Frost on air-cooling evaporators

Febru a ry 2009 ASHRAE Jou rna l 27

By Douglas T. Reindl, Ph.D, P.E., and Todd B. Jekel, Ph.D, P.E.

Air-cooling evaporators are refrigerant-to-air heat exchangers widely used in industrial refrigeration, commercial refrig-eration, and heat pump systems. Known as forced-circulation air coolers1 or air-cooling evaporators, these heat exchang-ers use tubes to carry refrigerant with fins

applied to the tube exterior. Individual tubes of the heat exchanger are arranged in multiple rows of parallel circuits to achieve increased thermal performance. Refrigerant evaporates inside the tubes as it absorbs heat from air flowing over the outside surface of the finned tubes.

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.

FrostOn Air-Cooling Evaporators

In this article, we focus on the operation of air-cooling evaporators in industrial refrigeration applications at temperatures below freezing. These heat exchangers are generally applied to control the environmental conditions in holding freezers,

dynamic blast freezers, stationary blast cells, refrigerated docks, as well as other

lower temperature conditioned spaces found in food manufacturing and distribution

facilities. We review the factors that influence the formation of frost on the evapora-

tor surfaces and discuss how frost accumulation impacts coil performance.

When air-cooling evaporators operate with both coil surface temperatures below 32F (0C) and entering air dew-point temperatures above the coil surface tem-perature, moisture from the air being cooled will accumulate on the fins and tubes of the coil in the form of frost. The formation and growth of frost on the evaporator will decrease the coils cooling capability.What is the root cause of capacity decrease from frosted coil?

a. Increased air-side pressure drop; thereby, decreasing airflow through the coil.

b. Increased resistance to heat transfer between the air and the

The following article was published in ASHRAE Journal, February 2009. Copyright 2009 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. It is presented for educational purposes only. This article may not be copied and/or distributed electronically or in paper form without permission of ASHRAE.

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refrigerant due to the insulating effects of the frost.c. Both of the above.d. None of the above.

If you answered c, you are correct. A number of papers highlighting this phenomena have been published.2,3,4 Aljuwa-hel, et al,5 confirmed that the single greatest factor reducing evaporator capacity due to frost accumulation is the decrease in airflow rate due to its effect on air-side pressure drop as originally suggested by Stoecker.2

Factors Influencing Coil Capacity During FrostingWhat are the factors that control how fast my evaporators

capacity will decrease due to frosting?A number of factors influence the rate of frost

accumulation on a coil resulting in increased air-side pressure drop and reduced airflow rate through the coil.

Fin spacing. Sometimes referred to as fin pitch, the density of fins applied to an evaporator will have a dramatic effect on the susceptibility of coil frosting and the resulting capacity loss due to blocking airflow. Although increased fin density (decreased fin spacing) may be desirable because it increases the sur-face area available for heat transfer, the reduced spacing between fins will result in a decrease of the open area available for air to flow as frost accumulates on the coil. A coil design with narrow fin spacing will plug with frost more rapidly, requir-ing more frequent defrosts. A balance between fin spacing that provides adequate coil heat transfer surface area, but that does not lead to a rapid buildup of frost, is required.

In low-temperature freezing systems, some coil designs have fin spacing that varies from the air-entering side of the coil to the air-leaving side of the coil to more effectively manage the effects of frost accumulation. Figure 1 shows an evaporator for a low-temperature blast freezer where the first two rows of the coil are bare tubes (no fins). Because moisture is being extracted from the airstream as it moves from the entering to the leaving side of the evaporator, successive rows in this coil have increased fin density to provide surface area while miti-gating the plugging effects of the frost accumulation. Table 1

provides some basic guidance for typical fin spacing on coils over a range of operating conditions.

Table 1 provides some basic guidance for typical fin spacing on coils over a range of operating conditions.

It is important to work with your evaporator manufacturer to select an appropriate coil once you understand the operating environment for that coil.

