Experimental Quantification of Liquid Line Temperature Drop As a Feature to Detect Liquid Line Restriction Faults in a Residential Heat Pump
Liquid line restrictions, Residential air source heat pump, Fault detection and diagnosis, Simultaneous faults
Liquid line restrictions (LL) in a vapor compression refrigeration cycle cause abnormal pressure drops in the liquid line and reduce the refrigerant mass flow. Although a thermostatic expansion valve (TXV) in the system can compensate for small restrictions, larger restrictions cause the TXV to fully open, at which point the LL can create significant impacts on system performance. Furthermore, in the presence of other faults, like undercharge (UC) or non-condensable gas (NC) in the refrigerant, this impact becomes more pronounced. Therefore, effectively detecting this fault and addressing it can improve the efficiency of system and reduce the cost of operation of the air conditioning system. Existing methods to detect this fault either require a large amount of data to train a model, or apply only to a specific system, making them impractical for a technician to use in the field. The proposed method is based on the phase change of refrigerant inside the liquid line caused by LL faults. The temperature of the refrigerant drops during phase change, so the temperature difference across the liquid line can be regarded as a decoupling feature for LL faults. Four types of common installation faults -improper evaporator airflow (EA), improper refrigerant charge (CH), NC, and LL faults -were imposed on a TXV-equipped split system heat pump, individually with four different operating conditions, and simultaneously with sets of two, three and four faults at one operating condition. Testing was conducted in a set of psychrometric chambers, to tightly control the thermal environment indoors and outdoors. The decoupling feature was calculated for each of the test scenarios. The results showed that this feature is sensitive to the presence of LL faults and insensitive to other faults, either singly or collectively. In the presence of UC or NC faults, the feature became more sensitive to LL faults, but it became less sensitive in the presence of overcharge (OC) faults. The threshold for this feature varied from condition to condition, but a general threshold of 2 °C is proposed to detect severe LL faults and moderate LL faults without the presence of OC faults.