Deshielo Evaporadores Industriales Aug2009

8/13/2019 Deshielo Evaporadores Industriales Aug2009

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3 0 A S H R A E J o u r n a l a s h r a e . o r g A u g u s t 2 0 0 9

By Douglas T. Reindl, Ph.D, P.E., Fellow ASHRAE; and Todd B. Jekel, Ph.D., P.E., Member ASHRAE

The accumulation of frost on forced-circulation air coolers 1 or air-coolingevaporators leads to a continual decreasein cooling capability; thereby, requiringthe periodic removal of accumulated frostto avoid a complete loss of refrigerationcapacity. The removal of frost from an

evaporator is accomplished through the

use of a defrost process. There are anumber of alternative means availablefor defrosting coils including: electric,off-cycle, secondary uid, water, hot-gas,and continuous defrost through the useof sprayed liquid desiccants. With theexception of the liquid desiccant option,

all of these defrost strategies require in-

terrupting the coils normal cooling modeoperation to allow warming of its surfacesto melt accumulated frost.

Electric defrost uses resistance heatingelements interlaced throughout the coilto warm the coil surfaces sufciently tomelt accumulated frost. For evaporatorsoperating in spaces with air temperaturesabove freezing (e.g., a cooler or dock areamaintained at 38F [3.3C]), an off-cycledefrost can be accomplished by shutting

off the refrigerant feed for an extendedperiod of time while continuing to oper-ate the fans. The heat from the relativelywarmer room air heat melts the accumu-lated frost on the unit. A secondary uiddefrost relies on the use of a separate uidcircuit within the evaporator. In this case,

About the AuthorsDouglas T. Reindl, Ph.D., P.E., is a professorand director and Todd B. Jekel, Ph.D., P.E., isassistant director at the University of Wisconsin-Madisons Industrial Refrigeration Consortium in

Madison, Wis.

T his article discusses techniques for removing accumulated frost on air-cooling evaporators in industrial refrigeration applications. Although wereview alternative approaches to defrosting coils, our primary focus is on the use

of hot-gas for defrost, including valve group arrangements and their sequences

of operation. Due to past incidents, particular emphasis is placed on valve group

designs that offer enhanced plant safety. The article concludes with a discussion

of the parasitic energy effects associated with the defrost process with an eye

toward using this information to enhance the energy performance of defrosting.

DEFROSTING Industrial Ref rigeration Evaporators

This article was published in ASHRAE Journal, August 2009. Copyright 2009 American Society of Heating, Refrigerating and Air-ConditioningEngineers, Inc. Posted at www.ashrae.org. This article may not be copied and/or distributed electronically or in paper form without permissionof ASHRAE. For more information about ASHRAE Journal, visit www.ashrae.org.


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plants, some practitioners have exploredalternative methods to determine whena particular unit requires defrost includ-ing: timers that accumulate liquid feedsolenoid open time, frost sensors, air

pressure drop sensors, and others. Theaccumulated liquid feed solenoid opentime can be effective since it is somewhatadaptive to the coils load (sensible andlatent). The other sensors mentioned pre-viously have not proven suitably robust tond signicant penetration in industrialapplications. Once it has been determinedthat a coil requires defrosting, a controlsequence is triggered to initiate and com-plete several steps in a defrost sequence.

The following individual steps are typi-cal of the sequences used for defrostingforced air circulation evaporators.

Step 1: Pump-Out The pump-out period is used to prepare the coil for receiving

hot-gas. The purpose of the pump-out period is to evaporate asmuch of the residual cold liquid refrigerant contained within

Figure 1: Valve positi ons and fan operation dur ing pump-out for a typical li quid overf ed coil.