Coil Location. Coils located in regions with supersaturated moisture will experience a much higher rate of frost accumula-tion and plugging. Figure 2 shows an example of warm, moist air from a dock infiltrating into a freezer. Because the infiltrating air is more buoyant, it rises to the ceiling. The rapid cooling of the supersaturated moist infiltrating air causes crystals of ice to

form in the air rather than on the coil surface. If an evaporator is located immediately above the door opening, it will see a significant load of supersaturated moisture and will quickly plug the face of the coil with frost. The accelerated rate of frost accumulation on the entering side of the coil surface is due to impaction and interception of the ice crystals onto the coil surface. If the moist dock air had an opportunity to blend with the colder drier freezer air, the rate of frost-induced plugging on the coil would be reduced.

Moisture Load. In applications with sig-nificant moisture loading, the rate of frost ac-cumulation can rise dramatically, accelerating the capacity loss. The severity of the moisture

load can be characterized in terms of the sensible heat ratio (SHR). The sensible heat ratio represents the ratio of the space sensible load to the space total load as given by:

(1)

The term Qsensible represents that portion of a heat load that causes the air temperature to rise while Qlatent is the portion of the heat load attributable to a space moisture increase. As the moisture or latent load in a temperature-controlled space increases, the sensible heat ratio decreases and operating evaporators will experience increased difficulty removing the moisture needed to meet the latent load. This is especially

Operating Temperature Range Moisture/Frost LoadTypical Fin Pitch

(Fins per in.) Comments

25F (32C) and Colder Heavy Moderate 0 3 Consider a Variable Fin Pitch Coil

25F (32C) to +10F (12C)Heavy 0 3 Consider a Variable Fin Pitch Coil

Moderate Light 2 3

+10F (12C) to +35F (2C)Heavy Moderate 3

Light 4

+35F (2C) to +50F (10C) Heavy Light 4 6Avoid using the high fin density coils in applications with airborne particulates (e.g., packaging areas).

Table 1: Review of typical evaporator fin spacing over a range of space operating conditions.

Figure 1: An evaporator coil with the first two tube rows fin free.


relevant for those evaporators operating in low-temperature environments (below 32F [0C]).

A number of past investigators have char-acterized envelopes of operating conditions that lead to moisture and frost problems in conditioned spaces. Figure 3 shows a series of three process lines on a section of a low-temperature psychrometric chart. The situa-tion is typical of what happens when higher temperature and humidity air from a dock space infiltrates to a lower temperature and humidity storage freezer space in an airflow pattern similar to that shown in Figure 2.

The psychrometric processes involved with this situation are illustrated in three separate situations shown in Figure 3. In each case the infiltrating air from the dock

leads to a supersaturated moisture condition where ice crystals will form in the air. Smith6,7 identified the unfavorable frost condition and noted how the presence of this condition adversely affected evaporator performance, as well as causing significant icing effects on other cold surfaces within the freezer. Case 1 would be reflective of entering air condition for an evaporator located in close proximity to the dock-freezer doors where the coils entering air (a mixed-air condition) is closer to the dock condition due to the relatively narrow opportunity of the infiltrating air to fully mix with the freezer air.

In Case 2, the dock relative humidity is lower (approximately 40%), and the coil entering air is much closer to the freezer condition, indicating that a more thorough mixing process has occurred as would be the case for evaporators located further away from the dock-freezer doors. In this case, the process line is approaching, but not crossing, the saturation curve leading to

Case 1: Unfavorable Process Line

Case 3: Favorable Process Line

Case 2: Borderline Favorable Process Line

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Figure 3: Psychrometric chart showing process lines that range from unfavorable to favor-able conditions for frosting.3

a borderline favorable coil frosting condition. Case 3 is a lower dock humidity condition (30%) with a similar dock-freezer air mixing ratio as Case 2. In this case, the process line stays below the saturation curve yielding a favorable frost condition. We should note that the assumed freezer set temperature of 10F (12C) and the temperature difference between the freezer and coil refrigerant temperature (approximately the ADP) of 20F (11C) are both quite high. These conditions were used to illustrate the concepts of unfavorable and favorable frost-ing conditions. Set temperatures for freezers range from 5F (20C) to 20F (29C), while a 10F (6C) temperature difference is more typical.

