Page 167 - Boiler_Operators_Handbook,_Second_Edition

152 Boiler Operator’s Handbook

ing between the evaporator and the compressor. It has a are in effect by operating ice-making equipment at night
specific purpose when the compressor is higher than the when electricity rates are lower. Proper use of ice storage
evaporator. It accumulates coalesced oil until the trap is to limit peak electrical costs, and to absorb heat loads
full and the gas pushes it up the piping to the compres- that exceed the capacity of the chilled water system, can
sor. In most systems some lubricating oil circulates and significantly reduce the owners operating costs while
those little traps make sure it doesn’t get trapped in the ensuring the occupants of the building are always com-
system and always returns to the compressor. fortable.
Ice storage systems don’t work like the icemaker in
your refrigerator. In order to absorb the heat the ice and
FREEZING AND ICE STORAGE chilled water have to be in contact and the rate of heat
transfer between the two is proportional to the surface
I doubt that there are many of you who can remem- area where the water contacts the ice. In other words,
ber an icebox. The name sort of says it, an icebox was a you cannot simply create one huge block of ice then melt
wooden box with doors and an opening in the top where it. The only system I’m aware of that works well is made
the Iceman dumped a chunk of ice every few days. Be- by Baltimore Aircoil and consists of a heat exchanger en-
fore refrigeration the only way we had to obtain ice, oth- closed in a tank; the outer surface of the tubing of the
er than in the winter, was to draw on reserves of ice cut exchanger is exposed to water that flows through the
from the lake surface and stored under a huge pile of tank and ice is formed on the outside of the tubes by
straw during the summer. No, I’m not that old, but my cooling using a refrigerant, brine, or glycol water mix-
grandmother still had her icebox so I know what they ture inside the tubes. The ice builds up on the surfaces
look like. The ability to make ice and freeze things is one of the heat exchanger when making ice and is melted by
of the major contributions to better health and longer water pumped through the tank that chills that water.
living that we enjoy today. To ensure an even build up of ice on the surfaces of the
On the other hand, freezing of water has contribut- heat exchanger air is blown into the tank to bubble up
ed to considerable damage to refrigeration equipment, through the water thereby increasing turbulence to help
piping, and other equipment that contains water. I’m improve heat transfer.
sure you know that ice expands as it cools; a contradic-
tion of most substances because they shrink as they cool
and expand when heated. It’s a good thing in actuality COMPRESSORS
because, by expanding, the density of ice is less. Frozen
water floats. If it were not for this phenomenon earth The compressor takes in the refrigerant after leav-
could be an ice planet because the sunlight couldn’t get ing the evaporator and pumps it up to a higher pres-
at the ice to melt it. An operator has to be conscious of sure to raise the saturation temperature to a point where
the fact that ice expands when cooled so he prevents the heat absorbed by the evaporator can be dumped
damage to piping and equipment by preventing its tem- to another substance in the condenser. A low-pressure
perature dropping below 32°F or draining the equip- with a correspondingly low saturation temperature is
ment and piping. maintained in the evaporator because the compressor
When making ice the operator also has to consider removes the refrigerant vapor from the evaporator.
the fact that ice is a solid and heat transfer through ice Work is performed on the refrigerant by the compres-
is by conduction only. The rate of flow of heat through sor to squeeze it up to a higher pressure and the energy
a solid, like ice, is proportional to the thickness of the of the compression increases the heat in the refrigerant
solid; the thicker the solid the slower the heat transfer. increasing its superheat. Because many compressor mo-
Freezing water requires removal of 80 Btu from tors are cooled by the refrigerant, friction and heat from
each pound of ice and that ice will absorb the same the motor windings typically adds to the superheat as
amount of heat when it’s melted. Ice, therefore, increas- well.
es the capacity of the system to absorb heat within 1¼% High evaporator pressures or high temperatures at
of the mass of water. Storing ice, principally for air-con- the outlet of the compressor are indicative of inefficient
ditioning systems, permits the installation of smaller compression. A noisy compressor, that is one noisier
equipment for cooling chilled water that is used in most than normal, may indicate liquid flooding through the
facilities. Ice storage can also significantly reduce elec- evaporator to the inlet of the compressor. It could also
tricity cost for cooling when time-of-use electricity rates indicate loss of lubrication in the compressor. Because