Page 192 - Boiler_Operators_Handbook,_Second_Edition

Refrigeration & AC 177

tion chillers. Facilities that have both absorbers and cen-
trifugal chillers can take advantage of the swings in the
cost of energy to operate the chillers that are most eco-
nomical. First there’s the opportunity to reduce demand
charges by operating an absorber during peak load pe-
riods then there’s the constantly changing difference in
energy prices. An absorber can be cheaper to operate at
certain times of the day but starting and stopping one
takes time and energy so there’s clearly going to be a fair
amount of calculating to determine how to operate since
electricity prices can change every hour.

COOLING TOWERS

For refrigeration systems cooling towers are used to
reduce the pressure differential a refrigerant compressor
has to overcome in order to pump the refrigerant from
the evaporator to the condenser. It’s accomplished by
cooling the water used in the water cooled condensers Figure 5-36. Blow thru cooling tower
of refrigeration equipment, especially chillers. A cooling
tower helps cool the water by evaporating some of it. wet bulb temperature of the atmospheric air (see psy-
A boiler operator knows that it takes about 1,000 Btu to chometrics in the section on air conditioning) and that’s
evaporate a pound of water. In addition to reducing the seldom higher than 85°F. The cooling tower operates by
cooling tower water temperature by 10°F the water is distributing the water over a fill which has atmospheric
cooled by evaporation of some of the water. A good rule air flowing through it, normally forced by a fan. The fan
of thumb is the cooling tower will evaporate 2 gpm of can be on the outlet of the tower, the common location,
water for every one million Btuh removed from the cool- or force air into the tower where it’s commonly referred
ing water. The tower can cool the water almost to the to as a blow-thru cooling tower like Figure 5-36. Oth-
er terms for cooling towers are
cross-flow and counter-flow; a
blow-thru is commonly count-
er-flow where the air is pushed
up through the tower as the wa-
ter drops down through. In a
cross flow tower the air enters
the sides or ends crossing the
flow of flowing water then rises
to the fan inlet as in Figure 5-37.
Normally the water is
delivered to perforated trays
at the top of the cooling tower.
Some use sprays but the perfo-
rated tray serves to distribute
the water evenly over the top of
the fill. Fill is the word used to
describe the material in the tow-
er that the water runs over and
around as it drops from the inlet
trays or sprays to the sump at
Figure 5-37. Cross flow cooling towers the bottom of the cooling tower.