[Closed] BleedSolenoid

Hand Valve

Plot PressureRegulator

Suction Stop Valve [Open]

Defrost ReliefRegulator

Suction Stop PilotSolenoid [Closed]

Wet Suction Return

Liquid Feed Solenoid

Mode Valve(s) Position

PumpOut

Suction Stop Valve

Suction StopPilot SolenoidBleed Solenoid

Liquid FeedSolenoid

Soft-Gas Solenoid

Hot-Gas Solenoid

Open

Closed

Closed

Closed

Closed

Closed

[Evaporator Fans On ]

Soft-Gas Solenoid[Closed]

Regulated Hot Gas

Defrost Return (Medium Pressure)

Defrost Condensate

Recirculated Liquid/ Vapor Return

Recirculated Liquid Supply

Defrost Hot-Gas Supply

E v a p o r

a t o r F a

n s

[ O n ]

[Closed]

PumpedLiquid Supply

Hot-Gas Solenoid[Closed]

P a n

the coil as possible prior to supplying hot-gas to the coil. Byremoving residual liquid refrigerant, the hot-gas will morequickly and effectively warm the coil to melt accumulated frost.


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Figure 2: Coi l capacity decrease durin g pump-out. 6

E v a p o r a t o r

C a p a c

i t y ( t o n

)

30

25

20

05

10

5

0 0 2 4 6 8 10 12 14 16 18 20Pump-Out Dwell Time (min)

The pump-out period begins by de-energizing (closing) the evaporatorsliquid feed solenoid valve while thesuction stop valve remains open, and theunits fans operate as shown in Figure

1 . Heat from the fan motors and room(or product) causes the residual liquidrefrigerant within the coil to evaporatewith the refrigerant vapor returning tothe engine room via the wet suctionreturn (also referred to as recirculatedsuction).

The amount of time scheduled forpump-out varies from an extremelyshort duration, more typical for gravityooded recirculation and direct-expan-sion unit designs (zero to ve minutes),to a longer period for liquid overfed unitdesigns (10 to 15 minutes 2). A shortpump-out period for a gravity ooded

design requires a short pump-out period because its normalliquid refrigerant inventory within the unit during coolingmode operation is low. Liquid overfed coil designs requirea longer pump-out period due to a combination of effects.

evaporator is made possible because the low refrigerant-sidepressure drop of the coil allows any residual liquid refrigerant(and liquid condensate) to be readily cleared when hot-gasis supplied to the coil for defrost. The direct-expansion coil


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A u g u s t 2 0 0 9 A S H R A E J o u r n a l 3 5

Figure 3: Valve positions and fan operation duri ng soft-gas peri od for typical li quid overfed coil.


Hand Valve


Suction Stop Valve [Closed]


Suction Stop PilotSolenoid [Open]

Wet Suction Return



Soft-Gas

Suction Stop Valve


Liquid FeedSolenoid

Soft-Gas Solenoid

Hot-Gas Solenoid

Closed

Closed

Open

Closed

Open

Closed

[Evaporator Fans Off ]

Soft-Gas Solenoid[Open]

Regulated Hot Gas


Defrost Condensate




E v a p o r

a t o r F a

n s

[ O f f ]

[Closed]

PumpedLiquid Supply


P a n

First, the liquid refrigerant inventorywithin the coil is higher compared toa direct-expansion evaporator. Second,the refrigerant-side coil pressure dropis relatively high due to the presence

of button orices located within eachcircuit on the refrigerant feed-side of thecoil (typical for mechanically pumpedoverfed designs).

Because a longer pump-out period isrequired for overfed coil designs, it isnatural to ask how long of a pump-outperiod is suff icient? The pump-outperiod should be long enough to evapo-rate the majority of residual liquid inthe coil but not too long that parasiticheat load effects to the space becomesignicant. The parasitic heat load ef-fects during pump-out arise because thesupply of liquid refrigerant to the coilhas been interrupted; the evaporatorsfans continue to run; it is heat fromthe fans that are a parasitic space load.In addition, longer pump-out periodsextend the time the unit is unavailableto meet space loads.

Aljuwayhel, et al., 3 reported exten-sive data collected on a eld-installedevaporator unit located in a penthousefor a low temperature holding freezer.

The coil in this particular unit has arated capacity of 37 tons (130 kW t) withve fans that deliver 60,000 cfm (102000 m 3/h) of air during cooling modeoperation, but that result in approxi-mately 5 tons (17.6 kW t) of parasiticheat load during fan operation. Datawere collected on the units refrigera-tion capacity during the pump-out pe-riod and the units decrease in capacityover ve separate pump-out cycles isshown in Figure 2 . At the end of the 20

minute pump-out period, the coils capacity has decreasedto a level approaching a break-even capacity to just meetthe fan heat gain.

