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Table of Contents

Lesson One Purging Air and Noncondensables...................................................3

Lesson Two Ammonia System Piping................................................................17

Lesson Three Ammonia System Safety Codes and Guidelines............................35

Lesson Four OSHA Process Safety Management...............................................49

Lesson Five EPA Regulations and Ammonia Safety..........................................63

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Purging, Piping,and Safety

PURGING, PIPING, AND SAFETY

Lesson One

Purging Air andNoncondensables

46401

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1

4

Lesson

Purging Air and Noncondensables

Materials to Be PurgedEffects of NoncondensablesPower PenaltyDetermining If Noncondensables Are Present

Removing Noncondensables (Purging)Purge Point LocationsAutomatic PurgingEconomics of Purging

TOPICS

After studying this Lesson, you should be able to…

• List common noncondensable vapors and dis-cuss their effects in a refrigeration system.

• Discuss the power penalty resulting from noncon-densable gases in terms of compression and lossof refrigeration capacity.

• Explain how to determine the presence of non-condensables.

• Explain how to minimize the entrance of noncon-densables and describe common entry points.

• Compare the features and operation of manualand automatic purging equipment and name thebest connection points for the purge unit.

• Discuss the economic benefits of the purge unit interms of typical payback times.

OBJECTIVES

Purging 1.01 the process of removing noncon-densable gases from a refrigeration system

Dalton’s law 1.07 the physical principle thatstates that when two or more gases are mixed inthe same volume, the total pressure exerted bythe gases is equal to the sum of the partial pres-sures of each gas

Foul gas line 1.34 the line carrying the vapor tobe purged

KEY TECHNICAL TERMS

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Materials to Be Purged

1.01 Purging is the process that removes noncon-densable gases from the refrigeration system. Thesegases do not contribute to the refrigeration capacity.Instead they reduce the capacity, increase the systempower requirement, and lead to system degradation.

1.02 The noncondensable gases involved in ammo-nia refrigeration systems are primarily air, nitrogen,and hydrogen. Because the gases do not condense atthe temperatures and pressures existing in the ammo-nia condenser, they remain as a gas in the condenserand receiver areas. They are also trapped in theseareas because they cannot travel beyond the liquidseal in the receiver. Therefore, the noncondensablegases eventually accumulate to the point where theybecome a problem.

1.03 Air as a noncondensable in the system is rela-tively easy to account for. It enters whenever the sys-tem is opened for service or repair, when filters arechanged, or when screens are cleaned. Air enters anytime the sealed ammonia system is violated, especial-ly in systems operating under vacuum conditions.

1.04 The air is about 80 percent nitrogen. Howev-er, additional nitrogen and hydrogen may also befound as noncondensables. These come directly froma breakdown of the ammonia refrigerant, which is acompound containing nitrogen and hydrogen (NH3).The breakdown occurs mainly in systems with recip-rocating compressors operating at discharge tempera-tures approaching 300°F. At these temperatures, asmall portion of the ammonia disassociates (chemi-cally breaks down) into the primary nitrogen andhydrogen.

1.05 This problem is almost nonexistent with screwcompressor units, because the discharge temperature iscontrolled to less than 200°F at all times. Recall fromprevious Units that this cooling is provided by thelarge quantities of oil that are circulated through thecompressor. The circulating oil effectively removes theheat of compression.

1.06 Other minor noncondensable gases includethe trace gases that are part of the air that enters thecompressor. In addition, some hydrocarbon gas prod-ucts are generated by the breakdown of the oil. Theseproducts are most likely to be found when moisture ispresent in the system along with the high tempera-tures possible with reciprocating compressors.

Effects of Noncondensables

1.07 Over time, noncondensable gases collect in thecondenser and receiver vapor spaces. A physical lawapplies when more than one gas fills a fixed volume.This is known as Dalton’s law, which states that whentwo or more gases are mixed in the same volume, thetotal pressure exerted by the gases is equal to the sumof the partial pressures of each gas.

1.08 What Dalton’s law means is that the total pres-sure of the mixture is equal to the pressure that one gaswould have in that volume plus the pressure that thesecond gas would have if each gas were the only gas inthat volume. Figure 1-1 on the following page illustratesDalton’s law, showing schematically how the total pres-sure is equal to the partial pressures of the refrigerantand the noncondensable gases. You can see that anyadditional noncondensables, whether air or hydrogen ornitrogen, will raise the condensing pressure above thatof the ammonia alone without the noncondensables.

