defrosting ind. refrigeration evaporators

30 A SHRAE Jou rna l ash rae .o rg 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 coolers1 or air-cooling evaporators leads to a continual decrease in cooling capability; thereby, requiring the periodic removal of accumulated frost to avoid a complete loss of refrigeration capacity. The removal of frost from an evaporator is accomplished through the

use of a defrost process. There are a number of alternative means available for defrosting coils including: electric, off-cycle, secondary fluid, water, hot-gas, and continuous defrost through the use of sprayed liquid desiccants. With the exception of the liquid desiccant option, all of these defrost strategies require in-

terrupting the coils normal cooling mode operation to allow warming of its surfaces to melt accumulated frost.

Electric defrost uses resistance heating elements interlaced throughout the coil to warm the coil surfaces sufficiently to melt accumulated frost. For evaporators operating in spaces with air temperatures above freezing (e.g., a cooler or dock area maintained at 38F [3.3C]), an off-cycle defrost can be accomplished by shutting off the refrigerant feed for an extended period of time while continuing to oper-ate the fans. The heat from the relatively warmer room air heat melts the accumu-lated frost on the unit. A secondary fluid defrost relies on the use of a separate fluid circuit within the evaporator. In this case,

About the AuthorsDouglas T. Reindl, Ph.D., P.E., is a professor and director and Todd B. Jekel, Ph.D., P.E., is assistant 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 we review 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 Refrigeration Evaporators

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

Augus t 2009 ASHRAE Jou rna l 31

a warm secondary fluid is circulated through the defrost coil to raise the evaporators surface temperature and melt accumulated frost. Water can also be used for defrosting evaporators. With water defrost, the refrigerant feed to the coil is interrupted and water is sprayed directly on the external surfaces of the coil to melt the frost.

A hot-gas defrost process redirects a portion of the high pres-sure discharge gas from the outlet of high stage compressors to the evaporator and a heating circuit embedded in its defrost condensate drain pan. As the high pressure gas flows to the unit, it desuperheats and condenses giving up both sensible and latent heat of condensation as it warms the surfaces of the evaporator and the drain pan. The warm evaporator coil causes the accumulated frost to melt and the warm drain pan permits the water to drain out of the unit without refreezing. The liquid refrigerant condensed during the defrost process is returned to a protected lower suction pressure through a re-seating pres-sure relief regulator. This pressure of the regulator is set in the range of 70 to 90 psig (4.8 to 6.2 bar), which corresponds to a

refrigerant saturation (condensing) temperature of 47F to 58F (8C to 14C). For industrial refrigeration systems, hot-gas is the most widely used technique for defrost.

Although there are other defrost techniques such as the use of a warm liquid refrigerant, these do not find widespread use in industrial systems so their coverage is not included here. Ad-vantages and disadvantages of the above-mentioned industrial refrigeration system defrost strategies are highlighted in Table 1.

Because of its widespread use in industrial refrigeration systems, our focus in this article is on the use of hot-gas for coil defrosting. Lets first look at the steps involved in a typical defrost sequence. Then, we explore energy considerations as-sociated with the entire cooling and defrost processes.

Defrost Sequence of ControlDue to its simplicity, a time clock is the most common method

used to initiate and terminate the defrosting of individual units. With a time clock, a defrost sequence is initiated a prescribed fixed intervals in time. In attempts to improve the efficiency of

Defrost Approach

Applications Advantages Disadvantages

Hot-Gas

Widely used in most industrial and some

commercial refrigeration systems (direct refrigerant).

Able to achieve effective defrost.

Uses lower grade of energy (waste heat from the refrigeration system).

Can be effective at scavenging and returning oil that may have

accumulated in an evaporator.

Increased safety risks due to hydraulic ham-mering from condensation-induced shock and vapor-propelled liquid slugs if defrost sequences are not properly managed and proper piping practices not implemented.

Extremely high working pressures required for some refrigerants such as CO2.

Can lead to increased parasitic energy con-sumption with improper valve group design and poorly adjusted defrost sequence times.

Electric

Used in some commercial refrigeration systems and in

industrial refrigeration systems where CO2 is used as a cascade

refrigerant or secondary loop phase change fluid.

Decreased risk of damage from events such as hydraulic hammer.

Minimizes parasitic load.

