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.

Defrosting

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

T

his article discusses techniques for removing accumulated frost on aircooling 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. The accumulation of frost on forcedcirculation 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 30

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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 coil’s 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 38°F [3.3°C]), an off-cycle defrost can be accomplished by shutting off the refrigerant feed for an extended period of time while continuing to operate the fans. The heat from the relatively warmer room air heat melts the accumulated 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 Authors Douglas 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 WisconsinMadison’s Industrial Refrigeration Consortium in Madison, Wis.

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Defrost Approach

Applications

Widely used in most industrial and some commercial refrigeration systems (direct refrigerant).

Hot-Gas

Electric

Off-Cycle

Water

Advantages

Disadvantages

Able to achieve effective defrost.

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

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.

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.

Used in industrial and commercial refrigeration systems for spaces operating above freezing point (typically >38°F [3.3°C]).

Efficient means of defrost.

Found in some lower-temperature refrigeration systems. This form of defrost can also be integrated into the normal sanitation operations.

Secondary Fluid (Indirect)

An alternative to electric defrost in CO2 cascade and secondary phase change systems.

Minimizes parasitic load. Avoid extreme refrigerant-side pressure (CO2 refrigerants).

Simple implementation. Inherently safe. Lower capital and maintenance costs. Applies heat directly to the accumulated frost. The defrost process can be integrated into a normal sanitation cycle. Able to achieve fast defrost.

Efficient means of defrost. Conceptually simple. Avoids risks of hydraulic hammering on refrigerant-side of coil.

Extremely high working pressures required for some refrigerants such as CO2. Can lead to increased parasitic energy consumption with improper valve group design and poorly adjusted defrost sequence times. 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. Not relevant in applications where space temperatures are below freezing. Not effective at removing oil accumulation from evaporators. Difficult to apply for “defrost on the fly” during operation for low temperature applications. Not effective at removing oil accumulation from evaporators. Increases plant water use. Additional secondary fluid system and circuiting, which makes the coil larger, heavier, and more costly. Not effective at removing oil accumulation from evaporators. 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.

a warm secondary fluid is circulated through the defrost coil to raise the evaporator’s 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 pressure 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 pressure 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 August 2009

refrigerant saturation (condensing) temperature of 47°F to 58°F (8°C to 14°C). 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. Advantages 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. Let’s first look at the steps involved in a typical defrost sequence. Then, we explore energy considerations associated with the entire cooling and defrost processes. Defrost Sequence of Control

Due 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 ASHRAE Journal

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plants, some practitioners have explored alternative methods to determine when a particular unit requires defrost including: 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 coil’s load (sensible and latent). The other sensors mentioned previously 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 complete several steps in a defrost sequence. The following individual steps are typical of the sequences used for defrosting forced air circulation evaporators.

[Closed] Bleed

Solenoid Plot Pressure Regulator

Hand Valve

Suction Stop Pilot Solenoid [Closed]

Valve(s)

Mode

Suction Stop Valve

Suction Stop Valve [Open]

Wet Suction Return Liquid Feed Solenoid [Closed]

Pumped Liquid Supply

Soft-Gas Solenoid [Closed]

Pump Out

Suction Stop Pilot Solenoid Bleed Solenoid

Hot-Gas Solenoid [Closed]

Position Open Closed Closed

Liquid Feed Solenoid Soft-Gas Solenoid

Closed

Hot-Gas Solenoid

Closed

Closed

[Evaporator Fans – On ] Regulated Hot Gas

Defrost Return (Medium Pressure) Defrost Condensate Defrost Relief Regulator

Recirculated Liquid/ Vapor Return Recirculated Liquid Supply Defrost Hot-Gas Supply

or

t ora

p

Eva

s

Fan

n]

[O

n Pa

Figure 1: Valve positions and fan operation during pump-out for a typical liquid overfed coil. 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 as much of the residual cold liquid refrigerant contained within

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.