5

Purging is a process that should be used on all industrial ammonia refrigerationsystems, from the smallest to the largest, regardless of their operating condi-tions. Many benefits result from routine purging, and many harmful effects resultfrom the failure to purge. One of the main harmful effects is economic.

This Lesson describes the effects of allowing air and other noncondensables toremain in the system. You may be surprised to see how expensive theseunwanted contaminants can be, and also how fast the payback is with properpurge equipment. This Lesson explains how to determine if noncondensablesare present. You will also read about ways to minimize the entry of noncondens-ables as well as various methods and connection points for removing them fromthe system.

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1.09 This increase in pressure has several harmfuleffects. A main problem arises because the saturatedcondensing temperature is equivalent to the ammo-nia pressure without the additional noncondensablepressure. However, the actual pressure is not at thesaturation value—it is higher. The compressor has towork harder to reach the higher-than-normal con-densing pressure. This requires more horsepowerand results in a lower refrigeration capacity.

1.10 The higher condensing pressure caused by thenoncondensables also results in higher discharge tem-peratures, especially in systems with reciprocatingcompressors. Because the screw compressor dis-charge temperature is controlled by the oil flow andcooling methods, its discharge temperature remainsrelatively stable.

1.11 Operation against a higher condensing pressureand the resulting higher discharge temperatures leads toincreased bearing loads, reduced lubrication quality, andmechanical stresses on the operating components.These together generally reduce compressor life or cer-tainly the time periods between major maintenancerequirements.

1.12 A second main disadvantage of air and non-condensables in the condenser is that the air acts as aninsulating shield between the condenser tube surfaceand the ammonia vapor to be condensed. This occurson the inside of the tubes of an evaporative condenserand also on the external surfaces of a shell-and-tubecondenser in which the refrigerant is on the outside ofthe tubes. Figure 1-2 shows how the air acts as aninsulator, inhibiting heat transfer and reducing thecondenser capacity.

Power Penalty

1.13 It is estimated that for each 4 psi of additionalcondensing pressure caused by air and noncondens-ables, the compressor power requirement increases bytwo percent and the compressor capacity decreases byabout one percent. However, when the air continues toaccumulate, the pressure may rise to 20 psi or moreabove the normal condensing pressure. At 20 psiabove normal operation, the increased power require-ment is about 10 percent or more.

1.14 Table 1-1 shows the additional operatingcosts for electric power associated with a 1000-tonsystem operating 6500 hours per year. At a moderate

6 Lesson One

Partial pressuredue to air

Partial pressuredue to ammoniarefrigerant

Total condensingpressure = theaddition of air plusammonia partialpressures

Total condensing pressure is equal to the pressure dueto ammonia plus the pressure due to air and othernoncondensable vapors

Fig. 1-1. Dalton’s law illustrated

Refrigerant gasis hindered frommaking contactwith condensingsurface

Air moleculesconcentrated oncool wall act asinsulators

Refrigerant gasis hindered frommaking contactwith condensingsurface

Air molecules concentratedon cool wall act as insulators

Coolingwater

Cooling water in tubes

Evaporativecondenser

Shell-and-tubecondenser

Fig. 1-2. Air insulation of condenser tubes

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electrical rate of $0.07 kWh, excess air of only 5 psicosts an additional $11,500 per year. If the noncon-densables rise to an excess of 20 psig, at the sameelectrical rate of $0.07 kWh, the additional cost tooperate the system will be $46,000 per year. Theseadditional costs are significant and need not beincurred.

Determining If Noncondensables Are Present

1.15 The presence of noncondensables increasesthe condenser pressure above the normal saturationpressure. The most convenient way to detect noncon-densables is to measure the temperature of the liquidrefrigerant leaving the condenser or in the receiver. Itis also necessary to measure the pressure in the con-denser or receiver and compare the pressure againstthe saturation pressure at the measured temperature.Table 1-2 shows the saturation temperatures and pres-sures for ammonia.

1.16 To determine if noncondensables are pre-sent, compare the measured pressure against the sat-uration pressure. Then analyze the situation as fol-lows:

• If the pressures are equal, there are no non-condensables in the vapor. The temperatureand pressure are at the saturated condition.

• If the measured pressure is above the saturat-ed pressure, the pressure difference indicatesthat noncondensables are present in theammonia vapor.

