ASHRAE Journal - February 2009 - 29

relevant for those evaporators operating in low-temperature environments (below Frost Accumulation 32°F [0°C]). Ice Crystals A number of past investigators have characterized 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 lowLoading Dock temperature psychrometric chart. The situation is typical of what happens when higher temperature and humidity air from a dock space infiltrates to a lower temperature and Warm Inflow humidity storage freezer space in an airflow Freezer pattern similar to that shown in Figure 2. The psychrometric processes involved with this situation are illustrated in three Fog Cold Outflow separate situations shown in Figure 3. In Iced Floor each case the infiltrating air from the dock is at a constant dry-bulb temperature of Figure 2: Airflow patterns from a dock to a freezer.14 50°F (10°C), while the relative humidity 0.012 varies from 60% (Case 1) to 30% (Case 3), which mixes with a freezer with a Saturation Line space temperature of 10°F (–12°C). 0.010 The lowest surface temperature in the freezer space is the evaporator coil, which 0.8 0.008 operates at –10°F (–23°C)—a point commonly referred to as the apparatus 0.6 0.006 40°F dew-point temperature (ADP). Case 1: Unfavorable Process Line Mixed Air When the air from the dock enters into Case 2: Borderline Favorable Process Line 30°F the freezer, a mixing process occurs. As 0.4 0.004 Case 3: Favorable Process Line the warmer dock air blends with the cold20°F er air within the freezer, the mixed-air 0.2 0.002 ADP 10°F 0°F condition progressively moves down the –10°F tie line that connects the dock condition 0.000 to the freezer condition. Case 1 illustrates –20 –10 0 10 20 30 40 50 60 Dry-Bulb Temperature (°F) an “unfavorable process line” due to portions of the process line that connects Figure 3: Psychrometric chart showing process lines that range from unfavorable to favorthe mixed-air condition to the coil ADP able conditions for frosting.3 being above the saturation curve, which leads to a supersaturated moisture condition where ice crystals a borderline favorable coil frosting condition. Case 3 is a lower will form in the air. Smith6,7 identified the “unfavorable” frost dock humidity condition (30%) with a similar dock-freezer air condition and noted how the presence of this condition adversely mixing ratio as Case 2. In this case, the process line stays below affected evaporator performance, as well as causing significant the saturation curve yielding a favorable frost condition. We icing effects on other cold surfaces within the freezer. Case 1 should note that the assumed freezer set temperature of 10°F would be reflective of entering air condition for an evaporator (–12°C) and the temperature difference between the freezer located in close proximity to the dock-freezer doors where and coil refrigerant temperature (approximately the ADP) of the coil’s entering air (a mixed-air condition) is closer to the 20°F (11°C) are both quite high. These conditions were used dock condition due to the relatively narrow opportunity of the to illustrate the concepts of unfavorable and favorable frostinfiltrating air to fully mix with the freezer air. ing conditions. Set temperatures for freezers range from –5°F In Case 2, the dock relative humidity is lower (approximately (–20°C) to –20°F (–29°C), while a 10°F (6°C) temperature 40%), and the coil entering air is much closer to the freezer difference is more typical. condition, indicating that a more thorough mixing process has How can I minimize or avoid conditions that lead to unfavoroccurred as would be the case for evaporators located further able frosting? away from the dock-freezer doors. In this case, the process line Obviously, the source of the moisture should be identified and is approaching, but not crossing, the saturation curve leading to minimized. In cases where moisture originates from infiltrating February 2009 ASHRAE Journal Dock Condition 29 Humidity Ratio

ASHRAE Journal - February 2009

Table of Contents for the Digital Edition of ASHRAE Journal - February 2009