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Evaporator Capacity (ton)

The pump-out period begins by de30 energizing (closing) the evaporator’s liquid feed solenoid valve while the 25 suction stop valve remains open, and the unit’s fans operate as shown in Figure 1. Heat from the fan motors and room 20 (or product) causes the residual liquid refrigerant within the coil to evaporate 05 with the refrigerant vapor returning to the engine room via the wet suction return (also referred to as recirculated 10 suction). The amount of time scheduled for 5 pump-out varies from an extremely short duration, more typical for gravity flooded recirculation and direct-expan0 0 2 4 6 8 10 12 14 16 18 20 sion unit designs (zero to five minutes), Pump-Out Dwell Time (min) to a longer period for liquid overfed unit designs (10 to 15 minutes 2). A short Figure 2: Coil capacity decrease during pump-out.6 pump-out period for a gravity flooded evaporator is made possible because the low refrigerant-side design requires a short pump-out period because its normal pressure drop of the coil allows any residual liquid refrigerant liquid refrigerant inventory within the unit during cooling (and liquid condensate) to be readily cleared when hot-gas mode operation is low. Liquid overfed coil designs require is supplied to the coil for defrost. The direct-expansion coil a longer pump-out period due to a combination of effects.


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First, the liquid refrigerant inventory [Closed] Bleed Hand Solenoid Valve Suction Stop Pilot within the coil is higher compared to Solenoid [Open] Valve(s) Plot Pressure Mode Position a direct-expansion evaporator. Second, Regulator Suction Stop Valve Closed the refrigerant-side coil pressure drop Suction Stop Open Pilot Solenoid is relatively high due to the presence Suction Stop Valve [Closed] Wet Suction Return Bleed Solenoid Closed of button orifices located within each Liquid Feed Solenoid Soft-Gas Liquid Feed [Closed] Closed Solenoid circuit on the refrigerant feed-side of the Pumped Open Soft-Gas Solenoid coil (typical for mechanically pumped Soft-Gas Solenoid Liquid Supply Hot-Gas Solenoid Closed [Open] overfed designs). Hot-Gas Solenoid [Evaporator Fans – Off ] [Closed] Because a longer pump-out period is Regulated Hot Gas required for overfed coil designs, it is Defrost Return (Medium Pressure) natural to ask “how long of a pump-out period is sufficient?” The pump-out s Fan period should be long enough to evapoDefrost Condensate tor a r po Defrost Relief rate the majority of residual liquid in Eva Recirculated Liquid/ Regulator Vapor Return the coil but not too long that parasitic ff ] Recirculated Liquid Supply heat load effects to the space become [O an P significant. The parasitic heat load efDefrost Hot-Gas Supply fects during pump-out arise because the supply of liquid refrigerant to the coil Figure 3: Valve positions and fan operation during soft-gas period for typical liquid overfed coil. has been interrupted; the evaporator’s fans continue to run; it is heat from [Closed] Bleed Hand Suction Stop Pilot Solenoid Valve the fans that are a parasitic space load. Solenoid [Open] Valve(s) Plot Pressure Mode Position In addition, longer pump-out periods Regulator Suction Stop Valve Closed extend the time the unit is unavailable Suction Stop Open Pilot Solenoid Suction Stop Valve [Closed] to meet space loads. Wet Suction Return Bleed Solenoid Closed Aljuwayhel, et al.,3 reported extenLiquid Feed Solenoid Hot Gas Liquid Feed [Closed] Closed Solenoid sive data collected on a field-installed Pumped Soft-Gas Solenoid Closed Soft-Gas Solenoid evaporator unit located in a penthouse Liquid Supply Hot-Gas Solenoid Open [Closed] for a low temperature holding freezer. Hot-Gas Solenoid [Evaporator Fans – Off ] [Open] The coil in this particular unit has a Regulated Hot Gas rated capacity of 37 tons (130 kWt) with Defrost Return (Medium Pressure) five fans that deliver 60,000 cfm (102 000 m3/h) of air during cooling mode s Fan Defrost Condensate or operation, but that result in approxit a r po Defrost Relief Eva mately 5 tons (17.6 kWt) of parasitic Recirculated Liquid/ Regulator Vapor Return heat load during fan operation. Data ff ] Recirculated Liquid Supply [O were collected on the unit’s refrigeran Pa Defrost Hot-Gas Supply tion capacity during the pump-out period and the unit’s decrease in capacity over five separate pump-out cycles is Figure 4: Valve positions and fan operation during hot-gas period for typical liquid overfed coil. shown in Figure 2. At the end of the 20 minute pump-out period, the coil’s capacity has decreased Step 2: Soft-Gas to a level approaching a break-even capacity to just meet The use of a soft-gas step in the defrost sequence is recomthe fan heat gain. mended for evaporator coils with 15 tons (53 kWt) of capacity A pump-out period longer than 20 minutes is usually not or greater.2,4,5 The soft-gas period of the defrost sequence required. Shorter pump-out periods should be validated by begins by shutting off the evaporator fans and energizing the observing the frost melt pattern on the coil during the hot-gas pilot solenoid for the suction stop valve. The pilot solenoid supply period of the defrost sequence. Assuming the coil is applies hot-gas pressure to the top of the suction stop valve’s top-fed with hot-gas (typical), an adequate pump-out period is piston, forcing this normally open valve closed. likely established when the bottom rows of the coil completely With the coil now isolated from the system’s suction pressure, release their frost during the hot-gas dwell period and when no a small ported (e.g., 0.5 in. [13 mm]) soft-gas solenoid valve is audible effects of hydraulic hammering are observed on the coil opened to allow a low flow rate of hot-gas into the coil—usuand its connected piping during the early part of the hot-gas ally after flowing first through the drain pan warming circuit; supply period. slowly raising the pressure of refrigerant in the coil. The softAugust 2009