• The higher the difference in measured pressureabove the saturated pressure, the greater is thepower requirement to compress the ammoniaand the greater is the loss in capacity.

1.17 Notice that, when noncondensables are pre-sent in the ammonia vapor, the measured pressure isalways higher than the saturation pressure for the par-ticular temperature. However, it is important toremember that not all high condensing pressures arethe result of noncondensables in the system. If themeasured pressure agrees with the saturation table,then the high pressures are caused by a problem otherthan noncondensables. Some possible causes for ahigh condensing pressure include insufficient con-denser capacity, fouled condensing surfaces, clogged

Purging Air and Noncondensables 7

Table 1-1. Operating costs

Excess pressuredue to air (psig) $0.05 kWh $0.07 kWh $0.10 kWh $0.12 kWh

5101520

$8,200$16,400$24,600$32,800

$11,500$23,000$34,500$46,000

$16,500$33,000$49,500$66,000

$19,750$39,500$59,250$79,000

Table 1-2. Ammonia saturation temperatures and pressures

Temp. (°F)

Sat.pressure

(psig) Temp. (°F)

Sat.pressure

(psig) Temp. (°F)

Sat.pressure

(psig)

7273747576777879

118.7121.0123.4125.8128.3130.7133.2135.8

138.3140.9143.6146.3149.0151.7154.5157.3

160.1163.0165.9168.9171.9174.9178.0181.1

8081828384858687

8889909192939495

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spray nozzles, or liquid backing up into the condenserdue to improper condenser drain design.

Removing Noncondensables (Purging)

1.18 Additional costs due to noncondensablescan be avoided in two ways. The first means of con-trol is to minimize the amount of air entering thesystem. The second is to use a purge process thatremoves air and noncondensables from the systemeffectively.

1.19 To minimize air entering the refrigerationsystem, take special care while performing the fol-lowing maintenance procedures:

• adding additional refrigerant from theammonia supplier’s tank truck

• adding oil as required to the system

• draining oil from systems in a vacuum

• changing oil filters

• cleaning strainers

• servicing compressors and components

• operating the system in a vacuum.

By simply being careful to prevent contaminationwhile charging with oil and refrigerant, you willgreatly reduce the chance of introducing noncondens-ables into the system.

1.20 It is surprising how much air enters a refrig-eration system during service maintenance. Air thatenters the system can be evacuated after the servicework is completed, but before introducing ammoniato that area. A second method is to bleed someammonia through the portion of the system that wasopen to flush out the air. This method, although com-mon, does not ensure that all of the air is expelled,and it also wastes ammonia in the process. In addi-

tion, it is not generally recommended because ofsafety considerations.

1.21 The best method by far is to use the methodroutinely practiced by the air-conditioning techni-cian—that is, to connect a vacuum pump to any areathat has been opened for service. The vacuum pumpshould be operated long enough to pull the pressureinto a high vacuum range. This ensures that all airand moisture have been expelled. Note that the vacu-um pump must be made of materials suitable for usewith ammonia systems.

1.22 Years ago it was common to purge air andnoncondensables from ammonia systems by openinga valve on the condenser or receiver. Technicians letthe system vent to the atmosphere for a period of timewhile they watched the condensing pressure. Whenthe pressure returned to its normal range, they closedthe valve. This method was wasteful because itreleased much more ammonia than air. Ammonia wascheap then, and there was little or no concern for theenvironmental impact in comparison with the OSHAand EPA regulations of today.

1.23 Some systems still are purged of the noncon-densables by venting the ammonia vapor from thecondenser and receiver areas. However, the venting isinto water. The water absorbs the ammonia vapor,preventing what would otherwise be considered anammonia release. If enough water is used, all of theammonia is completely absorbed in the water, andonly the air or noncondensables bubble up throughthe water. When bubbles are no longer evident, youcan assume that all of the air has been removed fromthe system. Great care must be taken during this pro-cedure because of safety considerations.

The Programmed Exercises on the next page willtell you how well you understand the material youhave just read. Before starting the exercises,remove the Reveal Key from the back of yourbook. Read the instructions printed on the RevealKey. Follow these instructions as you workthrough the Programmed Exercises.

8 Lesson One

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1-1. Noncondensable gases sys-tem power needs and systemcapacity.

1-2. The main noncondensables in ammoniarefrigeration systems are ,

, and .

