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Heat Pumps Part 2: Heat Pump Systems

In this section, we will confine our discussion to the various expansion device configurations for heat pumps.  (Complete system design is far beyond the scope of this discussion.  A good heat pump system must take into consideration many factors, such as duct work, air flow, insulation, etc.)

Large commercial and industrial heat pumps are custom tailored to a specific job.  Small commercial and residential heat pumps are turned out production-line style.  Most of these are of the “air to air” type.  This article will cover the residential air-to-air heat pump, as it is the predominant configuration.

Probably the most popular configuration is one that has a TEV on the outside coil and a cap tube on the inside coil.  See Figures 1 and 2.  The use of a TXV on the outside coil results in controlled flow at all outside ambient conditions.  The TXV’s used for heat pump applications have special low superheat ranges.  Alco’s heat pump TXV’s have an “HCA” bulb charge.  Above 40 degrees F ambient, this charge operates the same as any regular air conditioning bulb charge with standard superheat.  Below 40 degrees F, the HCA bulb charge provides 0 degrees F superheat.  This results in controllable, wet suction gas to cool the compressor.  Hermetic compressors have been “beefed up” to withstand the heavy-duty use imposed by heat pumps.

Figure 1.

Figure 2.

Note:   When replacing a TXV on a heat pump outdoor coil, do not use a standard TXV. A TXV with a low superheat range is needed.

The capillary tube used on the indoor coil keeps down expense.  On some “deluxe” units, the cap tube will be replaced with a standard air conditioning TXV.

Low cost packaged heat pumps (overgrown window units) typically use two cap tubes, a larger cap tube for heating, and a smaller cap tube for cooling.  See Figures 3 and 4.

Figure 3.

Figure 4.

Another system consists of one common TXV with bi-directional flow, as in Figure 5.

Figure 5.

The Alco TLE is a bi-flow TXV with a CA charge.  It is sold only to OEM’s but can be field serviced using TCLE parts and reusing the TLE body flange.

The development of the bi-flow TXV required a bi-flow filter dryer.  Alco BFK dryers are bi-flow dryers.  As you can see, this bi-flow system eliminates check valves and one other expansion device.  This means lower cost to the OEM’s so these systems are quite popular.

Note that the bulb of the bi-flow TXV is located on the suction line between the reversing valve and the compressor. In all our illustrations, no accessories were shown for clarity’s sake.  Many other devices will be on a heat pump system.  Sight glasses, accumulators, distributors, even discharge mufflers may be part of the heat pump refrigerant system.  Note that one thing always was absolutely necessary – the reversing valve.

In Part III, we will discuss heat pump controls.

Go to Part III

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