18 May 2017
The in situ monitoring method for variable refrigerant flow (VRF) multi-split air conditioning and heat pump equipment (VRF system) has not yet been established, and actual capacity and efficiency (performance) cannot be sufficiently understood. A manufacturer understands actual performance by means of the compressor curve (CC) method. This method, however, cannot be applied to all models of all manufacturers because details of the method are not disclosed.
The refrigerant enthalpy method is expected to become a common monitoring method. In order to measure refrigerant flow rate, it is necessary to cut the refrigerant piping, but this is avoided, so as not to inconvenience customers. Recently, refrigerant flow rate measurement has become possible without cutting the piping by using a clamp-on ultrasonic flowmeter, however the accuracy has not been verified.
In this study, an in situ monitoring instrument is installed in the VRF system; the instrument includes an ultrasonic flow meter (UFM), a Coriolis flow meter (CFM), 12 thermocouples and an absolute pressure sensor. The mutual verification between the UFM and the CFM shows that an acceptably accurate flow rate is measured by the ultrasonic flow meter, though a constant error margin is measured. In addition, three kinds of estimated capacity are compared: (1) capacity from the CC method, (2) capacity from monitoring, and (3) capacity from flow rate correlation. The capacity from the correlation agrees with that from the CC method with an accuracy of +/-20%.