1-3. Hydrocarbon noncondensables causedby oil breakdown are most commonwhen there is in the systemalong with temperatures.

1-4. Noncondensables increase condensingpressures because of the principleknown as law.

1-5. Air and noncondensables act as a(n)in the condenser, inhibiting

heat transfer.

1-6. For each 4 psi of additional condensingpressure, system power needsincrease by about percentand capacity decreases by about

percent.

1-7. To determine if noncondensables arepresent, first measure the ofthe liquid refrigerant at the condenser

or in the .

1-8. The best way to minimize the entranceof air into the system is to use a(n)

after a service procedure.

1-1. INCREASE; REDUCE

Ref: 1.01

1-2. AIR; NITROGEN; HYDROGEN

Ref: 1.02

1-3. MOISTURE; HIGH

Ref: 1.06

1-4. DALTON’S

Ref: 1.07

1-5. INSULATOR

Ref: 1.12

1-6. 2; 1

Ref: 1.13

1-7. TEMPERATURE; OUTLET;RECEIVER

Ref: 1.15

1-8. VACUUM PUMP

Ref: 1.21

Programmed Exercises 9

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10 Lesson One

Purge Point Locations

1.24 Because the noncondensables travel to thecondenser and receiver vapor spaces, the places fromwhich they may be purged are all found in theseareas. However, some locations within these areascontain higher accumulations of the noncondensablegases. These locations are the obvious choices forpoints at which to connect purging vents.

1.25 The air is always located on the vapor side ofthe condenser and receiver. It may also be found on thevapor areas of high pressure oil separators, demisters,and heat recovery coils. The best location for the purgeconnection on an evaporative condenser in service is atthe top of the liquid outlet line. This point has the high-est concentration of air because it is a place of lowvelocity and is at the coolest portion of the condenser.Manufacturers of evaporative condensers normally pro-vide purge connections at the top of the liquid outletheader for each circuit to make it easy for customers toconnect their purge units.

1.26 When the condenser is shut down, the air gen-erally rises and is found at the top of the condenser.Purging is most effective from connections at the vaporinlet to the condenser. Manufacturers also supply purgeconnections at the top of the inlet header.

1.27 In shell-and-tube condensers where the vaporrefrigerant is on the shell side and the cooling water

runs through the tubes, the purge connection shouldbe located at the top of the shell away from the hotgas inlet. This location has the lowest velocity and thecoolest sections.

1.28 Receivers must also be fitted with purge con-nections, following the same basic rules of lowestvelocity and coolest areas. For receivers as well ascondensers, the purge connection should be in thevapor portion of the vessel and not in the liquid por-tion.

1.29 Horizontal receivers with the inlet at one endhave a single purge point on the opposite end of thereceiver at the top. Horizontal receivers with the inletat the center have two purge connections, one at eitherend of the receiver, again at the top. Figure 1-3 showspurge connection locations on evaporative condensersand receivers.

Automatic Purging

1.30 In order to minimize the noncondensables inthe refrigeration system at all times, it is necessary touse an automatic purge unit. The automatic purgertakes a small amount of vapor from the purge point. Itruns the vapor through a low-temperature heatexchanger to condense the ammonia from the vapor.The remaining vapor that is not condensed is mainlythe noncondensable gas that was drawn along withthe ammonia vapor. The condensed ammonia is

Purging at inlet is notefficient because gasvelocity is too high toaccumulate air whencompressor is running

Air is concentrated at thecoolest, lowest-velocitylocation in the condenser

Purge point at outletof condenser

Outlet

Outlet

OutletEvaporative condenser

Inlet

Inlet

Receiver

Purge pointconnection

Purge pointconnection

Purge pointconnection

Air collects at cool,low-velocity end ofreceiver

Air collects at cooler,low-velocity areas ofreceiver

Receiver with inlet at one end

Receiver with center inlet

Fig. 1-3. Purge connections

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Purging Air and Noncondensables 11

returned to the refrigeration system, and the noncon-densables are bubbled through water and vented to theatmosphere.

1.31 Most industrial systems are large and typicallyhave several evaporative condensers and often morethan one receiver. Thus, there are many points wherethe purge unit can be connected. Automatic purgeunits are available for either a single purge point ormultiple purge points. Some units can handle up to 24locations.