How can I minimize or avoid conditions that lead to unfavor-able frosting?

Obviously, the source of the moisture should be identified and minimized. In cases where moisture originates from infiltrating

Figure 2: Airflow patterns from a dock to a freezer.14

Loading Dock

Fog

Frost Accumulation

Ice Crystals

Warm Inflow

Freezer

Cold OutflowIced Floor

is at a constant dry-bulb temperature of 50F (10C), while the relative humidity varies from 60% (Case 1) to 30% (Case 3), which mixes with a freezer with a space temperature of 10F (12C). The lowest surface temperature in the freezer space is the evaporator coil, which operates at 10F (23C)a point commonly referred to as the apparatus dew-point temperature (ADP).

When the air from the dock enters into the freezer, a mixing process occurs. As the warmer dock air blends with the cold-er air within the freezer, the mixed-air condition progressively moves down the tie line that connects the dock condition to the freezer condition. Case 1 illustrates an unfavorable process line due to por-tions of the process line that connects the mixed-air condition to the coil ADP being above the saturation curve, which

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air, appropriate techniques to reduce that infiltration rate should be pursued. This may require repairing seals, maintaining door control, reducing openings (e.g., for conveyors), and ensuring an appropriate pressure balance between spaces at dif-fering conditions. Further information on these strategies can be found in several papers.3,8,9

If the source of moisture is from prod-ucts that are being processed, consider alternative means that can reduce mois-ture loss. Strategies for reducing product moisture loss can include: packaging prior to cooling/freezing; pre-cooling product; and crust-freeze product using a cryogenic fluid prior to finish-freezing with a mechanical freezing system (flash freezing will create a crust on the product surface to minimize desiccation). Tech-niques to reduce moisture loss have the

Figure 4: Psychrometric chart showing process lines for a lower dock set temperature to improve favorability of the freezer coil frosting condition.

Process Line With Lower

Dock Set Temperature Unfavorable

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added benefit of increasing product yield. Yield savings almost certainly will far outweigh energy cost benefits from reduced moisture loads on evaporators. For many low-temperature freez-

ing systems, the extent of unfavorable frost conditions can be minimized but not eliminated.

For spaces such as holding freezers, one common approach

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for avoiding unfavorable frost conditions is to lower the setpoint temperature of the dock to increase the level of moisture re-moval at a higher evaporator temperature (when compared to the freezer). Reduc-ing the dock setpoint temperature to something in the range of 35F (2C), will permit air defrosting while provid-ing significantly more moisture removal when compared to a 50F (10C) space setpoint. In some cases, hot-gas reheat is added at the dock evaporator to further increase the space sensible heat ratio.

Figure 5: Low density frost forming on an evaporator due to high coil temperature difference and presence of supersaturated air.

capacity decrease due to airflow blockage. Unfavorable frost-ing conditions leads to the formation of ice crystals directly in the airstream. There is a tendency for these ice crystals to precipitate onto cold surfaces within the space; however, they will ride along on air currents created by operating evaporator fans. The frost crystals will readily adhere to the coil surface by physical impaction or interception; thereby, blocking airflow. Figure 5 shows the structure of unfavorable frost adhering to the surfaces of a variably finned, low-temperature evaporator freezing unpacked product and operating with a moderately high TD (difference in temperature between the entering air and the evaporating refrigerant). In this case, the structure of

Figure 4 shows a dock maintained at a 35F (2C) dry-bulb temperature with a relative humidity of 60%. A process line from the entering coil condition to the ADP shows it to be just favorable as it approaches but does not cross the saturation curve. Adding reheat at the dock door can further increase the dry-bulb temperature of infiltrating air, driving the process to an increasingly favorable frost condition process line.

Cleland3 offers other strategies for avoiding unfavorable frosting conditions but rightly places a particular emphasis on preventing the infiltration using door protection devices.