Avoid extreme refrigerant-side pressure (CO2 refrigerants).

Poor use of high grade primary energy (electricity).

High maintenance due to frequent failure of resistance heating elements.

Not effective at removing oil accumulation from evaporators.

Off-Cycle

Used in industrial and commercial refrigeration systems

for spaces operating above freezing point (typically >38F

[3.3C]).

Efficient means of defrost.

Simple implementation.

Inherently safe.

Lower capital and maintenance costs.

Not relevant in applications where space temperatures are below freezing.


Water

Found in some lower-temperature refrigeration systems. This form

of defrost can also be inte-grated into the normal sanitation

operations.

Applies heat directly to the accumulated frost.

The defrost process can be integrated into a normal sanitation cycle.

Able to achieve fast defrost.

Difficult to apply for defrost on the fly during operation for low temperature

applications.


Increases plant water use.

Secondary Fluid (Indirect)

An alternative to electric defrost in CO2 cascade and secondary

phase change systems.

Efficient means of defrost.

Conceptually simple.

Avoids risks of hydraulic hammering on refrigerant-side of coil.

Additional secondary fluid system and circuiting, which makes the coil larger,

heavier, and more costly.


Secondary fluid circuit in the coil can fail (freeze) if the secondary fluid concentration

is not properly maintained.

Table 1: Advantages and disadvantages of various defrost alternatives.

32 A SHRAE Jou rna l A u g u s t 2 0 0 9

plants, some practitioners have explored alternative methods to determine when a particular unit requires defrost includ-ing: timers that accumulate liquid feed solenoid open time, frost sensors, air pressure drop sensors, and others. The accumulated liquid feed solenoid open time can be effective since it is somewhat adaptive to the coils load (sensible and latent). The other sensors mentioned pre-viously have not proven suitably robust to find significant penetration in industrial applications. Once it has been determined that a coil requires defrosting, a control sequence 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 defrosting forced air circulation evaporators.

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

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

Figure 1: Valve positions and fan operation during pump-out for a typical liquid overfed coil.

[Closed] Bleed Solenoid

Hand Valve

Plot Pressure Regulator

Suction Stop Valve [Open]

Defrost Relief Regulator

Suction Stop Pilot Solenoid [Closed]

Wet Suction ReturnLiquid Feed Solenoid

Mode Valve(s) Position

Pump Out

Suction Stop Valve

Suction Stop Pilot SolenoidBleed Solenoid

Liquid Feed Solenoid

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

Evapor

ator Fa

ns

[On]

[Closed]

PumpedLiquid Supply

Hot-Gas Solenoid[Closed]

Pan

the coil as possible prior to supplying hot-gas to the coil. By removing residual liquid refrigerant, the hot-gas will more quickly and effectively warm the coil to melt accumulated frost.

Advertisement formerly in this space.


Figure 2: Coil capacity decrease during pump-out.6

Evap

orat

or C

apac

ity

(ton

)

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 evaporators liquid feed solenoid valve while the suction stop valve remains open, and the units fans operate as shown in Figure 1. Heat from the fan motors and room (or product) causes the residual liquid refrigerant within the coil to evaporate with the refrigerant vapor returning to the engine room via the wet suction return (also referred to as recirculated suction).

The amount of time scheduled for pump-out varies from an extremely short duration, more typical for gravity flooded recirculation and direct-expan-sion unit designs (zero to five minutes), to a longer period for liquid overfed unit designs (10 to 15 minutes2). A short pump-out period for a gravity flooded

design requires a short pump-out period because its normal liquid refrigerant inventory within the unit during cooling mode operation is low. Liquid overfed coil designs require a longer pump-out period due to a combination of effects.

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



Figure 3: Valve positions and fan operation during soft-gas period for typical liquid overfed coil.


Hand Valve


Suction Stop Valve [Closed]


Suction Stop Pilot Solenoid [Open]



Soft-Gas

Suction Stop Valve



Soft-Gas Solenoid

Hot-Gas Solenoid

Closed

Closed

Open

Closed

Open

Closed

[Evaporator Fans Off ]

Soft-Gas Solenoid [Open]

Regulated Hot Gas


Defrost Condensate




Evapor

ator Fa

ns

[Off ]

[Closed]

PumpedLiquid Supply


Pan

First, the liquid refrigerant inventory within the coil is higher compared to a direct-expansion evaporator. Second, the refrigerant-side coil pressure drop is relatively high due to the presence of button orifices located within each circuit on the refrigerant feed-side of the coil (typical for mechanically pumped overfed designs).

