DEVELOPMENT OF REFRIGERATION Modern refrigeration has m
any applications. The first, and probably still most important,
is the preservation of food. Most food kept at room temperature
spoil rapidly. This is due to the rapid growth of bacteria. At us
ual refrigeration temperature of about 40°F (4°C), bacteria grow
quite slowly. Food at this temperature will be kept longer. Refri
geration preserves food longer by keeping it cool. The refrigerat
ion industry became commercially important during the 18th centur
y. Early refrigeration was obtained by use of ice. Ice from lakes
and ponds was cut and stored in the winter in insulated storeroo
ms for summer use. The use of natural ice is required to build in
sulated containers or iceboxes for stores, restaurants, and homes
. These units appeared on a large scale during the 19th century.
Ice was first made artificially about 1820 as an ex
periment. Not until 1834 did artificial ice manufacturing become
practical. Jacob Perkins, an American engineer, invented the appa
ratus which was the forerunner of our modern compression systems.
In 1855 a German engineer produced the first absorption type of
refrigerating mechanism although Michael Faraday had discovered t
he principles in 1824. HOW A MECHANICAL REFRIGERATOR OPERATES
Removing heat from inside refrigerator is somewhat like
removing water from a Leaking canoe. A sponge may be used to soa
k up the water. The sponge is held over the side, squeezed, and t
he water is released overboard. The operation may be repeated as
often as necessary to transfer the water from the canoe into the
lake. In a refrigerator, heat instead of water is t
ransferred. Inside the refrigerating mechanism, heat is absorbed,
"soaked up" by evaporating the liquid refrigerant in the evapora
tor (cooling unit). This occurs as the refrigerant changes from a
liquid to a vapor (gas).The Removal and Relocation of Heat HEAT
Heat is a form of energy. It has a relationship to th
e atom, the smallest indivisible part of an element. (Indivisible
means if one broke the atom down into more pieces it would no lo
nger be that element.) All substances are made up of tiny atoms w
hich are combined to make molecules. All the atoms are in a state
of rapid motion. As the temperature of the substance
increases, the atoms move more rapidly. As the temperature drops
, they slow down. If all heat is removed from a substance (absolu
te zero), all molecular motion stops. The U.S. conven
tional unit of heat is the British thermal unit (BTU). The metric
unit of heat is the joule (J). If a substance is warmed, heat is
added; if cooled, heat is removed. How do things get colder ? Th
e States of Matter SolidLiquidGasPlasmaThe Magic of Latent Heat 4
Main Components in a Mechanical Refrigeration System:s.w. cool.p
umpSENSING VALVEEVAPORATOREXP. VALVESOLENOID VALVETEMP. SENSORTEM
P. CONT.SWL.O. press.S WS.W. PRESS. SWOIL SEP.COMP.MOTORH.P.L.P.F
ILTERCHARGING VALVEPOWER SUPPLYMOTOR CONT.CONDENSERSUC.DISC.EVAPO
RATORBRINE PUMPBRINE TANKCOOLING CHAMBERTEMPERATURESENSORTEMP. CO
NTROLHeat Transfer Rates Copper and aluminum materials are used b
ecause of very good thermal conductivityIncreasing surface areaRe
frigeration Components and EquipmentCompressorCondenserEvaporator
sAuxiliary EquipmentMetering Device The Evaporator The Compressor
The Condenser Superheat and Slugging Accessories AccessoriesCond
ensing UnitHeat FlowReefer RoomTypes of CompressorCentrifugalReci
procatingScrewScroll typeVane typeGear typeSystem FaultsOvercharg
eAir in the systemUnder chargeMoisture in the systemAir Condition
ing SystemReefer CompressorAir DryerVentilationsNatural Ventilati
onsForced VentilationsEngine Room Fan FwdQUIZHow do things get co
lder? Explain. 10 pts.Essay1. What is the function of a TXV? 5 pt
s.2. Where is the thermal bulb of a TXV located? Explain why is
it located at that point? 5 pts.3. Name 3 safety devices of a ref
rigerating system and explain their function. 5 pts each.1 Body flange cap screw (2) 3 Cage assembly 5 Seat gasket 2 Power assembly 4 Body flange gasket (2) 6 Body flange Figure 7-6. ?Liquid quench valve disassembly, exploded view. |
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