A pump-out period longer than 20 minutes is usually notrequired. Shorter pump-out periods should be validated byobserving the frost melt pattern on the coil during the hot-gassupply period of the defrost sequence. Assuming the coil istop-fed with hot-gas (typical), an adequate pump-out period islikely established when the bottom rows of the coil completelyrelease their frost during the hot-gas dwell period and when noaudible effects of hydraulic hammering are observed on the coiland its connected piping during the early part of the hot-gas

supply period.

Step 2: Soft-Gas The use of a soft-gas step in the defrost sequence is recom-

mended for evaporator coils with 15 tons (53 kW t) of capacityor greater. 2,4,5 The soft-gas period of the defrost sequencebegins by shutting off the evaporator fans and energizing thepilot solenoid for the suction stop valve. The pilot solenoidapplies hot-gas pressure to the top of the suction stop valvespiston, forcing this normally open valve closed.

With the coil now isolated from the systems suction pressure,a small ported (e.g., 0.5 in. [13 mm]) soft-gas solenoid valve isopened to allow a low ow rate of hot-gas into the coilusu-ally after owing rst through the drain pan warming circuit;

slowly raising the pressure of refrigerant in the coil. The soft-

Figure 4: Valve positions and fan operation dur ing hot-gas peri od for typical liquid overfed coil.


Hand Valve





Wet Suction ReturnLiquid Feed Solenoid


Hot Gas

Suction Stop ValveSuction StopPilot Solenoid

Bleed SolenoidLiquid Feed

SolenoidSoft-Gas Solenoid

Hot-Gas Solenoid

Closed

Open

Closed

Closed

Open

Closed



Regulated Hot Gas

Defrost Return ( Medium Pressure)

Defrost Condensate




E v a p o r

a t o r F a

n s

[ O f f ]

[Closed]

PumpedLiquid Supply

Hot-Gas Solenoid[Open]

P a n


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gas cycle is intended to reduce the risk ofhydraulic hammer that can occur on thecoil or connected piping by reducing thepressure difference between the coil andthe hot-gas main. The reduced pressure

difference will decrease the rapid in-rushof hot-gas when the larger main hot-gassolenoid opens. Briley 5 recommends siz-ing the soft-gas solenoid at 20% to 25%of the main hot-gas solenoid valve.

Figure 3 shows the valve positions andthe evaporator fan state during the soft-gas period. The the soft gas dwell time isgenerally set to last for a period rangingfrom ve to 10 minutes. 4 Soft-gas dwellperiods up to 20 minutes may be requiredfor larger liquid overfed evaporators or inapplications having large operating pres-sure differences between the hot-gas mainand the evaporator. The soft-gas dwelltime period should be field-adjustedto raise the evaporator pressure to ap-proximately 35 to 40 psig (2.4 to 2.8bar) before moving to the next mode inthe sequence of defrost operation. Notall evaporators have a soft-gas solenoid.While it is benecial for all evaporators,it is more common on larger capacity,low-temperature evaporators.

Step 3: Hot-Gas Thus far, the individual segments of

the defrost sequence have focused onpreparing the coil to receive hot-gas to melt the accumulated frost. In thisportion of the defrost sequence, thelarger hot-gas solenoid opens to deliverhot-gas rst through the coils drain panand then the evaporator coil, as shownin Figure 4 . During the hot-gas supplyperiod, the smaller soft-gas solenoid caneither remain open or closed since the

ally 70 to 90 psig (4.8 to 6.2 barg) (equivalent to a saturationtemperature of 47F to 58F [8C to 14C] for ammonia). Thedefrost relief regulator will modulate to maintain the evapora-tor at the regulators pressure setting and it will fully reseat atthe conclusion of the hot-gas dwell period. A check valve isrequired on the outlet of the defrost relief regulator when thedefrost condensate return is piped to a suction pressure higherthan the evaporators normal operating pressure.

