Practically all refrigeration systems today operate on the ”vapor compression” principle. This term simply means that the heat being removed and transported to another location is accomplished by the alternate evaporation and condensation of a refrigerant usually a man made chemical commonly referred to as a Freon.
The state of this refrigerant ( i.e. whether it exists as a liquid or vapor) is controlled by a compressor which varies the pressure on the refrigerant. To use a simple word picture, the Freon gas which transfers heat in a refrigerating machine or heat pump, has the ability to absorb heat when it changes state from a liquid to a gas much the same way a sponge soaks up water. Similarly, when the Freon changes back into a liquid (in the air coil inside your home) it releases its heat much like wringing out the sponge. It is this process that allows us to absorb heat from ground water, concentrate it through the heat pump system, and deliver it to the home in the form of warm air.
To properly grasp the concept, one must bear in mind that the heat pump does not produce heat by itself. It is merely a mechanism that moves heat from one place to another. Its simplicity is based on a heating only refrigeration system with direct groundwater cooling coils and domestic hot water generator as built in added features. Most of the parameters which may possibly lead to failure of a refrigeration system have been designed out of the units over the years and those which remain have their functions carefully controlled by internal safety mechanisms. We can begin the explanation of the actual operation of a heat pump at any point in the system. However, for our purposes, we shall begin at the evaporator. American Dream uses a counter flow coaxial heat exchanger which is highly resistant to freeze damage. High turbulence created by the water flow makes them relatively self-cleaning, keeping the heat pump in peak operating condition. The layout and design of the water-to-refrigerant heat exchanger is such that our units can maintain a suction pressure equal to most conventional models on less than half the water requirement; or, if equal water is available to each unit, the ultra performance geothermal unit will continue to operate safely on lower temperature water than most competitors.
This feature allows our unit to operate in a much more northerly climate without danger of freeze-up. Normal suction pressures are in the range of 50 to 60 psig with corresponding evaporator temperatures approximately 30 to 32° F. Once the Freon leaves the evaporator, it is routed to a large suction line accumulator whose purpose is to ensure that absolutely no liquid Freon, which is the # 1 killer of compressors, reaches our compressor. Suction gas temperatures at a point 6” from the compressor are usually in the area of 37 to 39° F giving us a superheat of from 5 to 7° F. Should the water supply be interrupted for some reason, our unit incorporates a low suction temperature (or pressure) cut-out switch which will automatically shut off the unit before any freeze-up can occur inside the water heat exchanger. Although the low temperature cut-out thermostat is of the automatic reset variety, a lock-out relay prevents the compressor from restarting until either the main power is interrupted or the room thermostat is turned down and then back up again.
The Freon gas travels from the suction accumulator to the compressor which makes the gas more dense and hot by the concentration of a large amount of thermal energy into a small space. The compressor used in our unit is a heavy duty Copeland® scroll compressor with one of the highest energy efficiency ratios (EER) on the market today. Long life and dependable service can be expected from this compressor since it performs a job much easier on it than what it was designed for. Typical head pressures run in the range of 200 to 250 psig which gives a compression ratio of approximately 3.5 to one.
The manufacturer has designed this compressor to work under the much more adverse conditions of an air source heat pump which could often see a compression ratio of 8 to 1, therefore current draw and wear on the compressor when used in our unit is significantly less and results in long service life. Discharge temperatures experienced by our unit are usually between 190 and 200° F. These discharge temperatures are well below accepted standards and there is no danger of internal scoring or breakdown of the oil in the system. From the compressor the hot 198° F. discharge gas enters our tube-by-tube domestic hot water coil which is simply a desuperheater coil that drops the temperature of the dry discharge Freon vapor to approximately 150° F. This domestic hot water generator actually improves the performance of the entire system by initiating slight condensation of the Freon. Operation of the condenser becomes more efficient since all it’s surface is wet before it reaches it’s actual condensation temperature. The condenser coil used in our units, has been specially designed to act as a transfer surface for both heating and cooling the household air. The coil has sufficient extra capacity to allow for dirty air filters poor air flow etc, which can produce poor performance until the situation is corrected.
The return liquid line is equipped with a large filter-drier to absorb any moisture droplets that may become present in a system. Factory procedures in silver soldering and evacuating our unit will ensure that our systems are clean and dry; however the field technician may not observe all accepted practices when he services the unit, thus admitting contaminants which the filter will catch. American Dream uses the most efficient expansion system – a TXV to vaporize the return liquid freon. Air distribution is handled by a high efficiency, multi-speed ECM fan.