NORDIC® DX Series - Direct Expansion

A direct earth coupled heat pump or “DX” heat pump is one that has its refrigerant evaporator / condenser in direct thermal contact with the earth from which heat is either extracted from during the heating mode or introduced to during the cooling mode of operation.

The general refrigeration cycle of our DX machine is similar in nature to a conventional water-to-air or water-to-water heat pump in that there exist a compressor, expansion device, reversing valve, and refrigerant-to-air heat exchanger. Conventional technology concerned with heat pumps relies upon the transfer of heat from the ground by means of a secondary heat exchanger system and working fluid, e.g., water, which is pumped to the geothermal unit located in the heated structure. The conventional heat pump has it’s own internal primary heat exchanger which extracts heat (heating mode) or rejects heat (cooling mode) from this water, which is then pumped back to the earth to be re heated.

DX systems similarly use a ground coil system, however, the working fluid is a refrigerant and the copper ground loop is the primary heat exchanger. Such geothermal heat exchange is an efficient and effective way of achieving heat exchange in heating and air conditioning systems, and especially heat pump type systems. Since the ground temperature is relatively constant at 48 °F at a depth below the frost line, the available heat is constant. The elimination of the secondary earth heat exchanger (typically plastic in nature) and its associated working fluid reduces the temperature difference required between the ground and the evaporating refrigerant yielding a higher suction pressure than a conventional system under similar circumstances and thus a higher efficiency.

 Some Shortcomings Included

• Inadequate evaporator length and spacing for properly extracting heat from the earth resulting in low capacity and low efficiency of the systems.
• Inadequate oil return to the compressor primarily in the heating mode.
• Refrigerant charges in the range of 10 times greater than a similar capacity conventional geothermal heat pump.
• Approximately 3 times as much refrigerant required in the cooling mode as is required in the heating mode.
• Lack of a proper means to store additional refrigerant required during the cooling operation but not needed during the heating mode.
• Inefficient and ineffective method to account for vastly varying condenser capability depending on ground temperature.
• Difficulty in providing an easy to install system of earth exchanger loops.

The NORDIC Solution

The NORDIC® solution has been to start with a clean new concept and to design a unit from the ground up. We started by developing an evaporator system that would yield the best performance to pressure drop factor and which would impact enough area to maintain a minimum suction pressure above 40 Psig. The current horizontal ground loop comprises  350’ per ton of 1/2” OD copper tubing. A 3 ton system would have 3 such loops working in parallel during the heating mode. Refrigerant charge had been determined to be 5 lbs. of R-22 per loop. These 1/2” copper loops maintain sufficient velocity at all times to insure adequate oil return.

During cooling mode the machine automatically selects one or more loops based on discharge pressure to act as the condenser. As the discharge pressure builds to a predetermined point the on-board computer selects the most appropriate combination of groundloops to dissipate the heat at the lowest cost to the homeowner. By intelligently controlling the manner in which the condenser is utilized our total system charge does not have to be altered nor does an excess charge have to be stored anywhere.

"DX" - Better than Conventional Technology

There are several advantages of “DX” technology that are superior to conventional geothermal heat pumps of both the “open loop” and “closed loop” variety. Listed below are some of the reasons why “DX” technology is becoming more attractive to home owners, Dealers and Utilities. 

More Reliable

• Fewer parts to the system
• Does not require a supply and return well.
• Does not require a well pump or circulation pumps.
• No water heat exchanger and associated valving to corrode, freeze and break.
• More Efficient

The direct expansion principle allows the refrigerant to come directly into contact with the earth, separated only by copper tubing. During winter,maximum heat transfer takes place at higher temperature than conventional ground loop technology without the maintenance and electrical cost of circulation pumps.

Less Maintenance

Only a sealed refrigeration circuit to maintain.

More Versatile

“DX” systems can be installed in a more confined area than a conventional groundloop system, primarily because the heat exchanger coil is much more efficient at transferring heat to the refrigerant than a plastic earth exchanger. Normal loop lengths for a “DX” machine are nominally 350’ per ton as opposed to 450’ to 500’ per ton for a plastic earth exchanger. Similarly, vertical systems require only a 3” bore hole to a normal depth of 120’ per ton.

Easier to sell