ASHRAE Journal - February 2009 - 32

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 10°F (6°C), moisture from the air forms on the coil by a diffusion 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 Frosting Figure 6: Higher density frost forming on an evaporator. Parameter Fin Pitch Face Area Tube Diameter Tube Length Number of Fans Fan Power at –30°F (–34°C) Air Temperature Rated cfm Number of Tube Rows Saturated Evaporator Temperature Coil Temperature Difference Rated Coil Capacity Fin and Tube Material Evaporator Coil Type Value 3 fins/in. (0.85 cm) 88.6 ft2 (8.23 m2) 3/4 in. (19.05 mm) 18 ft (5.5 m) 5 3.125 HP (2.33 kW) 60,000 cfm (1,699 m3/min) 10 –30°F (–34.4°C) 10°F (5.6°C) 37 tons (130 kW) Aluminum CPR-fed Liquid Overfeed How 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 storage 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 unit’s 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 leaving the evaporator during frosting operation. The average entering air temperature (i.e., space temperature) is relatively constant at –17.5°F (–28°C) while the leaving temperature decreased from –24°F (–31°C) to –26°F (–33°C) 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 coil’s 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 unit’s rated capacity. Second, the measured evapora32 ASHRAE Journal Table 2: Geometry and operating conditions of the experimentally monitored air-cooling evaporator. tor capacity is gross because it does not include fan heat gains. The net effect is that an evaporator’s capacity, while operating under frosting conditions will decrease and the system’s operating 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 Approaches Are there other approaches that can further reduce or eliminate the need for defrosting evaporators? The short answer to this question is “not really.” Some alternative 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 equivalent 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 ashrae.org February 2009

ASHRAE Journal - February 2009

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