ASHRAE Journal - February 2009 - (Page 28) 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 accumulation and plugging. Figure 2 shows an example of warm, moist Factors Influencing Coil Capacity During Frosting air from a dock infiltrating into a freezer. Because the infiltrating What are the factors that control how fast my evaporator’s air is more buoyant, it rises to the ceiling. The rapid cooling of capacity will decrease due to frosting? the supersaturated moist infiltrating air causes crystals of ice to A number of factors influence the rate of frost form in the air rather than on the coil surface. If accumulation on a coil resulting in increased an evaporator is located immediately above the air-side pressure drop and reduced airflow rate door opening, it will see a significant load of through the coil. supersaturated moisture and will quickly plug the Fin spacing. Sometimes referred to as face of the coil with frost. The accelerated rate of “fin pitch,” the density of fins applied to an frost accumulation on the entering side of the coil surface is due to impaction and interception of evaporator will have a dramatic effect on the the ice crystals onto the coil surface. If the moist susceptibility of coil frosting and the resulting dock air had an opportunity to blend with the capacity loss due to blocking airflow. Although colder drier freezer air, the rate of frost-induced increased fin density (decreased fin spacing) plugging on the coil would be reduced. may be desirable because it increases the surface area available for heat transfer, the reduced Moisture Load. In applications with sigspacing between fins will result in a decrease of nificant moisture loading, the rate of frost acFigure 1: An evaporator coil with the open area available for air to flow as frost cumulation can rise dramatically, accelerating the first two tube rows fin free. accumulates on the coil. A coil design with the capacity loss. The severity of the moisture narrow fin spacing will “plug” with frost more rapidly, requir- load can be characterized in terms of the sensible heat ratio ing more frequent defrosts. A balance between fin spacing that (SHR). The sensible heat ratio represents the ratio of the space provides adequate coil heat transfer surface area, but that does sensible load to the space total load as given by: not lead to a rapid buildup of frost, is required. In low-temperature freezing systems, some coil designs have (1) 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 The term Qsensible represents that portion of a heat load that for a low-temperature blast freezer where the first two rows causes the air temperature to rise while Qlatent is the portion of the coil are bare tubes (no fins). Because moisture is being of the heat load attributable to a space moisture increase. As extracted from the airstream as it moves from the entering to the moisture or latent load in a temperature-controlled space the leaving side of the evaporator, successive rows in this coil increases, the sensible heat ratio decreases and operating have increased fin density to provide surface area while miti- evaporators will experience increased difficulty removing the gating the plugging effects of the frost accumulation. Table 1 moisture needed to meet the latent load. This is especially 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 Aljuwahel, 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 Operating Temperature Range –25°F (–32°C) and Colder 25°F (–32°C) to +10°F (–12°C) +10°F (–12°C) to +35°F (2°C) +35°F (2°C) to +50°F (10°C) Moisture/Frost Load Heavy – Moderate Heavy Moderate – Light Heavy – Moderate Light Heavy – Light Typical Fin Pitch (Fins per in.) 0–3 0–3 2–3 3 4 4–6 Avoid using the high fin density coils in applications with airborne particulates (e.g., packaging areas). Comments Consider a Variable Fin Pitch Coil Consider a Variable Fin Pitch Coil Table 1: Review of typical evaporator fin spacing over a range of space operating conditions. 28 ASHRAE Journal ashrae.org February 2009 http://www.ashrae.org
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