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LESSON

3 LECTURE

REFRIGERATION SYSTEMSOPERATION

SUB-OBJECTIVE

At the end of this Lesson the T !inee "i## $e !$#e to%

&' Identif( the )! ts * f+n,tions diffe ent Ref i e !tion S(ste.s'

/' T o+$#e shoot fo .ino * .!0o ) o$#e.s in the !$o1e'

3' Co e,t#( .!int!in the !$o1e s(ste.s'

&'2 CASCA E REFRIGERATING SYSTEMS

In a cascade refrigerating, two or more refrigerating systems are connected asshown in Fig. 3-3-1. Both systems operate at the same time.

System A (on the right) has its evaporator, A, (heat a sor ing part) arranged tocoo! the condenser B for the system.

B. "he evaporator for system B s#pp!ies the coo!ing effect desired. $achsystem has a thermostatic e%pansion va!ve ("$&) for refrigerant contro!.

"he !ow-press#re !i'#id of system A coo!s the high-press#re vapor of system

B.ascade systems are often #sed in ind#stria! processes where o ects m#ste coo!ed to temperat#res e!ow - *+ F. (- .)

ne motor contro! is #sed for oth motors. It is connected to a temperat#resensing #! on evaporator B.

/otors #sed on cascade systems m#st e capa !e of starting #nder !oad.0ith the #se of thermostatic e%pansion va!ves, the press#re does not a!anceon the off cyc!e. "he condenser-evaporator is #s#a!!y of the she!!-and-t# e

f!ooded evaporator type.Since these systems operate at very !ow temperat#res, the refrigerant m#st every dry. therwise, any moist#re wo#!d condense at the need!e-seat of the"$& and stop the f!ow of refrigerant. System A m#st have specia! refrigerantoi! (wa% free, moist#re free and f!ows at e%tra !ow temperat#res).

i! separators sho#!d e insta!!ed in the compressor-to-condenser !ines onoth of these condensing #nits to he!p eep the oi! in the compressors.

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Fi ' 3-3-&' C!s,!de ef i e !tin s(ste.'

/'2 MO ULATING REFRIGERATION CYCLE

In most refrigeration insta!!ations, the coo!ing or refrigerating capacity iseno#gh to maintain the desired temperat#re #nder the heaviest !oad. thistemperat#re is maintained y the motor contro!.

It starts the motor compressor when coo!ing (or heat remova!) is re'#ired andsh#ts it off as soon as the desired temperat#re is reached.

;owever, if the heat !oad is !ight, this sing!e system may e over capacity for the o .

"he operating e%pense is greater than it wo#!d e if the machine capacitymore near!y matched the needed !oad.

"he system a!so tends to coo! too fast and it operates on and off too '#ic !y.

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A mod#!ating (varying capacity) system has een deve!oped to fit the machinecapacity more c!ose!y to the needed heat !oad.

sing two or more compressors connected in para!!e! sometimes does this.$ach compressor is operated y a motor contro!.

2#ring operation, if the heat !oad increases and the temperat#re starts to rise,one compressor wi!! contin#e to r#n.

B#t if the temperat#re eeps on rising, the second compressor wi!! start tooperate. Additiona! compressor may c#t in #nti! eno#gh capacity is o tained.

Fig. 1-=-< i!!#strates a typica! cyc!e diagram for a mod#!ated insta!!ation.

"his insta!!ation has three compressors. A press#re contro! connected to thes#ction !ines operates the motors.

"he contro! contains a specia! switching device, which rotates the service of the vario#s compressors.

"h#s, each compressor wi!! e #sed a o#t the same amo#nt of time.

"he mod#!ating cyc!e maintains #niform temperat#res and operateseconomica!!y.

Any conventiona! refrigerant contro! can e #sed. ;owever, the thermostatice%pansion va!ve is most common.

A!! the compressors may #se the same condenser and !i'#id receiver, or eachmay have its own.

"he same evaporator is connected to a!! the compressors '

A mod#!ating system may #se a m#!tip!e cy!inder compressor, each cy!inder eing e'#ipped with an 6-!oading device.

&aria !e speed motors are a!so #sed to provide a mod#!ated refrigerationcapacity.

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Fi ' 3-3-/' Mod+#!tin ef i e !tion ,(,#e .e,h!nis.'

0hich #ses a three (3) motor compressor.

:ress#re motor contro! is arranged to operate one or more compressors as need.

3'2 T4ERMOELECTRIC REFRIGERATION

Fig. 3-3-3 represents a simp!e thermoe!ectric co#p!e.

"he co#p!e moves heat from the inside of an ins#!ated space to a heate%changer on the o#tside.