Frost Type. Somewhat related to discussions in the previous section, the type of frost has an influence on the rate of coil

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the frost is extremely light and fluffy with minimal bonding to the coil surface. We postulate that, in this case, the coil plugged where the fins started and that the light, fluffy frost grew after the coil blockage. As mentioned previously, this type of frost degrades coil performance more rapidly than a higher density frost as shown in Figure 6.

The higher density frost forming on the coil shown in Figure 6 occurs in spaces with favorable frosting conditions. Due to the lack of supersaturated air, as well as an operating coil TD of 10F (6C), moisture from the air forms on the coil by a dif-fusion process creating a much more dense frost structure. The higher frost density allows the coil to accumulate significantly more mass of moisture (frost) before adversely affecting coil capacity due to airflow blockage.

Measuring Coil Capacity Decrease Due to FrostingHow significant is the rate of capacity loss due to frosting? As mentioned previously, the loss of coil capacity under

frosting operation is due to reduced airflow, as well as increased resistance to heat transfer. The more significant of these two factors is the capacity loss due to blockage of airflow.2,5,10,11,12 The effects of frost presenting an increased resistance to heat transfer are significantly less important.2,13

Aljuwahel5 monitored the performance of a single 37 ton (130 kW) evaporator located in a penthouse in a low-temperature stor-age freezer. Additional details on the coil are given in Table 2.

The in situ performance of the unit was determined using an extensive configuration of air-side instrumentation arranged to measure entering and leaving conditions (air temperature and moisture content), as well as the average velocity of air flowing through the coil. In addition, data was collected to determine the volume flow rate of air being conveyed by the units five fans.

Figure 7 shows the average face velocity of air across the coil during frosting operation over a 41 hour period. The average velocity of air across the frost-free coil is approximately 560 ft/min (2.85 m/s) but that average velocity decreases by nearly 50% to 315 ft/min (1.6 m/s) at the end of its operating cycle. Figure 8 shows the average dry-bulb temperature of air entering and leav-ing the evaporator during frosting operation. The average entering air temperature (i.e., space temperature) is relatively constant at 17.5F (28C) while the leaving temperature decreased from 24F (31C) to 26F (33C) as the coil accumulated frost. The drop in leaving air temperature is a byproduct of the decreased airflow rate through the coil, which allows longer dwell time to give up its heat to the refrigerant. Unfortunately, that decreased coil leaving air temperature is not sufficient enough to overcome the drop in airflow rate. Consequently, the coils refrigeration capacity decreases over time as frost accumulates on the coil. The actual measured gross capacity of the coil is shown in Figure 9.

The average clean coil capacity over four separate runs is 33 tons (116 kW) and the capacity of the unit decreases to 27 tons (95 kW) after 41 hours of operation, representing a capacity loss of nearly 20%. Two other observations were made regarding the measured coil capacity. First, the field-measured capacity is 8% less than the units rated capacity. Second, the measured evapora-

Parameter Value

Fin Pitch 3 fins/in. (0.85 cm)

Face Area 88.6 ft2 (8.23 m2)

Tube Diameter 3/4 in. (19.05 mm)

Tube Length 18 ft (5.5 m)

Number of Fans 5

Fan Power at 30F (34C) Air Temperature

3.125 HP (2.33 kW)

Rated cfm 60,000 cfm (1,699 m3/min)

Number of Tube Rows 10

Saturated Evaporator Temperature

30F (34.4C)

Coil Temperature Difference 10F (5.6C)

Rated Coil Capacity 37 tons (130 kW)

Fin and Tube Material Aluminum

Evaporator Coil Type CPR-fed Liquid Overfeed

Table 2: Geometry and operating conditions of the experimentally monitored air-cooling evaporator.

Figure 6: Higher density frost forming on an evaporator.

tor capacity is gross because it does not include fan heat gains. The net effect is that an evaporators capacity, while operating under frosting conditions will decrease and the systems operat-ing efficiency suffers as a result. To counter these effects, the accumulated frost must be removed from the evaporator surface on either a continuous or intermittent basis.

