Page 189 - Boiler_Operators_Handbook,_Second_Edition

174 Boiler Operator’s Handbook

combining those droplets until they’re large enough to Sometimes people experienced with the need for
drop back into the liquid refrigerant pool. The refriger- superheat to control the flow through the throttling de-
ant is superheated as it is worked on by the compressor vice insist that the gas leaving the evaporator of a chiller
and monitoring that superheat can detect problems with has to be superheated but that’s not the case with cen-
the compressor. trifugal chillers. The throttling device in a centrifugal
Despite the fact that the chiller barrel operates in a chiller is a float valve, Figure 5-22, and normally consists
vacuum it’s possible for the refrigerant in an idle chiller of two float valves which separate two different pres-
to reach the setting of the pressure relief device, which is sures. The system of two float valves and the chamber
typically a rupture disc which looks like a large version between them is commonly called an economizer be-
of the lid on one of your grandmother’s canning jars. cause it captures some of the vapor that flashes as the
It’s a convex sheet of metal that is bowed in toward the pressure is dropped and uses it to cool the hermetic mo-
chiller barrel and, when the pressure reaches the design tor of the chiller, entering the compressor between two
of the rupture disc it pops out (like the lid on that jar) stages. That system saves the compressor pumping all of
but, in the process of doing that impinges on some sharp the gas from evaporator pressure to condenser pressure.
steel cutters that cut it so it peels back and dumps all Being a large, expensive machine they are normally
the refrigerant. Temperatures above 120°F can create a insured and the insurers typically require a breakdown
saturation pressure that exceeds the typical rupture disc inspection of a chiller every fifteen years. Examinations
design of 15 psig. A full charge of refrigerant in a typical include electrical insulation testing, leak testing, and
chiller costs more than $25,000 so, don’t let those chillers eddy current testing of the tubes. There’s a hint here,
get too warm. I’ve always questioned some locations for the typical chiller is expected to operate for fifteen years
them, especially over boiler rooms. without concern so it’s a highly reliable piece of equip-
Since the centrifugal chiller operates in a vacuum ment.
it’s possible for ambient air to leak into it. You’ll proba- Why chillers? A centrifugal chiller system can pro-
bly notice what looks like part of a window unit hang- duce cooling for a fraction of the cost of the typical air
ing around the condenser. Its purpose is to pull any air to air cooling system. One chiller can produce chilled
that leaks in off the top of the condenser and remove it, water at a lower first cost than self-contained units, all
dumping it to atmosphere along with any moisture the in a smaller space, for as little as half the operating cost.
air carried along. Centrifugal chillers are made with capacities, at the time
Another device that can be applied to a centrifugal of this writing, as high as 3,000 tons. That one would
chiller is an oil separator which recovers oil that leaked be a fraction of the size of 3,000 one ton window units
into the evaporator and return it to the compressor stacked up.
sump. The oil can coat the tubes and block heat transfer Just like a boiler, the load for a chiller will change
so considerable effort is expended to ensure it doesn’t with outside air conditions. To accommodate reduced
leak into the evaporator. loads many chillers use inlet vanes that throttle the flow
The lubricating oil for a centrifugal chiller isn’t cir- of the evaporated refrigerant to the compressor while
culated with the refrigerant like it is with other compres- causing it to swirl thereby reducing the horsepow-
sors. The oil is sealed at the bearings and circulated in er requirements of the compressor (Figure 5-14). More
its own system with a small portion of the chilled water modern chillers control the evaporator temperature by
used to cool the oil. If the seals leak so oil ends up in the varying the speed of the compressor. When the vanes or
evaporator it coats the tubes to reduce heat transfer and, reduced speed cannot prevent the evaporator pressure
therefore, the efficiency of the chiller. A separate electric dropping to produce colder chilled water a low pressure
oil pump is used to provide lubrication to the compres- switch shuts down the compressor. The low pressure
sor bearings and it must be proven in operation before switch has a differential setting which allows the evap-
the compressor can be started. The oil sump is heated orator pressure to rise over that range before restarting
like other compressors and for the same reasons. The lu- the compressor. A chiller cycling on and off is not much
bricating oil is typically sampled and tested regularly to different than a boiler cycling and, whenever possible, a
determine the condition of the oil and the chiller itself. change to a smaller unit or shutting one down complete-
One concern you should have is properly labeling the oil ly can reduce power consumption and demand charges.
samples because you do not want to mix up samples from Other considerations when operating centrifugal
several units and, like what almost happened once with chillers include the monitoring of the temperature and
one of our customers, a rebuild of the wrong chiller. corrosive/scaling condition of the cooling tower water