Enhanced Vapor Injection for Air to Water Heat Pumps

As shown in the above schematic, the liquid out of the condenser is separated into two parts. A smaller part of the liquid (i), is expanded through an additional expansion valve, and then directed (or flows) into a counter-flow plate heat exchanger, HX. The main part of the liquid out of the condenser (m), is then cooled down through the economizer while evaporating and superheating the injection mass flow. This additional plate heat exchanger, more generally called economizer, acts therefore as a sub cooler for the main mass flow (m) and as an evaporator for the injection mass flow. Superheated vapor is then injected into the intermediate vapor injection port in the scroll compressor.

The additional subcooling increases the evaporator capacity by reducing the temperature of the liquid from TLI to TLO, thus reducing its enthalpy. The additional condenser mass flow (i), increases the heating capacity by the same amount.

Efficiency with vapor injection scroll compressor cycle is higher than that of a conventional single stage scroll delivering the same capacity because the added capacity is achieved with proportionally less power. The injection mass flow created in the subcooling process is compressed only from the higher inter-stage pressure rather than from the lower suction pressure.

The additional Sub-cooling effect of EVI configuration allows heat pump to draw heat from the outdoor at lower outdoor temperatures. That could explain why DC inverter (Non EVI) Heat Pumps operate between -20⁰C and 45⁰C (Outdoor BD Temperatures) while DC Inverter EVI Heat Pumps operate between -25⁰C and 45⁰C (Outdoor BD Temperatures).