| Parameter | Without Economizer | With Economizer | Gain |
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How to Use the Economizer Calculator
Pick a plate heat exchanger or a flash vessel, then select the refrigerant and enter the evaporating (dew point) and condensing (dew point) temperatures. The capacity gain grows with the temperature lift.
Suction superheat and condenser subcooling default to 5 K. Isentropic efficiency defaults to 70% and is applied the same way with and without the economizer. Subcooling is measured from the bubble point.
Leave the injection temperature blank to use the value that gives equal pressure ratios on both compression stages, or enter the saturation temperature at the compressor injection port. For a plate heat exchanger, set the approach and the injection superheat.
Enter the capacity and power of the compressor without the economizer at the same conditions, from the data sheet. The calculator scales them by the modelled capacity and power ratios and reports the new capacity, power, COP, injection flow and pressures.
Economizer cycles in Canadian refrigeration and heat pumps
An economizer adds a second, intermediate pressure level to a single compressor. Part of the liquid is expanded to that pressure and boiled, which cools the rest of the liquid before it reaches the expansion valve, and the vapour that was produced is injected into the compressor partway through compression. The evaporator receives colder liquid, so each kilogram of refrigerant absorbs more heat, and the injected vapour is compressed over a smaller pressure ratio than it would be from suction.
How the cycle works
Liquid leaves the condenser and splits. The main stream passes through the economizer and is cooled to the injection saturation temperature plus the approach. The side stream expands to the injection pressure, takes that heat from the main stream and leaves as vapour. The calculator balances the two: the injected mass per kilogram sent to the evaporator is the enthalpy drop of the main liquid divided by the enthalpy gained by the injected stream. The first compression stage runs from suction to the injection pressure, the injected vapour mixes in, and the second stage takes the larger flow to discharge pressure.
In the example (R-410A, -15°C evaporating, 40°C condensing, 5 K suction superheat and condenser subcooling, 70% isentropic efficiency, plate heat exchanger with a 5 K approach), the liquid reaching the expansion valve is cooled from 34.9°C to 14.7°C. That raises the refrigerating effect from 164.1 to 198.1 kJ/kg, a capacity gain of 20.7%, while the compressor power rises 11.4%.
Plate heat exchanger or flash vessel
A brazed plate heat exchanger keeps the streams separate and leaves a temperature approach between the cooled liquid and the boiling side stream. An expansion valve on the side stream sets the injection flow, so it can follow the load, and the injected vapour leaves with some superheat. A flash vessel lets the liquid separate at the injection pressure, so the liquid leaving it is at the saturation temperature and the vapour is saturated. It reaches slightly colder liquid than a plate exchanger can and has fewer parts, but injection flow follows the liquid supply and is harder to modulate. The calculator uses the same equations for both, with the approach and injection superheat set to zero for the flash vessel.
Choosing the injection pressure
For a two-stage cycle the pressure that gives equal pressure ratios on both stages is the geometric mean of suction and discharge pressure, and the calculator uses its saturation temperature when you leave the field blank. A real scroll or screw compressor does not let you pick the pressure freely. The injection port sits at a fixed point in the compression, so the pressure there follows from the operating conditions and the port design. If the data sheet gives the port pressure, enter its saturation temperature. The calculator warns when your value is more than 8 K from the automatic one.
What the calculator reports
Capacity, power and COP come from the cycle model, scaled by the base capacity and power you enter, so the result starts from your compressor's own numbers. Suction mass flow is taken as unchanged by the injection, and the isentropic efficiency is the same with and without it. Real compressors differ, so use the result as an estimate to compare with manufacturer ratings. Refrigerant properties are fitted to CoolProp real-fluid data. Against a full CoolProp cycle at evaporating temperatures of -35 to 5°C and condensing temperatures of 35 to 55°C, the COP change agreed to within about 3 percentage points. The heat pump COP calculator shows how a COP change translates to seasonal heating performance.
Cold-climate heat pumps
The reason economizers matter in Canada is low outdoor temperature, where suction pressure is low and the pressure ratio is high. Researchers at Oak Ridge National Laboratory describe a cold climate heat pump using tandem vapor-injection compressors and report that vapor-injection compressors provide better efficiency and larger capacity at low ambient temperatures. Their unit delivered 75% of its 8.3°C capacity at -35°C with a COP of 1.8. Those are results for one design, and they show the direction of the effect, not a number to expect from every unit.
Field results in Canada
Natural Resources Canada tested a cold climate air source heat pump at the Canadian Centre for Housing Technology in Ottawa, and its performance assessment recorded an average heating COP of 1.5 at -21.1°C and 3.0 at 4.9°C, with a 10 kW electric backup heater. It shows how far efficiency falls as the lift grows, which is the gap an economizer is meant to narrow. The study does not say the unit used vapour injection.
Cost and payback
An economizer is specified with the compressor, so the cost is the price difference between a vapour-injection compressor with its heat exchanger or vessel and injection valve, and a standard one. The saving comes from capacity and efficiency at low temperature, and in a heat pump from fewer hours on electric resistance backup. Put the extra capacity at your design outdoor temperature into the HVAC payback calculator with the added equipment cost to see whether the simple payback is acceptable.
Commissioning
After installation, read the injection line superheat and the discharge temperature at several operating points. Injection superheat should stay within the compressor manufacturer's range: too little risks liquid reaching the compressor, and too much reduces the injected flow and the gain. Check superheat with the superheat calculator and compare overall performance with the compressor efficiency calculator. For a full two-stage design with separate compressors, see the two-stage compression calculator.
Frequently Asked Questions
Only if the compressor was built with an injection port. The port is machined into the scroll set at the pressure location where the injected vapour joins the compression, and it cannot be added to a compressor that was not designed for it. Replacing the compressor with a vapour-injection model is the usual route. The rest of the economizer circuit has to be added at the same time: the plate heat exchanger or flash vessel, the injection expansion valve and the injection line. Check the compressor selection data for the port and its operating envelope before specifying the replacement.
Normally it lowers it. The injected vapour is much cooler than the partly compressed gas it joins, so the gas that finishes compression starts the second stage cooler than it would without injection. That is one reason vapour injection is used with R-32 and other refrigerants that run hot at high lift. The effect depends on the injection flow and on how much superheat the injected vapour carries, so it is not guaranteed. Read the discharge temperature at commissioning and compare it with the compressor manufacturer's limit and with the same operating point without injection.
A plate heat exchanger keeps the two refrigerant streams apart. A side stream is expanded to the injection pressure and boiled in the plates, which sub-cools the main liquid, and an expansion valve on that side stream can be modulated to match the compressor load. A flash vessel lets part of the liquid flash to saturated vapour at the injection pressure inside a small tank, and the liquid left in the tank is at the saturation temperature. The flash vessel therefore gives the coldest liquid and dry vapour but has no approach temperature to tune, while the plate heat exchanger gives up some of that gain for better control. In the calculator the flash vessel is the same cycle with zero approach and no injection superheat.
It depends on the climate and on what the economizer replaces. The extra cost is a vapour-injection compressor, the heat exchanger or vessel and the injection valve, and that equipment is specified when the unit is built or replaced. The saving comes mainly from heat pump capacity and efficiency at low outdoor temperatures, which cuts the hours when electric resistance backup has to run. A mild-winter location gets less from it than a prairie or northern one. Work out the extra capacity at your design outdoor temperature, convert it to avoided backup hours and compare that with the added equipment cost.