Because a longer pump-out period is required for overfed coil designs, it is natural to ask how long of a pump-out period is sufficient? The pump-out period should be long enough to evapo-rate the majority of residual liquid in the coil but not too long that parasitic heat load effects to the space become significant. The parasitic heat load ef-fects during pump-out arise because the supply of liquid refrigerant to the coil has been interrupted; the evaporators fans continue to run; it is heat from the fans that are a parasitic space load. In addition, longer pump-out periods extend the time the unit is unavailable to meet space loads.

Aljuwayhel, et al.,3 reported exten-sive data collected on a field-installed evaporator unit located in a penthouse for a low temperature holding freezer. The coil in this particular unit has a rated capacity of 37 tons (130 kWt) with five fans that deliver 60,000 cfm (102 000 m3/h) of air during cooling mode operation, but that result in approxi-mately 5 tons (17.6 kWt) of parasitic heat load during fan operation. Data were collected on the units refrigera-tion capacity during the pump-out pe-riod and the units decrease in capacity over five separate pump-out cycles is shown in Figure 2. At the end of the 20 minute pump-out period, the coils capacity has decreased to a level approaching a break-even capacity to just meet the fan heat gain.

A pump-out period longer than 20 minutes is usually not required. Shorter pump-out periods should be validated by observing the frost melt pattern on the coil during the hot-gas supply period of the defrost sequence. Assuming the coil is top-fed with hot-gas (typical), an adequate pump-out period is likely established when the bottom rows of the coil completely release their frost during the hot-gas dwell period and when no audible effects of hydraulic hammering are observed on the coil and its connected piping during the early part of the hot-gas supply period.

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

mended for evaporator coils with 15 tons (53 kWt) of capacity or greater.2,4,5 The soft-gas period of the defrost sequence begins by shutting off the evaporator fans and energizing the pilot solenoid for the suction stop valve. The pilot solenoid applies hot-gas pressure to the top of the suction stop valves piston, 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 is opened to allow a low flow rate of hot-gas into the coilusu-ally after flowing first through the drain pan warming circuit; slowly raising the pressure of refrigerant in the coil. The soft-

Figure 4: Valve positions and fan operation during hot-gas period for typical liquid overfed coil.


Hand Valve







Hot Gas

Suction Stop ValveSuction Stop Pilot SolenoidBleed Solenoid


Soft-Gas Solenoid

Hot-Gas Solenoid

Closed

Open

Closed

Closed

Open

Closed



Regulated Hot Gas


Defrost Condensate




Evapor

ator Fa

ns

[Off ]

[Closed]

PumpedLiquid Supply

Hot-Gas Solenoid[Open]

Pan


gas cycle is intended to reduce the risk of hydraulic hammer that can occur on the coil or connected piping by reducing the pressure difference between the coil and the hot-gas main. The reduced pressure difference will decrease the rapid in-rush of hot-gas when the larger main hot-gas solenoid opens. Briley5 recommends siz-ing the soft-gas solenoid at 20% to 25% of the main hot-gas solenoid valve.

Figure 3 shows the valve positions and the evaporator fan state during the soft-gas period. The the soft gas dwell time is generally set to last for a period ranging from five to 10 minutes.4 Soft-gas dwell periods up to 20 minutes may be required for larger liquid overfed evaporators or in applications having large operating pres-sure differences between the hot-gas main and the evaporator. The soft-gas dwell time period should be field-adjusted to raise the evaporator pressure to ap-proximately 35 to 40 psig (2.4 to 2.8 bar) before moving to the next mode in the sequence of defrost operation. Not all evaporators have a soft-gas solenoid. While it is beneficial for all evaporators, it is more common on larger capacity, low-temperature evaporators.