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Cooling Interval 24 Hours 40

Cooling Interval 48 Hours

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

35 30 Penthouse Air Temperature (°C)

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 sizing 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 softgas 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 pressure 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 approximately 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.

25

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

20 15 10 5 0 –5 –10 –15 –20 –25

Bleed 10 min

Hot-Gas Dwell = 40 min

–30 –35 –40 0

5

10

15

20

25

30 35 Time (min)

40

45

50

55

60

Figure 5: Measured and predicted average penthouse air temperatures during hot-gas defrost and bleed periods.6 Bleed Solenoid Plot Pressure Regulator [Open]

Hand Valve

Suction Stop Pilot Solenoid [Open]

Suction Stop Valve [Closed]

Mode

Wet Suction Return Liquid Feed Solenoid [Closed]


Pumped Liquid Supply Hot-Gas Solenoid

Step 3: Hot-Gas

[Closed]

Bleed

Valve(s)

Position

Suction Stop Valve

Closed


Open Open

Liquid Feed Closed Solenoid Soft-Gas Solenoid Closed

Hot-Gas Solenoid Closed

[Evaporator Fans – Off ]

Regulated Hot Gas Thus far, the individual segments of Defrost Return (Medium Pressure) the defrost sequence have focused on preparing the coil to receive hot-gas s Fan or Defrost Condensate t to melt the accumulated frost. In this a r po Defrost Relief Eva Recirculated Liquid/ portion of the defrost sequence, the Regulator Vapor Return larger hot-gas solenoid opens to deliver ff] [O Recirculated Liquid Supply hot-gas first through the coil’s drain pan n Pa Defrost Hot-Gas Supply and then the evaporator coil, as shown in Figure 4. During the hot-gas supply period, the smaller soft-gas solenoid can Figure 6:Valve positions and fan operation during the bleed period for a typical liquid overfed coil. either remain open or closed since the majority of gas flow will occur through the main hot-gas valve. ally 70 to 90 psig (4.8 to 6.2 barg) (equivalent to a saturation As high-pressure superheated refrigerant vapor flows first temperature of 47°F to 58°F [8°C to 14°C] for ammonia). The through the piping in the drain pan circuit and then into the coil, defrost relief regulator will modulate to maintain the evaporathe high-pressure vapor condenses as it gives up its latent heat tor at the regulator’s pressure setting and it will fully reseat at to warm both the drain pan and the evaporator coil surfaces. the conclusion of the hot-gas dwell period. A check valve is A warm drain pan will help prevent re-freezing of the water required on the outlet of the defrost relief regulator when the draining from the coil to the pan. As the coil surfaces warm, the defrost condensate return is piped to a suction pressure higher accumulated layer of frost will begin to melt—flowing by grav- than the evaporator’s normal operating pressure. ity down the coil and into the drain pan before leaving the unit How long should the hot-gas supply period be set? The dwell through a defrost condensate drain line. The condensed liquid period of the hot-gas supply must be sufficient to allow all the refrigerant is directed from the coil to a lower pressure level in accumulated frost on the coil to melt but not excessive to avoid the plant through a defrost relief regulating valve. The defrost creating a parasitic heat load external (to the space) and internal (to relief regulator is factory set at a user-specified pressure—usu- the refrigeration system) by returning uncondensed hot-gas back to

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KEEP GROWING YOUR LEED ACCREDITATION SETS THE COURSE FOR TRANSFORMING YOUR PRACTICE.

