Page 178 - Boiler_Operators_Handbook,_Second_Edition

Refrigeration & AC 163

adjusting the pressure on the spring. for symptoms of failure of the throttling device.
Because this control valve is a proportional de- Frequently the throttling device is referred to as
vice and water temperatures can change due to sea- a metering device because it regulates the flow of re-
son, storms, etc. the pressure in the condenser will frigerant. Since I interpret the word “metering” to refer
vary. Under extreme conditions you might choose to to items that actually measure flow I’ve avoided use
adjust the setting of the valve to keep the range at the of the word. Another label is “expansion valve” but I
lowest possible values. That’s because maintaining a choose to limit the use of that title to the devices that
low-pressure in the condenser reduces the amount of are normally labeled with those words.
effort on the part of the compressor so it costs less to Failure of the throttling device to throttle sufficient-
operate. The only time I ever considered it important ly can result in liquid flooding the compressor resulting
to make that adjustment was when we were entering in damage to valves, the bearings, broken piston rods
or leaving the Gulf Stream where seawater tempera- as in Figure 5-18, and occasionally result in crankcase
tures could swing by as much as 50°F. You might find rupture. On the other hand, excessive throttling will re-
it advisable to tweak the setting to increase the range sult in high superheat temperatures so the compressor
between winter and summer. The control valve may overheats, burns up valves, burns up the motor, or melts
completely block flow when the condenser is not in use parts as shown in Figure 5-19. If the throttling device
and pressures inside it are low so it’s always a good isn’t working properly insufficient flow will result in
idea to have a small valved bypass around it that can lower capacity cooling because heat transferred to the
be operated to maintain a minimum flow which helps boiling liquid is much higher than heat transfer to a gas.
discourage organic growth on the water sides. Organ-
ic growth can be a problem for any water cooled con-
denser so it’s always a good idea to maintain some flow
through them to discourage organic growth.
Why not just let the temperature of the condens-
er water drop in a refrigeration system? Primarily it’s
because the condenser can get so cold that there’s not
enough pressure differential to force the liquid through
the throttling device. Some modern chillers and refrig-
eration systems may take advantage of cold water (or
cold air for that matter) to condense the gas coming
from the evaporator without compressing it by operat-
ing like a heat pipe. Those applications require a com-
pressor bypass valve, a throttling device bypass, and
because the flow of refrigerant is due to convection cur-
rents, the condenser has to be mounted higher than the Figure 5-18. Broken compressor piston rods
evaporator.

THROTTLING DEVICES

A throttling device is required to separate the
high pressure of the condenser from the low-pressure
of the evaporator to ensure maintenance of the satu-
ration conditions in each of those devices. The types
of throttling devices vary considerably, from the very
simple to quite complex, and can incorporate new
high-technology methodology. The throttling device on
the equipment you operate was selected based on eco-
nomic factors. Most of them will operate with a mini-
mum of attention for years. That doesn’t mean that you
should ignore them; you should be constantly checking Figure 5-19. Overheated compressor parts