Study On The Effect Of Outdoor Heat Exchanger Fouling On Heating Performance Of Room Air Conditioner

Air Source Heat Pump(ASHP)is a kind of distributed heating method,which can be used in partial time and partial space,thus it can meet the different needs of different households for room temperature.At the same time,air can be used for heating,which can save the initial investment and operating cost,and play a role in protecting the environment.It has a broad application prospect and space.The outdoor heat exchanger of ASHP is exposed to the air,so dust accumulation is difficult to avoid.While in the actual installation process,outdoor units are often installed in outdoor grooves,or directly installed on the exterior wall.As a result,the outdoor heat exchanger cleaning for residents becomes very difficult,at the same time,the residents’ awareness of the outdoor heat exchanger cleaning is weak,so dust accumulation on the outdoor heat exchanger of ASHP is a common problem.With the accumulation of dust on outdoor heat exchanger of ASHP,the performance of the unit has changed.The thermal resistance was increased while the heat exchange capacity was reduced.In addition,both the frosting accumulating mass and the frosting time were affected by outdoor temperature and humidity.The condensation and the frosting performance in the enthalpy different laboratory,where the operating conditions are divided into three types(condensation region,moderate frosting zone,severe frosting zone)with non-ash,mild-ash,moderate-ash,and heavy-ash on outdoor heat exchanger.With the increase of dust accumulation,the results showed that the outside heat exchanger temperature dropped by 0.5～2.3℃(45.5%～59.0%),the inlet temperature of the compressor decreased by 0.1～4.7℃(11.1%～52.8%),the heating capacity decreased by 228～381W(5.9%～9.5%),the compressor power decreased by 31～78W(2.2%～6.7%),the COP decreased by 0.06～0.11(2.2%～3.7%),the heat absorption of outdoor coil management decreased by 185～303W(7.7%～10.7%),and the frost rate increases by 3.97～6.10g/min(37.7%～60.2%).With the increase of dust accumulation in the defrosting stage,the ASHP frosting mass,the defrost duration,and 8-hour defrosting frequency will increase.In the comparison of different frosting environment,dust accumulation has the greatest effect on severe frosting zone.with the accumulation of dust,the ASHP frosting mass in the severe frosting zone increases by 106g(16.7%),the defrost duration increases by 1.5min(45.5%),and the 8-hour defrosting frequency increases by 3.0(26.8%).According to the calculation of the temperature and the heat transfer coefficients of different measurement points of the ASHP outdoor heat exchanger,dust accumulation has a greater influence on the heat transfer coefficient of the liquid refrigerant zone(the middle of the heat exchanger).While in the gaseous refrigerant zone(The lower part of the heat exchanger)the accumulation of dust has more effected on the superheat of the refrigerant.We cleaned the outside heat exchanger after the operation of one year,three years and five years.After cleaning,energy saving of ASHP in winter can bereach 19.1、27.5、16.3 kgce(64.1、92.7、54.8 kwh),and the energy saving rate are 7.0%、10.0%、6.0%,equivalent to saving the electricity fee of 33.4、48.2、28.5 yuan.For situations where the outdoor heat exchanger installation location is difficult to clean,after understanding the influence of different frost zone and different dust accumulation degree on the performance of ASHP,this paper studies the optimization of"temperature-time" defrosting control strategy commonly used by ASHP under different frosting zone and dust accumulation conditions.Determine the degree of ash accumulation by monitoring the proportion of the total heat transfer coefficient of the outdoor heat exchanger.In moderate frosting zone,40,50,60,70 min defrosting control time is used,and in severe frosting zone,30,35,40,50 min defrosting control time is used for forced defrosting of ASHP under different ash deposition conditions.The optimal defrosting control time is obtained by analyzing the performance parameters and heating characteristic parameters of ASHP,which is compared with the original defrosting control time and temperature,and different frosting zone and optimization strategy of ASHP defrosting control with different degree of ash accumulation are obtained.In the moderate frosting zone,the effective heating coefficient can be increased by 5.1%,4.29%and 5.16%respectively after the defrosting control optimization under the conditions of mild ash accumulation,moderate ash accumulation and severe ash accumulation.And In the severe frosting zone,the effective heating coefficient can be increased by 4.35%,4.68%and 6.76%respectively after the defrosting control optimization under the conditions of mild ash accumulation,moderate ash accumulation and heavy ash accumulation.In conclusion,the dust accumulation on ASHP outdoor heat exchanger will affect the frosting rate and surface temperature of outdoor heat exchanger,which makes ASHP prone to false defrosting operation.In this paper,the performance degradation rules of ASHP under different frosting areas and ash deposition conditions are explored,and the defrosting control strategies of ASHP under different conditions are optimized,which ensure the safe and efficient operation of ASHP,and provide design thoughts for ASHP defrosting control strategy of high particle pollution air operation and inconvenient cleaning of outdoor heat exchanger.