1.32 A single-point purge unit must not be connect-ed to all of the possible connection points at once. Inthis case, connection to multiple points permits thevapor from only the highest pressure point to flow tothe purge unit. The other connection points will not bepurged. On the other hand, the multiple-point purger isconnected to the numerous purge points on the con-densers and receivers, using solenoid valves at eachlocation. The purge unit opens one of the purge pointsto supply refrigerant vapor to be purged from that area.

1.33 Figure 1-4 shows a typical piping schematicbetween the auto-purger and the refrigeration system.Notice that the solenoid valves are located adjacent tothe purge points and that shortly beyond these loca-tions the purge line may be manifolded to a single 1/2-in. pipe. This simplifies the piping between the multi-ple-point purger and the several locations from whichthe purge is taken.

1.34 A sketch of one manufacturer’s purge unit isshown in Fig. 1-5 on the following page. The line car-

rying the vapor to be purged, called the foul gas line,directs the vapor through a strainer and into the air ornoncondensable separator. Any liquid ammonia in thefoul gas line is caught in a liquid drainer and fed intothe evaporator chamber.

1.35 A 1/2-in. high-pressure ammonia liquid linefeeds ammonia through a metering valve into theevaporator chamber. A 3/4-in. suction line is con-nected to the point of lowest system suction pres-sure. The lower the pressure, the lower is the tem-perature in the evaporator chamber and the more airor noncondensables are trapped and removed fromthe system.

1.36 The foul gas is contained within a spiral tubewrapped around the horizontal air separator cham-ber, and the vapor condenses at the low temperature.The noncondensables remain in vapor form and arereleased through a float switch and bubbled througha water bubbler, which captures any ammoniavapors.

1.37 The purge unit is designed so that operation isautomatic. A control system maintains the liquid levelin the evaporator chamber. A timer in the purge unitdetermines from which purge point the foul gas istaken. The timer periodically changes the purge pointby opening the suitable solenoid valve and keepingthe remaining solenoids closed.

1.38 In normal operation, the purge unit functionscontinuously to prevent the buildup of noncondens-ables in the system. Remember that any noncondens-

SolenoidStrainer

Condenser withfour purge points

Receiver withtwo purge points

Auto-purger is programmed to actuate solenoid valvesin sequence to purge each suspected location of air

Single purge gasline to purger

Auto-purger

Manifold purge gas lines at condenser andreceiver to minimize piping

Fig. 1-4. Multipoint auto-purger schematic

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12 Lesson One

ables will raise the condensing pressure above thenormal saturation condition and will cost additionalcompression power.

1.39 Most industrial refrigeration purge units oper-ate in a similar manner to that just described. Someunits, however, change purge points not by a timer,but rather by the amount of air measured at the purgepoint. In this kind of control, the unit purges a partic-ular location until the measured value of noncondens-ables is below a specified value. Then the unit closesthe solenoid on that purge point and selects the next.It continues to purge the new point until the noncon-

densables are measured below the specified limit. Itcontinues in this manner, going from point to pointand even skipping those locations at which no sub-stantial noncondensables are found.

1.40 Figure 1-6 shows the effectiveness of vari-ous methods and degrees of purging. Line A showsthe worst case. This is for occasional manual purg-ing with a varied time period between purgings. LineB shows the use of a single-point or undersizedautomatic purger, which results in incompleteremoval of noncondensables. Therefore, the systemalways has some level of noncondensables remain-

Gaugevalve

1/2 in. foulgas line

1/2 in. high-pressureliquid line

1/2 in. water line

3/4 in. suction line

Strainer Strainer

Checkvalve

Check valve

Liquid levelcontrol

Overflowtube

Waterbubbler

1 in. drainto sewer

Liquiddrainer

30 psidcheckvalve

1 psidcheckvalve

6

4

5

2

3

1

Reliefcheckvalve

Connection foroptional pressurerelief valve

Orifice

Floatswitch

Meteringvalve

Thermostat

Air separatorchamber

Evaporator chamber

Oil drain andpumpoutOil drain and

pumpout

Liquid levelcontrolsensor

Gaugevalve

Solenoids 1 Liquid line 2 Vent 3 Liquid metering 4 Foul gas 5 Purge gas 6 Water

Fig. 1-5. Auto-purger

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Ele

ctri

city

co

st $

Time

$100,000

$80,000

$60,000

$40,000

$20,000

$10,000$13,000

0

$10,000purgertotal cost

$24,500

1 yr 2 yr 3 yr 4 yr

1000-ton ammoniarefrigeration system

Operates 6500 hr/yr

Electric rate $0.07/kWh

Purger cost $10,000

15 psi ∆P

10 psi ∆P

10 psi ∆P payback = 5.5 mo. First yearsavings after payback = $13,000.