How long should the hot-gas supply period be set? The dwellperiod of the hot-gas supply must be sufcient to allow all theaccumulated frost on the coil to melt but not excessive to avoidcreating a parasitic heat load external (to the space) and internal (to

the refrigeration system) by returning uncondensed hot-gas back to

majority of gas ow will occur through the main hot-gas valve.As high-pressure superheated refrigerant vapor ows rstthrough the piping in the drain pan circuit and then into the coil,the high-pressure vapor condenses as it gives up its latent heatto warm both the drain pan and the evaporator coil surfaces.A warm drain pan will help prevent re-freezing of the waterdraining from the coil to the pan. As the coil surfaces warm, theaccumulated layer of frost will begin to meltowing by grav-ity down the coil and into the drain pan before leaving the unitthrough a defrost condensate drain line. The condensed liquidrefrigerant is directed from the coil to a lower pressure level inthe plant through a defrost relief regulating valve. The defrost

relief regulator is factory set at a user-specied pressureusu-

Figure 6: Valve positions and fan operation during the bleed period for a typical liquid overfed coil.


[Open] BleedSolenoid

Hand Valve






Bleed

Suction Stop Valve

Suction StopPilot Solenoid

Bleed SolenoidLiquid Feed

SolenoidSoft-Gas Solenoid

Hot-Gas Solenoid

Closed

Closed

Closed

Closed

Open

Open Wet Suction ReturnLiquid Feed Solenoid

PumpedLiquid Supply




Regulated Hot Gas


Defrost Condensate



E v a p o r

a t o r F a n

s

[ O f f ]

[Closed]

P a n

Fi gure 5: M easured and predicted average penthouse air temperatur es dur ing hot-gasdefrost and bleed peri ods. 6

40

35

30

25

20

15

10

5

0

5

10

15

20

25

30

35

40

P e n t

h o u s e

A i r T e m p e r a

t u r e

( C )

0 5 10 15 20 25 30 35 40 45 50 55 60Time (min)

Hot-Gas Dwell = 40 min Bleed10 min

Cooling Interval 24 Hours

No Frost (Experiment Data 6.5 min)No Frost (Model Prediction 6.0 min)Run #2 24hRun #3 24h

Cooling Interval 48 Hours

No Frost (Experiment Data 10.5 min)No Frost (Model Prediction 10.8 min)Run #4 48hRun #5 48h


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hot-gas solenoid valve (and soft-gas solenoid if open) is closedand a small bleed solenoid valve opens to slowly depressurizethe coil by relieving the pressure in the coil back to suction. Thebleed solenoid valve is typically three to four sizes smaller thanthe main suction stop valve but not less than 0.5 in. (13 mm). 7 An optional hand valve in the bleed line can be used to eldadjust the rate of coil depressurization as shown in Figure 6 .

The bleed period is necessary, particularly on large coils(with coil volumes greater than 8 ft 3 (0.23 m 3) or suction pip-ing greater than 2 in. (65 mm), 2 to prevent what would be avery rapid depressurization of the coil when the suction stop

valve opens. Rapid coil depressurization increases the potential

for hydraulic hammering to the coil and the connected suctionpiping. The bleed period also prevents rapid swings in suctionpressure and compressor loading that would normally result asthe engine room responds to maintain a constant suction pres-sure. The duration of the bleed period is installation-dependentand should be adjusted so no audible hammering occurs and thetime is sufcient to decrease the coil pressure to within 5 to 10psid (0.3 to 0.6 bar) of the normal cooling mode evaporator pres-sure. 4 Generally, the bleed period will last ve to 10 minutes.

At the conclusion of the bleed period, the suction stop pilotsolenoid is de-energized allowing the main valve to open. As

congured in the evaporator schematics, the pilot pressure

suction through the defrost relief regulator.Aljuwayhel 6 collected data on a penthouse-mounted evaporator during both coolingmode and defrost mode of operation. Forthe evaporator defrost control as-found, the

hot-gas dwell period was 40 minutes.Figure 5 shows model-predicted andeld-measured average air temperatureswithin the penthouse during the hot-gas and subsequent bleed periods of thedefrost sequence for two cases. The rstcase allowed the evaporator to operatefor 24 hours before initiating a defrostcycle. Once hot gas owed to the coil,all the frost had melted in a period ofless than seven minutes. The second caseallowed the evaporator to operate for 48hours before initiating a defrost cycle.In this situation, the coil was completelycleared of accumulated frost in less than11 minutes during the hot-gas supply.

