Boosting Refrigeration Efficiencies with Liquid Pumping
Liquid pressure amplification can increase the COP in refrigeration cycles.
As most engineers are aware, the vapor compression cycle, whether reciprocating or
centrifugal, consists of four components:
The controls associated with this equipment are commonly used to help maintain relatively constant pressure within the compressor and downstream from the metering device. This regulation of pressure helps reduce or suppress the formation of flash gas.
The primary operating principles of both of these refrigeration machines are basically the same, with the exception of how the liquid refrigerant is metered into the evaporator and methods used to compress refrigerant vapors.
There are many methods used to increase the coefficient of performance (COP) of refrigeration machines. Most of these techniques are common knowledge to the experienced and well-read engineer.
Understanding the System
In Figure I, the refrigerant in the evaporator is divided into two parts: the vaporizing refrigerant and the superheated vapor. The part that actually does the work (provides us with cooling) is when liquid refrigerant changes from a liquid to a vapor. The more liquid refrigerant we are able to maintain in the evaporator, the higher the unit's efficiency.
The proportion of liquid to vapor is a function of expansion valve performance, the percent of flash gas passing through the metering device, and the temperature of the liquid refrigerant.
After the vapor leaves the evaporator, the temperature of the vapor will continue to rise while its pressure remains relatively constant. This increase in temperature while the pressure remains constant is referred to as superheat.
The ideal condition in most refrigeration systems is to have refrigerant vapor enter the compressor in a saturated condition, containing no liquid or superheat.
The condenser is divided into three separate stage. Prior to being converted back into a liquid, the refrigerant passes through the desuperheated section, the condensing section, and the subcooling section. The discharge temperature of refrigerant vapor leaving the compressor is the result of the superheat from the evaporator and the heat of compression from the compressor.
When superheated vapors enter the condenser, superheat first must be removed so refrigerant vapors can start condensing. As seen in Figure I, the portion of the condenser used is directly related to the temperature of the superheated vapors. These superheated vapors, when heated, will increase in pressure or expand (or both), occupying more of the condenser space. Rejection of heat from the desuperheated portion of the refrigerant vapor is the least effective means of heat transfer: vapor-to-vapor.
In the condensing section of a condenser, the refrigerant vapors are cooled to the point of saturation. At this point, when the temperature continues to drop, the vapors start to condense into a liquid state.
As the refrigerant vapors change state, pressure within the condenser remains constant. As more of the refrigerant continues to condense along the walls of the condenser, the efficiency of rejected heat increases due to the transfer of heat from a vapor to a liquid state.
The pressure that exists in the condenser is a function of the total condensing area (that is, the condensing area minus the superheated section and subcooling section). Therefore, it is reasoned, the less superheat within the condenser, the more area is available for condensing the vapors to a liquid.
The purpose of the subcooling section is to help suppress the formation of flash gas entering the expansion valve. As seen in Figure I, part of the condenser is used to help subcool the liquid refrigerant prior to its entering the expansion valve.
If we were to evaluate the effects of subcooling liquid refrigerant without regard to decreased condensing surface, we could potentially realize a 0.5% increase in system capacity per degree F of subcooling.
Figure II shows a refrigeration system utilizing liquid pressure amplification. As you can see, a pump can be installed at the outlet of the condenser and upstream of the expansion valve. The main purpose of this pump is to increase the pressure of the liquid refrigerant before it enters the expansion valve. This increase in pressure (approximately 8 to 15 lb) provides subcooling to the refrigerant, the same as reducing its temperature.
Figure III shows liquid pressure amplification and liquid injection circuit for discharge gas desuperheating.
In order for an evaporator to operate efficiently, it must operate with as high a liquid-to-vapor ration as possible. To accomplish this, the expansion valve must be able to regulate the flow of liquid refrigerant into the evaporator at the same rate as it evaporates.
In most systems, overfeeding and underfeeding the expansion valve drastically affects evaporator efficiency. With the liquid pressure amplification (LPA) system installed and running, the flow rate consistently higher with a more evenly modulated expansion valve.
This increase in refrigerant flow and improved controllability of the expansion valve provides better utilization of the evaporator. The increase in refrigerant flow also causes an increase in pressure and a reduction in superheat prior to entering the compressor.
Significant efficient gains can be made if refrigerant vapor temperatures entering the compressor are reduced; pressures of entering refrigerant vapors are increased; and pressures exiting the compressor are reduced.
With higher vapor pressures entering the compressor, and the reduced discharge pressure, the total lift or work of the compressor is reduced. Likewise, with the reduction in temperatures and increase in volume of vapor entering the compressor, the compressor's internal parts run cooler.
The compressor's lower temperature will reduce the the internal expansion of entering vapors, adding increased efficiency of the compressor to the higher efficiency of the efficiency associated with reduced compression ratio.
Along with these improvements on the suction side of the compressor, we are now able to float head pressure with ambient conditions to very low condensing temperatures. These changes can result in capacity increases, power reductions, and lower operating costs (See Figure IV).
In closing, LPA systems can provide the engineer with a means of improving chiller efficiency, increasing chiller capacity, and reducing operating costs. In most cases, LPAs show a 12-to-18 month payback.