$!ectrons, rather than refrigerants, carry away the heat.

Fins on the evaporator increase the heat f!ow. Fins on the o#tside of the heate%changer he!p give off the heat to the s#rro#nding air.

"he thermoe!ectric co#p!e wor s eca#se of the difference in the energy !eve!of the two semicond#ctors : and 6. "he : and 6 refer to the two differentsemicond#ctor materia!s #sed (and their characteristics). "his sho#!d not e

conf#sed with positive and negative po!arity of an e!ectrica! circ#it.

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Semicond#ctors are meta!!ic a!!oys and o%ides. "hese vary great!y in energy!eve!s.

"he e!ectrica! '#a!ities of semicond#ctors !ie etween those of ins#!ator andcond#cts. "hey have properties of oth, th#s they do not carry e!ectric c#rrentas we!! as cond#ctors and they do not stop f!ow of c#rrent as we!! asins#!ators.

"he choice of materia!s for the semicond#ctors : and 6 determines theefficiency of the device.

6ow a sing!e co#p!e can transfer m#ch heat.

"o increase the coo!ing effect, severa! co#p!es may e connected in series."his gro#p of co#p!es is ca!!ed a mod#!e. 7ro#ps of mod#!es may econnected together in para!!e!, to increase the capacity sti!! f#rther.

A thermostat inside the refrigerated space contro!s the c#rrent f!ow thro#gh thetransformer rectifier, which s#pp!ies a contro!!ed d.c. c#rrent to the mod#!es.In this way, the temperat#re inside the refrigerator is contro!!ed.

"here are no moving parts in this refrigerator. Aside from the constr#ction of the mod#!es, it is '#ite simp!e.

"herma! efficiency is !ow. "hat is, the amo#nt of refrigerating effect o tainedfor the e!ectrica! energy spent is !ess than with a conventiona! compressor type refrigeration system.

By reversing the direction of the f!ow of c#rrent thro#gh a thermoe!ectricdevice, the hot and co!d s#rfaces wi!! e reversed.

"h#s, the same device can e #sed for oth heating and coo!ing an ins#!atedspace.

ne app!ication of this thermoe!ectric device has een in the air conditioningand heating of n#c!ear s# marines.

It is a!so #sed e%tensive!y to contro! temperat#res in e!ectronic e'#ipment(comp#ters, aerospace devices and so forth).

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Fi ' 3-3-3' i! !. of si.)#e the .oe#e,t i, ,o+)#e5 +sed fo ef i e !tin !nins+#!ted s)!,e'

;eat a sor ed y thermoe!ectric co#p!e is re!eased to o#tside y finsattached to heat radiating s#rface (heat sin ).

6'2 ELECTRIC EFROST

$!ectric heating e!ements p!aced a!ongside the evaporator s#rfaces heat #p tome!t the frost and ice #i!d#p from the evaporator.

A timer or contro! mechanism operates the heater d#ring the time that therefrigerating mechanism is on the off cyc!e. Fig. 3-3-3A shows therefrigerating cyc!e> Fig. 3- -3B shows the defrost cyc!e.

In 3-3-3A, the e!ectric heating mechanism is in the refrigerating part of thecyc!e. 4i'#id refrigerating is vapori?ed in the evaporator. It a sor s heat andecomes a vapor.

0hi!e in the evaporator, it f!ows thro#gh an acc#m#!ator, passes on to thes#ction !ine ac to the compressor.

In the compressor, it is compressed to a high-press#re high temperat#re andf!ows into the condenser. ;ere the heat of vapori?ation is removed and the


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refrigerant ret#rns to a !i'#id f!owing into the !i'#id receiver. From here thecyc!e is repeated.

In Fig. 3- -3 the same system is in the defrost cyc!e.

"he compressor is stopped and, then defrosts the contro! mechanism and, !etse!ectric c#rrent f!ow thro#gh the resistance heating e!ements a!ongside theevaporator s#rface.

;eat warms the evaporator s#rfaces #nti! the frost and ice are me!ted and themoist#re empties into a drain pan.

"he operation of the resistance #nits is #s#a!!y timed to contro! oth thefre'#ency and the d#ration of the e!ectric heating.

"his timing provides for ade'#ate frost remova! and the system is he!ped tooperate efficient!y with !itt!e or no frost on the evaporator s#rfaces.

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Fi ' 3-3-6A' An e#e,t i, def ost s(ste. d+ in the ef i e !tin ,(,#e'


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Fi ' 3-3-6B' An e#e,t i, def ost s(ste. d+ in the def ost ,(,#e'

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7'2 TEST YOUR 8NO9LE GE

1. 0hat are some of the advantages of the capi!!ary t# e type refrigerantcontro!

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