This suggested that a 40 minute hot-gasdwell period was excessive.

Within 15 minutes of the main hot-gasvalve opening, the average penthouse airtemperature reached a balmy 68F (20C)and that temperature was maintained for25 of the 40 minutes, which suggests thatthe continued supply of hot-gas to the coilwas not resulting in the full condensing ofthe refrigerant vapor. Rather, a signicantportion of the hot-gas was owing back tosuction and creating a parasitic load (in-ternal) on the compressors. The parasiticeffect of excessive hot-gas dwell periodspresents an opportunity for improving thesystems energy efciency by simply re-ducing the scheduled hot-gas dwell period.

Step 4: BleedAt the conclusion of the hot-gas dwell

period, a bleed or equalize sequence is

initiated. During the bleed period, the

Fi gure 8: An il lu strat ion of the ti me-dependent energy ows for cool ing mode and defr ostmode of operation (note: th is graphic is not to scale in either capacity or time). 9

Coil InitialCondition(No Frost)

Coil CapacityDecreases As FrostContinues to Form

Coil Capacity Drops Rapidly as RefrigerantFlow is Stopped and the Pump Out ProcessProceeds, Preparing the Coil for Defrost

Parasitic Energy is Attributed to Warming the Coil Mass and Both Sensible and Latent

Losses to the Space

Hot-Gas Defrost Terminates and Coil Begins to Cool Down

Coil Transitions from aTemperature Warmer Than theSpace to a Temperature Cooler

Than the Space, So UsefulRefrigeration is Now Restored

Time

C

B

A

E v a p o r a t o r

C a p a c

i t y

D

Figure 7: Valve positi ons and fan operation duri ng re-chill peri od for typical l iquid overfed coil.

[Open] BleedSolenoid

Hand Valve


Suction Stop Valve [Open]


Suction Stop PilotSolenoid [Closed]

Wet Suction Return



Re-Chill

Suction Stop Valve


Liquid FeedSolenoid

Soft-Gas Solenoid

Hot-Gas Solenoid

Open

Closed

Closed

Open

Closed

Open



Regulated Hot Gas


Defrost Condensate




E v a p o r

a t o r F a

n s

[ O f f ]

[Open]

PumpedLiquid Supply


P a n


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A u g u s t 2 0 0 9 A S H R A E J o u r n a l 3 9

regulator located in a branch line taken from thesuction side of the coil will hold the main suctionstop valve for the coil closed until the set pres-sure of the pilot regulator is reached. This pilotregulator should be set to a pressure difference no

greater than 10 psid (0.6 bar). The addition of thisvalve (and other valve designs that provide simi-lar function) is a critical safety measure to avoidhydraulic hammer that is likely to occur from arapid opening of the suction stop valve when thecoil is under pressure. It is important to note thatif the bleed period is too short, the coil pressurewill remain high and the suction stop valve willcontinue to be held closed by the pilot pressureregulator bleeding pressure from the coil to the topof the suction stop valves piston. If the suctionstop valve does not open, it becomes impossibleto prepare the coil for re-chilling.

At rst glance, it appears that this regulator is redundantsince the bleed solenoid provides the slow depressurization ofthe coil to within 10 psid (0.6 bar) or less of normal evaporatorpressure. This is true under normal circumstances; however, therapid opening of the suction stop valve will occur if the coil isin the hot-gas dwell period and a power outage occurs causingall solenoids to go to their normal positions. In this situation,the suction stop pilot solenoid (which is holding the suction stopvalve closed by pressurizing the top of the valves piston) willclose; allowing the suction stop valve to rapidly open as it returnsto its normal position. The net result is an increased likelihoodof hydraulic hammering with the risk of failure of the evaporatoror connected piping.