Alternative ApproachesAre there other approaches that can further reduce or elimi-

nate the need for defrosting evaporators? The short answer to this question is not really. Some alterna-

tive approaches use a liquid desiccant media such as glycol, which is sprayed directly onto the evaporator surface to preferentially absorb the moisture into the freezing point depressed working fluid. As moisture from the air goes into the liquid solution, the concentration of glycol will be reduced and reconcentration becomes necessary to avoid freeze-ups. In this case, the equiva-lent to a hot gas defrost for a typical evaporator occurs remotely from the unit as heat is added to drive off the accumulated water; thereby, re-concentrating the glycol for reuse.

Another alternative that has been promoted to reduce latent loads is the use of solid desiccants. The solid desiccant system


approach can reduce latent loads in temperature controlled spaces but the added cost of the desic-cant system operation must be carefully evaluated to understand whether or not the total cost of operation will be lowered.

ConclusionsWith the exception of sprayed desiccant units,

evaporators operating at lower temperature conditions will result in frost formation on the coil surface. Many factors influence the rate and nature of frost formation including: evaporator unit fin spacing, coil location, latent (moisture) load, and frost type or structure. The accumulation of frost on a coil causes its capacity to decrease due to blockage of airflow, as well as the insulating effects of the frost layer itself. As a result, periodic removal of the accumulated frost layer is required to maintain system capacity and efficiency.

References 1. 2006 ASHRAE HandbookRefrigeration, Chapter

42, Forced-Circulation Air Coolers.Stoecker, W.F. 1957. How frost formation on coils 2.

affects refrigeration systems. Refrigerating Engineer-ing, 65(2):44 45.

Cleland, D.J. 2005. Implications of coil frosting 3. on system designs for low-temperature applications. ASHRAE Transactions, 111 (1):336 345.

Mago, P.J. and S.A. Sherif. 2005. Coil frosting and 4. defrosting issues at low freezer temperatures near satura-tion conditions. ASHRAE Transactions, 111(1):3 17.

Aljuwayhel, N.F., D.T. Reindl, S.A. Klein, G.F. 5. Nellis. 2008. Experimental investigation of the per-formance of industrial evaporator coils operating under frosting conditions. International Journal of Refrigera-tion, 31(1):98 106.

Smith, G.R. 1989. Theoretical cooling coil calcu-6. lations at freezer temperatures to avoid unfavorable coil frost. ASHRAE Transactions, 95(2):1138 1148.

Smith, G.R. 1992. Latent heat, equipment-related 7. load, and applied psychrometrics at freezer tempera-tures. ASHRAE Transactions, 98(2):649 657.

Reindl, D.T. and T.B. Jekel. 2008. Infiltration rate 8. determination for low temperature freezing systems. ASHRAE Transactions, 114(2):264 272.

IRC. 2005. Air balance issues in refrigerated 9. facilities. Cold Front. Industrial Refrigeration Consor-tium, University of Wisconsin-Madison, 5(4):1 8.

Barrow, H. 1985. A note on frosting of heat 10. pump evaporator surfaces. Heat Recovery Systems, 5(3):195 201.

Seker, D., H. Karatas, and N. Egrican. 2004. 11. Frost formation on fin-and-tube heat exchangers. Part

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Figure 9: Coil capacity (load) as the unit operates from clean to frosted condition.

Figure 8: Average coil inlet and outlet temperatures during frosting operation.

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Imodeling of frost formation on fin-and-tube heat exchangers. International Journal of Refrigeration, 27: 367 374.

Yao, Y., Y. Jiang, S. Deng, and Z. Ma. 2004. A study on the 12. performance of the airside heat exchanger under frosting in an air source heat pump water heater/chiller unit. International Journal of Heat and Mass Transfer.

Machielsen, C.H. and H.G. Kerschbaumer. 1989. Influence 13. of frost formation and defrosting on the performance of air coolers: standards and dimensionless coefficients for the system designer. International Journal of Refrigeration, 12(3):283 290.

14. 2006 ASHRAE HandbookRefrigeration, Chapter 13, Refrig-eration Load.

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Frost on air-cooling evaporators