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[Open] Bleed

Solenoid Plot Pressure Regulator

Hand Valve

Suction Stop Pilot Solenoid [Closed]

Valve(s)

Mode

Suction Stop Valve

Suction Stop Valve [Open]

Wet Suction Return Liquid Feed Solenoid [Open]

Re-Chill


Pumped Liquid Supply


Hot-Gas Solenoid [Closed]

Position Open Closed Open

Liquid Feed Solenoid Soft-Gas Solenoid

Closed

Hot-Gas Solenoid

Closed

Open

[Evaporator Fans – Off ] Regulated Hot Gas

Defrost Return (Medium Pressure) Defrost Condensate Defrost Relief Regulator

Recirculated Liquid/ Vapor Return Recirculated Liquid Supply Defrost Hot-Gas Supply

p

Eva

or

t ora

s

Fan

ff ] [O n Pa

Figure 7: Valve positions and fan operation during re-chill period for typical liquid overfed coil. A D Coil Initial Condition (No Frost) Evaporator Capacity

suction through the defrost relief regulator. Aljuwayhel6 collected data on a penthousemounted 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 68°F (20°C) 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 (internal) on the compressors. The parasitic effect of excessive hot-gas dwell periods presents an opportunity for improving the system’s energy efficiency by simply reducing the scheduled hot-gas dwell period.

B 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 Time 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 C Refrigeration is Now Restored

Step 4: Bleed

At the conclusion of the hot-gas dwell Figure 8: An illustration of the time-dependent energy flows for cooling mode and defrost period, a bleed or equalize sequence is mode of operation (note: this graphic is not to scale in either capacity or time).9 initiated. During the bleed period, the hot-gas solenoid valve (and soft-gas solenoid if open) is closed for hydraulic hammering to the coil and the connected suction and a small bleed solenoid valve opens to slowly depressurize piping. The bleed period also prevents rapid swings in suction the coil by relieving the pressure in the coil back to suction. The pressure and compressor loading that would normally result as bleed solenoid valve is typically three to four sizes smaller than the engine room responds to maintain a constant suction presthe main suction stop valve but not less than 0.5 in. (13 mm).7 sure. The duration of the bleed period is installation-dependent An optional hand valve in the bleed line can be used to field and should be adjusted so no audible hammering occurs and the adjust the rate of coil depressurization as shown in Figure 6. time is sufficient to decrease the coil pressure to within 5 to 10 The bleed period is necessary, particularly on large coils psid (0.3 to 0.6 bar) of the normal cooling mode evaporator pres(with coil volumes greater than 8 ft3 (0.23 m3) or suction pip- sure.4 Generally, the bleed period will last five to 10 minutes. ing greater than 2½ in. (65 mm),2 to prevent what would be a At the conclusion of the bleed period, the suction stop pilot very rapid depressurization of the coil when the suction stop solenoid is de-energized allowing the main valve to open. As valve opens. Rapid coil depressurization increases the potential configured in the evaporator schematics, the pilot pressure 38

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Overall System Efficiency (%)