Step 3: Hot-GasThus far, the individual segments of

the defrost sequence have focused on preparing the coil to receive hot-gas to melt the accumulated frost. In this portion of the defrost sequence, the larger hot-gas solenoid opens to deliver hot-gas first through the coils drain pan and then the evaporator coil, as shown in Figure 4. During the hot-gas supply period, the smaller soft-gas solenoid can either remain open or closed since the

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

How long should the hot-gas supply period be set? The dwell period of the hot-gas supply must be sufficient to allow all the accumulated frost on the coil to melt but not excessive to avoid creating a parasitic heat load external (to the space) and internal (to the refrigeration system) by returning uncondensed hot-gas back to

majority of gas flow will occur through the main hot-gas valve.As high-pressure superheated refrigerant vapor flows first

through the piping in the drain pan circuit and then into the coil, the high-pressure vapor condenses as it gives up its latent heat to warm both the drain pan and the evaporator coil surfaces. A warm drain pan will help prevent re-freezing of the water draining from the coil to the pan. As the coil surfaces warm, the accumulated layer of frost will begin to meltflowing by grav-ity down the coil and into the drain pan before leaving the unit through a defrost condensate drain line. The condensed liquid refrigerant is directed from the coil to a lower pressure level in the plant through a defrost relief regulating valve. The defrost relief regulator is factory set at a user-specified pressureusu-

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


[Open] Bleed Solenoid

Hand Valve






Bleed

Suction Stop Valve



Soft-Gas Solenoid

Hot-Gas Solenoid

Closed

Closed

Closed

Closed

Open

OpenWet Suction ReturnLiquid Feed Solenoid

Pumped Liquid Supply



Hot-Gas Solenoid [Closed]

Regulated Hot Gas


Defrost Condensate



Evapor

ator Fa

ns

[Off]

[Closed]

Pan

Figure 5: Measured and predicted average penthouse air temperatures during hot-gas defrost and bleed periods.6

40

35

30

25

20

15

10

5

0

5

10

15

20

25

30

35

40

Pent

hous

e A

ir T

empe

ratu

re (

C)

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

Hot-Gas Dwell = 40 min Bleed 10 min

Cooling Interval 24 Hours

No Frost (Experiment Data 6.5 min)No Frost (Model Prediction 6.0 min)Run #224hRun #324h

Cooling Interval 48 Hours

No Frost (Experiment Data 10.5 min)No Frost (Model Prediction 10.8 min)Run #448hRun #548h


hot-gas solenoid valve (and soft-gas solenoid if open) is closed and a small bleed solenoid valve opens to slowly depressurize the coil by relieving the pressure in the coil back to suction. The bleed solenoid valve is typically three to four sizes smaller than the 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 field adjust 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 ft3 (0.23 m3) or suction pip-ing greater than 2 in. (65 mm),2 to prevent what would be a very 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 suction piping. The bleed period also prevents rapid swings in suction pressure and compressor loading that would normally result as the engine room responds to maintain a constant suction pres-sure. The duration of the bleed period is installation-dependent and should be adjusted so no audible hammering occurs and the time is sufficient to decrease the coil pressure to within 5 to 10 psid (0.3 to 0.6 bar) of the normal cooling mode evaporator pres-sure.4 Generally, the bleed period will last five to 10 minutes.

At the conclusion of the bleed period, the suction stop pilot solenoid is de-energized allowing the main valve to open. As configured in the evaporator schematics, the pilot pressure

suction through the defrost relief regulator. Aljuwayhel6 collected data on a penthouse-mounted evaporator during both cooling mode and defrost mode of operation. For the evaporator defrost control as-found, the hot-gas dwell period was 40 minutes.

Figure 5 shows model-predicted and field-measured average air temperatures within the penthouse during the hot-gas and subsequent bleed periods of the defrost sequence for two cases. The first case allowed the evaporator to operate for 24 hours before initiating a defrost cycle. Once hot gas flowed to the coil, all the frost had melted in a period of less than seven minutes. The second case allowed the evaporator to operate for 48 hours before initiating a defrost cycle. In this situation, the coil was completely cleared of accumulated frost in less than 11 minutes during the hot-gas supply. This suggested that a 40 minute hot-gas dwell period was excessive.