15 psi ∆P payback = 3.5 mo. First yearsavings after payback = $24,500.

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Purging Air and Noncondensables 13

ing and operates at a pressure above the refrigerantsaturation pressure.

1.41 The most efficient use of the automatic purg-er is with sufficient purge points and continuouspurge unit operation. This is indicated by line C,which is also the saturation pressure of the refriger-ant. The system will remain free of noncondensablesand will operate at optimum efficiency as long as thepurge unit is adequately sized, the system is free ofvacuum leaks, and proper service techniques areapplied.

Economics of Purging

1.42 Earlier in this Lesson, you read about thehigh operating costs incurred by not using a purgesystem. Now consider the economics for a typicalsystem. How long will it take to pay back the initialcost of the automatic purge unit? Assume that thecost and installation of a suitable eight-point auto-matic purger for the 1000-ton ammonia system is$10,000.

1.43 From the data in Table 1-1, for a systemthat normally operates 6500 hours a year with 15psig excess pressure causedby noncondensables, thecost of electricity at $0.07kWh is $34,500 per year.This indicates that in lessthan half a year, the purgeunit wil l pay for i tself .Moreover, the additionalsavings accrued for the firstyear wil l be $24,500($34,500 – $10,000 =$24,500). Figure 1-7 showsthe payback period as wellas the savings accrued forthe first four years of opera-tion. This graph shows thesesavings for both the 15-psigdifferential and also for a10-psig differential causedby noncondensables.

1.44 As you can see, pay-back for the automaticpurge unit is typically lessthan a year for almost any

level of noncondensables or almost any electricpower rate. Keep in mind that this is a simple pay-back. There are generally additional electrical coststo consider—for example, time-of-day rates,demand charges, and ratchet costs. All of theseincrease the basic electricity cost and reduce thepayback time, making use of the auto-purger evenmore economical.

Co

nd

ensi

ng

pre

ssu

re

Time

Line C (saturation pressure withauto-purger operating continuously)

Line A (manual purging)

Line B (single-pointor undersized purger

Fig. 1-6. Purge method effectiveness

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Fig. 1-7. Simple payback analysis for auto-purger

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1-9. Air is always located on theside of the condenser and

receiver.

1-10. The best purge points are at theof the vessel in sections

where the is lowest and thevapor is .

1-11. A horizontal receiver with a center inletneeds purge connections,one at each at the

of the receiver.

1-12. The auto-purger operates ,passing ammonia through a(n)

where the ammonia con-denses and leaves the noncondens-ables to be vented.

1-13. Some multiple-point auto-purgers canhandle up to purge points.

1-14. The line that carries the vapor to bepurged is called the line.

1-15. Some purge units work by means ofa(n) , but others work by mea-suring the amount of at thepurge point.

1-16. The payback time for an auto-purger isusually a year.

1-9. VAPOR

Ref: 1.25

1-10. TOP; VELOCITY; COOLEST

Ref: 1.25

1-11. TWO; END; TOP

Ref: 1.29

1-12. CONTINUOUSLY; HEATEXCHANGER

Ref: 1.30

1-13. 24

Ref: 1.31

1-14. FOUL GAS

Ref: 1.34

1-15. TIMER; AIR

Ref: 1.39

1-16. LESS THAN

Ref: 1.44

14 Programmed Exercises

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1-1. Noncondensables caused by ammoniabreakdown are a problem, especially in sys-tems with compressors operating attemperatures .

� a. reciprocating; approaching 300°F� b. reciprocating; below freezing� c. screw; approaching 300°F� d. screw; below freezing

1-2. According to Dalton’s law, the pressure in aclosed vessel filled with three gases is equalto the

� a. average pressure of the gases� b. pressure of the gas with the highest

pressure� c. product of the highest and lowest

pressures� d. sum of the partial pressures

1-3. At a condensing pressure 12 psi above nor-mal, the increased power requirement isabout

� a. 4%� b. 6%� c. 8%� d. 10%

1-4. If the measured condensing pressure is higherthan normal but equals the saturation pressurefor the measured temperature, then noncon-densables

