Page 182 - Boiler_Operators_Handbook,_Second_Edition

Refrigeration & AC 167

heat transfer from the bulb to air. While it isn’t neces- MISCELLANEOUS COMPONENTS OF
sary it’s a good idea to insulate the bulb and suction A REFRIGERATION SYSTEM
line. On large tubing the bulb is mounted at the bot-
tom of a horizontal tube because boiling liquid will run A refrigeration system consisting of an evaporator,
along the bottom and superheated vapor could be at compressor, condenser, and throttling device contains
the top at low loads. The bulb is offset from the very everything necessary to transfer heat from a substance
bottom of the tube to eliminate the insulating effect of we want cool to a substance that can accept the rejected
the thin film of oil that flows along with the refrigerant. heat. However, there are many additional components
The bulb is mounted with the capillary connection on included in systems for protection of the system, ease of
top of vertical tubes to retain liquid in the bulb. If the maintenance, or to serve diverse loads.
connection were on the bottom vapor would form in There are also specific labels for certain parts of
the top of the bulb to reduce heat transfer. Normally a refrigeration system. Among those labels are specif-
the diaphragm and capillary contain some vapor be- ic names for different sections of piping or tubing. We
cause they are at a higher temperature than the bulb. use the word line generically to identify piping or tub-
The vapor in the capillary and diaphragm is usually ing. The line between the evaporator outlet and the in-
superheated and doesn’t materially affect the opera- let of the compressor is called the suction line. The line
tion of the TXV. connecting the compressor discharge to the inlet of the
Another possible problem with TXV operation is condenser is called the hot gas line. The line connecting
crimping of the capillary or the equalizing line. Crimp- the outlet of the condenser to the inlet of the throttling
ing of the capillary can block the transfer of bulb pres- device is called the liquid line. Since there is seldom any
sure to the diaphragm or restrict the flow to increase the significant length of line between the throttling device
response time of the valve. Crimping of the equalizing and the inlet of the evaporator there’s no specific label
line will result in a higher pressure on the underside of for that portion. In many pieces of equipment it’s actual-
the diaphragm to reduce the superheat. If the evapora- ly a sort of Christmas tree that feeds the evaporator with
tor has a high pressure drop that could result in liquid a lot of smaller lines.
surging to the compressor.
Technological advances and production methods Liquid Observation Port
have resulted in the introduction of electronic expan- In many older refrigeration systems you will nor-
sion valves. These are controlled by the electronic con- mally find a fitting on the liquid line before the throttling
trols which must sense evaporator outlet pressure and device that contains a glass window so you can look
temperature to determine the positioning of the valve. into the line. Most of the time you should see nothing
I haven’t had an opportunity to work with one of these because light passes straight through the liquid. If you
and I would expect the cheaper ones to be a solenoid have one you should always check it, because it only
that simply opens and closes (repeated clicking should takes a glance, to be sure you do not see bubbles forming
be noted) to control the refrigerant flow. in it. The presence of bubbles indicates the liquid is not
What might appear to be improper operation of a sufficiently subcooled and some of the liquid is vapor-
TXV may be attributable to problems with other piec- izing before it reaches the throttling device. Normally
es of the system. Evaporators and condensers can be bubbles indicate a loss of charge but they can also be an
fouled with dust and debris changing system perfor- indication of inadequate subcooling in the condenser.
mance so much that the TXV cannot correct it. A worn
out compressor may not produce enough differential. Filter dryer
There are other elements of a typical system that could Practically every system made includes a filter-dry-
contribute to a problem including the filter-dryer and er. A filter-dryer is always installed in the liquid line be-
other controls. fore the throttling device. The filter-dryer (Figure5-27)
Regardless of the type of throttling device it is im- performs those two functions. It contains an element
portant to remember what it’s supposed to do, what that consists of fabric or paper filtering media combined
harm can come to the equipment if it doesn’t do what it with silica gel. Its purpose is to trap any rust, flaking,
is supposed to do, and most importantly, how to mon- loose solder from installation or repair, and material
itor conditions to detect potential problems with the from wear in the compressor to prevent plugging of the
throttling device. For most systems that means making small orifices in the throttling device. Also, the desic-
regular checks of superheat and subcooling. cant, normally silica gel, absorbs water that gets into the