Step 5: Re-ChillOnce the coil is depressurized and the suction stop valve open,

the unit is ready to return to refrigeration mode. In the re-chillmode, the liquid feed solenoid is opened to allow cold liquidrefrigerant to ow into the coil. Early in the re-chill period, thecold liquid supply will more rapidly evaporate as it absorbs heatfrom the coil mass as it reduces the coil temperature. The fanson the unit will usually remain off. Some plants short-cycle (i.e.,bump) the fans on and off to allow any remaining water on theexternal surfaces of the coil to re-freeze while preventing the

carryover of liquid water into the space that would normallyoccur if the fans were allowed to run at their full ow. Figure7 shows the valve positions during the re-chill period, whichgenerally lasts three to ve minutes.

Now that we have discussed the sequences of operation as-sociated with initiating defrost of an air-cooling evaporator, letslook at the energy consequences of this process.

Energy Impacts and Net Cooling OptimizationAs discussed in the article on coil frosting, 8 the accumulation

of frost on a coil progressively decreases its cooling capacity;necessitating a defrost cycle. The defrost cycle is a source of

efciency loss to the system but necessary to restore the coils

Figure 9: Net cooling optimization results. 6

100

98

96

9492

90

88

86

84

82

80

O v e r a

l l S y s t e m

E f c i e n c y

( % )

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09

0 25 50 75 100 125 150 175 200 225 250 375 300 325 350Total Mass of Condensed Water (kg)

Defrost Number

Maximum System Efciency

RH = 80%

RH = 85%

RH = 90%

capacity by removing the accumulated frost. This fact raises thequestion: What is the appropriate balance between tolerating thecapacity loss for accumulated frost and the parasitic load effectsattributable to the defrost cycle? Figure 8 is an illustration ofthe time-dependent energy ows associated with the operationof a forced air circulation evaporator for both cooling mode anddefrost mode operation. The operation of the coil from Point Ato B is reective of the diminishing cooling capacity of the unitdue to frosting during normal cooling mode operation. At PointB the pump-out period begins, and the units capacity dropsrapidly as the coil is starved and the residual refrigerant withinthe coil is removed by evaporation. Following the pump-outperiod, the coils capacity actually becomes negative (it is heat-ing rather than cooling) as hot-gas is supplied to warm the coiland melt accumulated frost. After the hot-gas ow is terminated(Point C), the coil will gradually cool down during re-chill untilit reaches the point at which it can begin normal cooling modeoperation (Point D).

The concept of net cooling optimization introduced by Alju-wayhel aims to maximize the integrated heat removal capabilityof the evaporator during an entire operational cycle: coolingmode to defrost and back to cooling mode. This integrated heatremoval capacity is represented by the blue shaded region inFigure 8 . A part of maximizing the heat removal capability of

an evaporator involves minimizing the parasitic effects of thedefrost sequence. The red hatched area above the operatingcapacity line represents the integrated cooling decit belowthe coils rated capacity due to both frost accumulation andthat the coil is unavailable during the defrost sequence. Thered shaded portion of the illustration below the line of zero coilcapacity represents the parasitic effects of the coil heating thespace during the hot-gas dwell period. Aljuwayhel 6 exploredthe prospect of optimizing the entire cooling and defrost modeoperation, i.e., maximizing the blue-shaded portion under thecooling curve shown in Figure 8 .

To nondimensionally characterize the frost loading of a coil,

Aljuwayhel dened a dimensionless defrost number as:


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4 0 A S H R A E J o u r n a l A u g u s t 2 0 0 9


V condensate

Amin Ld []Defrost number =

(1)

where V condensate (ft 3 or m 3) represents the volume of water con-densate produced at the conclusion of a defrost cycle, Amin (ft 2 orm2) represents the minimum area available for air to ow through

the coil (coil face area minus the n face area and the tube projectedarea of all circuits for a single row) and Ld (ft or m) representsthe depth of the coil in the direction of airow. Aljuwayhel foundthat a defrost number of 0.03 yielded a maximum in net coolingcapacity. Figure 9 shows the net cooling optimization results usingoverall system efciency as a gure of merit over a range of spacelatent loads represented by the three separate curves indicating thespace relative humidity (RH) ranging from 80% to 90%.