� a. are not the cause of the problem� b. are raising the system pressure� c. are raising the system temperature� d. may or may not be present

1-5. When purging by venting through water, youcan assume that all air is removed when

� a. bubbles no longer appear� b. condensing pressure returns to

normal� c. refrigerant temperature returns to

normal� d. the vacuum pump draws a high

vacuum

1-6. The best purge point for an operating evapo-rative condenser is at the

� a. center of the condenser� b. drain connection� c. top of the liquid outlet line� d. top of the vapor inlet line

1-7. The ammonia drawn from the system by anauto-purger is condensed and

� a. bubbled through water� b. discharged to a purge receiver� c. returned to the refrigeration system� d. vented to atmosphere

1-8. The timer in a typical auto-purger changesthe purge point by

� a. opening and closing solenoid valves� b. repositioning three-way valves� c. responding to the changing liquid

level� d. sensing the amount of air in the

section

1-9. An undersized multiple-point auto-purger

� a. always leaves some nonconden-sables in the system

� b. is much more effective than a single-point auto-purger

� c. provides no benefits over manual purging

� d. removes all noncondensables insystems operating above freezing

1-10. The savings resulting from installation of anauto-purger increase as the

� a. electrical rates decrease� b. pressure due to noncondensables

increases� c. saturation pressure increases� d. system operating temperatures

decrease

Self-Check Quiz 15

Answer the following questions by marking an “X”in the box next to the best answer.

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COPYAnswers to Self-Check Quiz

1-1. a. Reciprocating; approaching 300°F. Ref: 1.04

1-2. d. Sum of the partial pressures.Ref: 1.07, 1.08

1-3. b. 6%. Ref: 1.13

1-4. a. Are not the cause of the problem. Ref: 1.17

1-5. a. Bubbles no longer appear.Ref: 1.23

1-6. c. Top of the liquid outlet line.Ref: 1.25

1-7. c. Returned to the refrigeration system.Ref: 1.30

1-8. a. Opening and closing solenoid valves.Ref: 1.37

1-9. a. Always leaves some nonconden-sables in the system.Ref: 1.40, Fig. 1-6

1-10. b. Pressure due to noncondensables increases. Ref: 1.43, Fig. 1-7

Contributions from the following source are appreciated:

Figure 1-5. Hansen Technologies Corporation

Purging is the process that removes air and othernoncondensables from refrigeration systems. Air,nitrogen, and hydrogen are common in ammoniasystems, especially those with high dischargetemperatures. Noncondensable gases accumulatein the condenser and receiver. They raise the con-densing pressure, because each noncondensablegas adds its own pressure to that of ammonia inthe same space. This principle is known as Dal-ton’s law. The higher pressure makes the com-pressor work harder, raising electrical costs andreducing efficiency. The power penalty is about2% increased power for each increase of 4 psi,with decreased capacity of about 1%. The powerpenalty causes surprisingly high avoidable extraannual costs.

To determine if noncondensables are present inthe system, measure the temperature of the liq-uid refrigerant leaving the condenser or in thereceiver and compare the condenser or receiverpressure to the saturation pressure at the mea-sured temperature. Noncondensables are notpresent if the pressures are equal, but are pre-sent if the measured pressure is higher than thesaturated pressure. The greater the difference,the greater the extra power required by the com-pressor.

The main ways to minimize the power penalty areto reduce the entrance of noncondensables andto purge them from the system. Pulling a vacuumafter repairs or service is very helpful, as is instal-lation of a correctly sized purge unit. Sometimesthe system is purged by venting ammonia vaporthrough water, which dissolves the ammonia andreleases the noncondensables. Purge pointswhere noncondensables are most likely to collectare those locations with the lowest velocity andthe coldest temperatures.

Automatic purgers (auto-purgers) are available assingle-point or multiple-point units. Single-pointunits must be connected to only one purge point ata time, proceeding throughout the system. Multi-ple-point units can be connected to as many as 24purge points. Noncondensables are trapped andremoved from the system, and any liquid ammoniacaught in the foul gas line is fed to the evaporator.Some purge units work by means of a timer, andothers work by measuring the amount of air ateach purge point. Purge points are opened by sole-noid valves regardless of the purge method. Instal-lation of an auto-purger can remove the noncon-densables very efficiently and economically, withunit payback generally in less than a year and con-siderable additional savings as well.

16 Lesson One

SUMMARY

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