Within 15 minutes of the main hot-gas valve opening, the average penthouse air temperature reached a balmy 68F (20C) and that temperature was maintained for 25 of the 40 minutes, which suggests that the continued supply of hot-gas to the coil was not resulting in the full condensing of the refrigerant vapor. Rather, a significant portion of the hot-gas was flowing back to suction and creating a parasitic load (in-ternal) on the compressors. The parasitic effect of excessive hot-gas dwell periods presents an opportunity for improving the systems energy efficiency 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

Figure 8: An illustration of the time-dependent energy flows for cooling mode and defrost mode of operation (note: this graphic is not to scale in either capacity or time).9

Coil Initial Condition (No Frost)

Coil Capacity Decreases As Frost Continues to Form

Coil Capacity Drops Rapidly as Refrigerant Flow is Stopped and the Pump Out Process

Proceeds, 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 a Temperature Warmer Than the Space to a Temperature Cooler

Than the Space, So Useful Refrigeration is Now Restored

Time

C

B

A

Evap

orat

or C

apac

ity

D

Figure 7: Valve positions and fan operation during re-chill period for typical liquid overfed coil.

[Open] Bleed Solenoid

Hand Valve


Suction Stop Valve [Open]


Suction Stop Pilot Solenoid [Closed]



Re-Chill

Suction Stop Valve



Soft-Gas Solenoid

Hot-Gas Solenoid

Open

Closed

Closed

Open

Closed

Open



Regulated Hot Gas


Defrost Condensate




Evapor

ator Fa

ns

[Off ]

[Open]

PumpedLiquid Supply


Pan


regulator located in a branch line taken from the suction side of the coil will hold the main suction stop valve for the coil closed until the set pres-sure of the pilot regulator is reached. This pilot regulator should be set to a pressure difference no greater than 10 psid (0.6 bar). The addition of this valve (and other valve designs that provide simi-lar function) is a critical safety measure to avoid hydraulic hammer that is likely to occur from a rapid opening of the suction stop valve when the coil is under pressure. It is important to note that if the bleed period is too short, the coil pressure will remain high and the suction stop valve will continue to be held closed by the pilot pressure regulator bleeding pressure from the coil to the top of the suction stop valves piston. If the suction stop valve does not open, it becomes impossible to prepare the coil for re-chilling.

At first glance, it appears that this regulator is redundant since the bleed solenoid provides the slow depressurization of the coil to within 10 psid (0.6 bar) or less of normal evaporator pressure. This is true under normal circumstances; however, the rapid opening of the suction stop valve will occur if the coil is in the hot-gas dwell period and a power outage occurs causing all solenoids to go to their normal positions. In this situation, the suction stop pilot solenoid (which is holding the suction stop valve closed by pressurizing the top of the valves piston) will close; allowing the suction stop valve to rapidly open as it returns to its normal position. The net result is an increased likelihood of hydraulic hammering with the risk of failure of the evaporator or 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-chill mode, the liquid feed solenoid is opened to allow cold liquid refrigerant to flow into the coil. Early in the re-chill period, the cold liquid supply will more rapidly evaporate as it absorbs heat from the coil mass as it reduces the coil temperature. The fans on the unit will usually remain off. Some plants short-cycle (i.e., bump) the fans on and off to allow any remaining water on the external surfaces of the coil to re-freeze while preventing the carryover of liquid water into the space that would normally occur if the fans were allowed to run at their full flow. Figure 7 shows the valve positions during the re-chill period, which generally lasts three to five minutes.

Now that we have discussed the sequences of operation as-sociated with initiating defrost of an air-cooling evaporator, lets look 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 efficiency loss to the system but necessary to restore the coils

Figure 9: Net cooling optimization results.6

100

98

96

94

92

90

88

86

84

82

80

Ove

rall

Syst

em E

ffici

ency

(%

)

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 Efficiency

RH = 80%

RH = 85%

RH = 90%

capacity by removing the accumulated frost. This fact raises the question: What is the appropriate balance between tolerating the capacity loss for accumulated frost and the parasitic load effects attributable to the defrost cycle? Figure 8 is an illustration of the time-dependent energy flows associated with the operation of a forced air circulation evaporator for both cooling mode and defrost mode operation. The operation of the coil from Point A to B is reflective of the diminishing cooling capacity of the unit due to frosting during normal cooling mode operation. At Point B the pump-out period begins, and the units capacity drops rapidly as the coil is starved and the residual refrigerant within the coil is removed by evaporation. Following the pump-out period, the coils capacity actually becomes negative (it is heat-ing rather than cooling) as hot-gas is supplied to warm the coil and melt accumulated frost. After the hot-gas flow is terminated (Point C), the coil will gradually cool down during re-chill until it reaches the point at which it can begin normal cooling mode operation (Point D).

