Constant Pressure Expansion Valve Application Notes

The use of constant pressure (automatic) expansion valves as the metering device in vapor compression refrigeration cycles employed in air conditioning systems offer functional advantages which result in first cost and operating cost benefits to the manufacturers and users of air conditioning equipment. Operating performance is improved at typical application air temperature conditions and power consumption is reduced during peak summertime demand periods. This control concept should be of interest to utilities as well as manufacturers and users of air conditioning equipment. INTRODUCTION The constant pressure or automatic expansion valve is the fore runner of the thermostatic expansion valve and has been in production since the early days of mechanical refrigeration. It was the second stage in the evolutionary process from hand expansion valves to thermostatic expansion valves and is still employed in a wide variety of air conditioning and refrigeration applications. The recent concern for energy conservation at minimum cost has greatly expanded the interest in and demand for this simple and reliable control valve. Constant pressure expansion valves are basically pressure regulating devices which respond to the pressure at the valve outlet. Installed at an evaporator inlet, as a device to control refrigerant flow, the valve meters the refrigerant flow to maintain a cons·tant evaporator pressure during system operation. * * * * * * * * Constant Pressure (Automatic) Expansion Valves, employed as the prime expansion device, control the flow of liquid refrigerant to the system evaporator by opening on a decrease in valve outlet pressure below the set point. This results in the air conditioning system performance shown in figure l, when tested in accordance with AHAM (Association 150 of Home Appliance Manufacturers) standard RAC-1. For reference and comparison purposes the typical capillary tube performance is shown on the same graph. This performance data indicates that for a given system, with a constant pressure expansion valve purposely adjusted to duplicate the capillary tube performance at rating conditions, the unit will operate at higher suction pressure at low load conditions (thus achieving freezeup protection) and will operate at lower suction pressure at maximum operating conditions (thus minimizing power consumption and protecting the compressor against overload). The example used in figures 1 through 7 is a 1 ·ton room air conditioner (RAC) or packaged ·terminal air conditioner (PrAC). Constant pressure expansion valves would have similar results when applied to any small commercial or residential central air conditioning unit through 5 ton R-22. FIGURE 1 TYPICAL PERFORMANCE RAC OR PTAC '109rll..~rwa~r JODrOiii._ll" ,,.._ .. 1/Jor..-u.tt-) ... ,~,' J'V&J•W.r '~t(M:M6r (~~ ~~.l.t;:l-) !MJI'F 1:111, ?'~FVfll ..... r IUI"r~~ (~'-La.4c:-tlir:~) Through the use of system balance charts, additional benefits of employing constant pressure expansion valves on air conditioning systems can be demonstrated. These additional benefits include: improved capacity and operating efficiency through more efficient compressor operation and increased evaporator loading; and reduced first cost through condenser surface reduction and/or selection of lower capacity compressors. The poten·tial cost reduction is more than sufficient to offset the cost of the expansion valve and increased evaporator loading. The effects of changing the evaporator pressure at rating conditions will be considered. A complete system balance is constructed by plotting capacity performance data for all system components on one graph. A system balance plot consists of three families of curves, one representing compressor performance, another represen·ting condenser performance and the third representing evaporator performance. All are plotted on co-ordinates of system capacity and saturated suction temperature. The family of curves representing evaporator performance can be modified by ·the selection of the expansion device to be employed in the system. With these curves plotted for a particular system, if any two of the following variables are known, the remaining variables can be determined: capacity, suction pressure or temperature, evaporator entering air temperature. condensing temperature or pressure, condenser enter-ing air temperature.