Page 188 - Boiler_Operators_Handbook,_Second_Edition

Refrigeration & AC 173

accumulation of debris doesn’t block the bottom two or compressor, are the most common but a chiller can use
three rows of coil. I also inspect it about four times a any type of compressor and there are modifications of
year to catch any unusual accumulations. the typical arrangement that permit air cooled condens-
ers and operation with a mix of water and anti-freeze
Ground Source Heat Pumps to permit production of cold water at temperatures as
I would love to be able to afford a ground source low as -25°F. They see applications like ice storage sys-
heat pump for my house but my lot is so small that there’s tems where the chilled water is cold enough to form the
not enough room to do it. A ground source heat pump is ice in the ice storage tanks. The normal chiller, however,
also a big investment which I would gladly pay because produces chilled water at what has become a de facto
it would provide a good return on the investment. The standard of 42°F with a return temperature of 52°F. It’s a
reason is rather simple. Knowing the refrigeration cycle standard because all manufacturer’s make chillers with
you can see a significant difference between the typical the same conditions to match users of the chilled water
air to air heat pump in a residence and a ground source with the same conditions. Needless to say, you’re always
heat pump. Instead of heating the house with refriger- monitoring the temperature to ensure it’s leaving the
ant boiling from heat in air at temperatures as low as chilled water plant at a temperature of 42°F.
5°F you heat it with refrigerant heated by the ground at The typical centrifugal chiller, Figure 5-32, is de-
an average temperature of 55°F. That defrost cycle isn’t signed to dump the heat to cooling tower water sup-
required. As for cooling, instead of condensing refrig- plied at 85°F returning it to the cooling tower at 95°F.
erant with air temperatures of 90°F or more you’re con- Again, these are adopted standards so that equipment
densing it by dumping heat to the ground at that same of different manufacturers can be used in a system. A
55°F. Lower temperature differentials with heat pumps typical centrifugal chiller also operates with a vacuum
reduce the pressure differential the compressor has to in the evaporator, 15 to 16 inches Hg. Pressure in the
overcome with lower power consumption and electrici- condenser is typically 6 to 8 psig. The evaporator, com-
ty costs. Most residential ground source heat pumps are monly called a “chiller barrel” is a shell and tube heat
also used to heat or at least preheat the domestic hot exchanger with the chilled water flowing through the
water. tubes and liquid refrigerant surrounding the tubes in-
There are office buildings, schools, and other fa- side the shell. The condenser is similar with (normally)
cilities larger than a residence that are using ground cooling tower water flowing through the tubes. Unlike
source heat pumps, taking advantage of the ability of most refrigerant systems the refrigerant is not super-
the ground under their parking lots to absorb and give heated in the evaporator because all the heat is absorbed
up heat to either the refrigerant or, more commonly, wa- by boiling the liquid refrigerant. The evaporator is locat-
ter circulated from the condenser/absorber of the heat ed below the inlet of the compressor and a demister in
pump through wells in the ground under the adjacent the top of the evaporator captures any droplets of liquid
parking lots. The systems are operated and maintained that are carried up from the surface of the boiling liquid
just like any other refrigeration system with the added
duties of monitoring and recording the ground water
circulating pump, flow and temperatures.

Chillers
Hospitals, office buildings, retirement homes,
schools, and similar buildings are commonly serviced
with chillers for cooling and boilers for heating. Some
hotels and motels are but many of those opt for what’s
the equivalent of a window unit to provide heating and
cooling of the guest rooms. That’s because the units
serving empty rooms can be turned off to reduce operat-
ing cost when the hotel or motel isn’t fully occupied.
Chillers are used to cool chilled water that is circu-
lated to air handling units, convectors, beams and other
heat transfer devices in buildings that cool and dehu-
Figure 5-32. Centrifugal chiller with two stage com-
midify the air. Centrifugal chillers, using a centrifugal
pressor