The concept of net cooling optimization introduced by Alju-wayhel aims to maximize the integrated heat removal capability of the evaporator during an entire operational cycle: cooling mode to defrost and back to cooling mode. This integrated heat removal capacity is represented by the blue shaded region in Figure 8. A part of maximizing the heat removal capability of an evaporator involves minimizing the parasitic effects of the defrost sequence. The red hatched area above the operating capacity line represents the integrated cooling deficit below the coils rated capacity due to both frost accumulation and that the coil is unavailable during the defrost sequence. The red shaded portion of the illustration below the line of zero coil capacity represents the parasitic effects of the coil heating the space during the hot-gas dwell period. Aljuwayhel6 explored the prospect of optimizing the entire cooling and defrost mode operation, i.e., maximizing the blue-shaded portion under the cooling curve shown in Figure 8.

To nondimensionally characterize the frost loading of a coil, Aljuwayhel defined a dimensionless defrost number as:


VcondensateAmin Ld

[]Defrost number = (1)

where Vcondensate (ft3 or m3) represents the volume of water con-

densate produced at the conclusion of a defrost cycle, Amin (ft2 or

m2) represents the minimum area available for air to flow through the coil (coil face area minus the fin face area and the tube projected area of all circuits for a single row) and Ld (ft or m) represents the depth of the coil in the direction of airflow. Aljuwayhel found that a defrost number of 0.03 yielded a maximum in net cooling capacity. Figure 9 shows the net cooling optimization results using overall system efficiency as a figure of merit over a range of space latent loads represented by the three separate curves indicating the space relative humidity (RH) ranging from 80% to 90%.

Aljuwayhel defines the overall system efficiency as the ratio of the actual integrated evaporator coil cooling capacity to the ideal cooling capacity during an entire operational cycle. The actual integrated evaporator cooling capacity includes the performance degrading effects of frost accumulation, as well as the defrost process. The ideal cooling capacity assumes that the coils clean cooling capacity is maintained during the entire cycle. Aljuway-hel found that the defrost number was a useful figure-of-merit because it scales the volume of water condensate a coil produced during defrost to the volume of frost the coil is capable of hold-ing. The finding of net cooling optimization for a defrost number of 0.03 translates to a coil accumulating approximately 3% of a

representative volume before initiating a defrost sequence. As an example, consider a coil with a face area of 45 ft2 (4.18 m2), three fins per inch (one fin per 1.1 cm), 7/8 in. (22 mm) OD tubes in the first row, and a coil depth of 30 in. (0.76 m). A defrost number of 0.03 results in approximately 23 gallons (88 l) of water drained from the coil. Interestingly, the defrost number was found to be independent 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 defrost sequence involves five steps including: pump-out, soft-gas, hot-gas, bleed, and re-chill modes. Some of these steps may be omitted from defrost sequences based on the coils refrigerant feed configuration or size. A key consideration in field-tuning defrost sequence time settings is obtaining an effective defrost without audible hammering of the coil or its connected piping. We also introduced some key features relating to the function of the suction stop valve to prevent its 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 of many defrosting evaporators. One of the easiest adjustments to consider for improving the efficiency of the defrost process is the adjustment of the hot-gas dwell period. Coils with hot-gas dwell periods in excess of 15 minutes may be candidates for efficiency improvement by decreasing the hot-gas dwell period. The concept of net cooling optimization is introduced. Net cooling optimization aims to maximize the time-dependent heat extraction capability of an air-cooling evaporator during both cooling mode operation and defrost. Aljuwayhel defined a defrost number as an appropri-ate metric for optimizing the combined cooling mode and defrost mode operation of an evaporator. A defrost number of 0.03 yielded optimum performanceindependent of the coils latent load.

References1. 2006 ASHRAE HandbookRefrigeration, Chapter 42.

2. IIAR. 1992. Bulletin 116 Guidelines for: Avoiding Component Fail-ure in Industrial Refrigeration System Caused by Abnormal Pressure or Shock, 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 Journal of Refrigeration 31(1):98 106.

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

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

6. Aljuwayhel, N.F. 2006. Numerical and Experimental Study of the Influence 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.


defrosting ind. refrigeration evaporators

Documents