Aljuwayhel denes the overall system efciency as the ratio ofthe actual integrated evaporator coil cooling capacity to the ideal cooling capacity during an entire operational cycle. The actualintegrated evaporator cooling capacity includes the performancedegrading effects of frost accumulation, as well as the defrostprocess. The ideal cooling capacity assumes that the coils cleancooling capacity is maintained during the entire cycle. Aljuway-hel found that the defrost number was a useful gure-of-meritbecause it scales the volume of water condensate a coil producedduring defrost to the volume of frost the coil is capable of hold-ing. The nding of net cooling optimization for a defrost numberof 0.03 translates to a coil accumulating approximately 3% of a

representative volume before initiating a defrost sequence. As anexample, consider a coil with a face area of 45 ft 2 (4.18 m 2), threens per inch (one n per 1.1 cm), 7/8 in. (22 mm) OD tubes in therst row, and a coil depth of 30 in. (0.76 m). A defrost number of0.03 results in approximately 23 gallons (88 l) of water drained

from the coil. Interestingly, the defrost number was found to beindependent of the coils latent load as shown in Figure 9 .

ConclusionsIn this article, we review the basic sequences of operation for

defrosting forced-air cooling evaporators. The most common defrostsequence involves ve steps including: pump-out , soft-gas , hot-gas ,bleed , and re-chill modes. Some of these steps may be omitted fromdefrost sequences based on the coils refrigerant feed congurationor size. A key consideration in eld-tuning defrost sequence timesettings is obtaining an effective defrost without audible hammeringof the coil or its connected piping. We also introduced some keyfeatures relating to the function of the suction stop valve to preventits rapid opening when there is greater than a 10 psid (or lower)(0.6 bar) pressure difference between the evaporator and suction.

There is an opportunity to improve the energy performance ofmany defrosting evaporators. One of the easiest adjustments toconsider for improving the efciency of the defrost process is theadjustment of the hot-gas dwell period. Coils with hot-gas dwellperiods in excess of 15 minutes may be candidates for efciencyimprovement by decreasing the hot-gas dwell period. The conceptof net cooling optimization is introduced. Net cooling optimizationaims to maximize the time-dependent heat extraction capabilityof an air-cooling evaporator during both cooling mode operationand defrost. Aljuwayhel dened a defrost number as an appropri-ate metric for optimizing the combined cooling mode and defrostmode operation of an evaporator. A defrost number of 0.03 yieldedoptimum performanceindependent of the coils latent load.

References1. 2006 ASHRAE HandbookRefr igerat ion, Chapter 42.2. IIAR. 1992. Bulletin 116 Guidelines for: Avoiding Component Fail -

ure in Industr ial Refr igeration System Caused by Abnormal Pressure orShock , International Institute of Ammonia Refrigeration, Arlington, Va.

3. Aljuwayhel, N.F., D.T. Reindl, S.A. Klein, G.F. Nellis. 2008.Experimental investigation of the performance of industrial evapora-tor coils operating under frosting conditions. International Journalof Refr igeration 31(1):98 106.

4. IIAR. 2000. Ammonia Refr igeration Piping Handbook. Arlington,Va.: International Institute of Ammonia Refrigeration.

5. Briley, G.C. 2004. Optimizing defrost systems, part 3. ProcessCooli ng and Equipment (1).

6. Aljuwayhel, N.F. 2006. Numerical and Experimental Study of theInuence of Frost Formation and Defrosting on the Performance of Indus-trial Evaporator Coils, Ph.D. Thesis, University of Wisconsin-Madison.

7. Hansen. 2006. Collection of Instructions. Burr Ridge, Ill.:Hansen Technologies Coporation. p. 78.

8. Reindl, D.T. and T.B. Jekel. 2009. Frost on air-cooling evapora-tors. ASHRAE Journal 51(2):27 33.

9. Aljuwayhel, N.F. 2006. Optimizing Air-Cooling Evaporators.Presented at the IRC Research and Technology Forum, Madison, Wis.


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Deshielo Evaporadores Industriales Aug2009