW |
| Ex4 | = | exergy at evaporator inlet, kW |
| h | = | enthalpy of refrigerant in certain condition, kJ·kg−1 |
| hout | = | enthalpy of refrigerant in ambient condition, kJ·(k·kg)−1 |
| I | = | exergy loss, kW |
| Icom | = | exergy loss in compressor, kW |
| Icsa | = | exergy loss in condensing and sub-cooling assembly, kW |
| Ieva | = | exergy loss in evaporator, kW |
| Iexp | = | exergy loss in expansion valve, kW |
| Iae | = | exergy loss in the unit in year-round operation, kW |
| Isys,i | = | exergy in the unit in specific working condition, kW |
| mr | = | mass flow rate of refrigerant, kg·s−1 |
| Qcsa | = | heating capacity of condensing and sub-cooling assembly, kW |
| Qcsa | = | refrigeration capacity of evaporator, kW |
| s | = | entropy of refrigerant in certain condition, kJ·(K·kg)−1 |
| sout | = | enthalpy of refrigerant in ambient condition, kJ·(K·kg)−1 |
| Tout | = | ambient temperature, K |
| Tcon | = | condensing temperature, K |
| Teva | = | evaporating temperature, K |
| Wcom | = | input power of compressor, kW |
| ηae | = | exergy efficiency of the unit in year-round operation, kW·kW−1 |
| ηsys,i | = | exergy efficiency of the unit in specific working condition, kW·kW−1 |
Subscripts
| ae | = | annual exergy |
| com | = | compressor |
| con | = | condenser |
| csa | = | condensing and sub-cooling assembly |
| eva | = | evaporator |
| exp | = | expansion valve |
| sub | = | sub-cooler |