regulator located in a branch line taken from the Defrost Number suction side of the coil will hold the main suction 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 100 stop valve for the coil closed until the set pres98 sure of the pilot regulator is reached. This pilot 96 regulator should be set to a pressure difference no RH = 80% greater than 10 psid (0.6 bar). The addition of this 94 valve (and other valve designs that provide simi92 RH = 85% lar function) is a critical safety measure to avoid 90 hydraulic hammer that is likely to occur from a 88 rapid opening of the suction stop valve when the 86 coil is under pressure. It is important to note that RH = 90% Maximum System Efficiency 84 if the bleed period is too short, the coil pressure will remain high and the suction stop valve will 82 continue to be held closed by the pilot pressure 80 0 25 50 75 100 125 150 175 200 225 250 375 300 325 350 regulator bleeding pressure from the coil to the top Total Mass of Condensed Water (kg) of the suction stop valve’s piston. If the suction stop valve does not open, it becomes impossible Figure 9: Net cooling optimization results.6 to prepare the coil for re-chilling. At first glance, it appears that this regulator is redundant capacity by removing the accumulated frost. This fact raises the since the bleed solenoid provides the slow depressurization of question: What is the appropriate balance between tolerating the the coil to within 10 psid (0.6 bar) or less of normal evaporator capacity loss for accumulated frost and the parasitic load effects pressure. This is true under normal circumstances; however, the attributable to the defrost cycle? Figure 8 is an illustration of rapid opening of the suction stop valve will occur if the coil is the time-dependent energy flows associated with the operation in the hot-gas dwell period and a power outage occurs causing of a forced air circulation evaporator for both cooling mode and all solenoids to go to their normal positions. In this situation, defrost mode operation. The operation of the coil from Point A the suction stop pilot solenoid (which is holding the suction stop to B is reflective of the diminishing cooling capacity of the unit valve closed by pressurizing the top of the valve’s piston) will due to frosting during normal cooling mode operation. At Point close; allowing the suction stop valve to rapidly open as it returns B the pump-out period begins, and the unit’s capacity drops to its normal position. The net result is an increased likelihood rapidly as the coil is starved and the residual refrigerant within of hydraulic hammering with the risk of failure of the evaporator the coil is removed by evaporation. Following the pump-out or connected piping. period, the coil’s capacity actually becomes negative (it is heating rather than cooling) as hot-gas is supplied to warm the coil Step 5: Re-Chill and melt accumulated frost. After the hot-gas flow is terminated Once the coil is depressurized and the suction stop valve open, (Point C), the coil will gradually cool down during re-chill until the unit is ready to return to refrigeration mode. In the re-chill it reaches the point at which it can begin normal cooling mode mode, the liquid feed solenoid is opened to allow cold liquid operation (Point D). refrigerant to flow into the coil. Early in the re-chill period, the The concept of net cooling optimization introduced by Aljucold liquid supply will more rapidly evaporate as it absorbs heat wayhel aims to maximize the integrated heat removal capability from the coil mass as it reduces the coil temperature. The fans of the evaporator during an entire operational cycle: cooling on the unit will usually remain off. Some plants short-cycle (i.e., mode to defrost and back to cooling mode. This integrated heat bump) the fans on and off to allow any remaining water on the removal capacity is represented by the blue shaded region in external surfaces of the coil to re-freeze while preventing the Figure 8. A part of maximizing the heat removal capability of carryover of liquid water into the space that would normally an evaporator involves minimizing the parasitic effects of the occur if the fans were allowed to run at their full flow. Figure defrost sequence. The red hatched area above the operating 7 shows the valve positions during the re-chill period, which capacity line represents the integrated cooling deficit below the coil’s rated capacity due to both frost accumulation and generally lasts three to five minutes. Now that we have discussed the sequences of operation as- that the coil is unavailable during the defrost sequence. The sociated with initiating defrost of an air-cooling evaporator, let’s red shaded portion of the illustration below the line of zero coil look at the energy consequences of this process. capacity represents the parasitic effects of the coil heating the space during the hot-gas dwell period. Aljuwayhel6 explored Energy Impacts and Net Cooling Optimization the prospect of optimizing the entire cooling and defrost mode As discussed in the article on coil frosting,8 the accumulation operation, i.e., maximizing the blue-shaded portion under the of frost on a coil progressively decreases its cooling capacity; cooling curve shown in Figure 8. necessitating a defrost cycle. The defrost cycle is a source of To nondimensionally characterize the frost loading of a coil, efficiency loss to the system but necessary to restore the coil’s Aljuwayhel defined a dimensionless defrost number as: August 2009

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Defrost number =

Vcondensate [–] Amin × Ld

(1)

where Vcondensate (ft3 or m3) represents the volume of water condensate 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 coil’s clean cooling capacity is maintained during the entire cycle. Aljuwayhel 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 holding. 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 coil’s latent load as shown in Figure 9. Conclusions

In 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 coil’s 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 appropriate metric for optimizing the combined cooling mode and defrost mode operation of an evaporator. A defrost number of 0.03 yielded optimum performance—independent of the coil’s latent load. References 1. 2006 ASHRAE Handbook—Refrigeration, Chapter 42. 2. IIAR. 1992. Bulletin 116 Guidelines for: Avoiding Component Failure 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 evaporator 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 Industrial 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 evaporators.” 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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