1、4772 Implications of Coil Frosting on System Designs for Low-Temperature Applications Donald J. Cleland, PhD Member ASHRAE ABSTRACT Air relative humidity (RH) in a refrigerated facility is determined by the balance between moisture entry (principally through doors) and moisture removal (principally
2、as frost on the air-cooling coil). It has been shown that coil frosting is “unfavorable,” leading to very rapid decline in performance, ifthe air becomes super-saturated as it is cooled. The condi- tions for the transition to unfavorable frost formation can be predicted from coil design in formation
3、 including air-on temperature and RH, evaporation temperature, and heat load sensible heat ratio (SHR). Using advanced models for air injil- tration through doors plus standard models for other heat loads, and assuming that air cooling follows a straight line approach from the air-on condition to th
4、e saturation at the coil surface temperature, the load SHR, coil SHR, and, hence, the balancedairRHwereestimatedfora typical refrigeratedfacil- ity with a warehouse, environmental loading area (ELA), and blast freezer. It was shown that unfavorable frost formation is likely to occur in low-temperatu
5、re facilities even if ambient conditions are not that extreme and moisture entry is reduced by doorway protection. Unfavorable frosting can be best avoided by a combination of improved coil design (e.g., smaller air to refrigerant temperature diflerence, TO) and enhanced door protection (especially
6、doors opening to the ambient), but occasionally active dehumidification or special coil design (e.g., staggeredjin spacing) must be used to avoid frosting problems. INTRODUCTION Infiltration of air and, hence, moisture into refrigerated warehouses is costly to the operation of the facility. If the f
7、acil- ity operated near or below 0C (32”F), the moisture deposits on the evaporator coil surfaces as frost, decreasing the refiig- eration system efficiency. In particular, the frost both reduces the airflow through the coil and insulates the coil heat transfer surface. Periodically, the coils must
8、be defrosted to remove the accumulated frost, incurring further energy costs and causing increased risk of temperature fluctuations. In addition, the moisture can deposit as icelcondensation on floors, doors, walls, ceilings, and product (especially near to doors), creating hazards to workers, impai
9、ring productivity, causing problems with product acceptability, and requiring regular removal. To reduce the frequency of the fiostingldefrost cycle and the magnitude of the undesirable effects, a number of actions are often taken: Loading dock external truck-trailer doors are protected to reduce th
10、e infiltration of ambient air (e.g., bump or inflat- able cushions, flap curtains). Loading docks are conditioned to reduce the infiltration of moisture into the warehouse (reheat andor dehumidifica- tion coils or dessicant dehumidification). Doors between the loading dock and the warehouse are prot
11、ected (e.g., strip curtains, rapid-action doors, air curtains, door vestibules, etc.). Heat sources are used to prevent frost formation on surfaces other than the evaporator coils. For evaporator coils adjacent to areas with high air infil- tration (e.g., in vestibules), there is anecdotal evidence
12、that the coil performance deteriorates extremely rapidly, requiring seemingly continuous defrost. Smith (1989, 1992) proposed the concept of unfavorable” frost formation to describe such situations and postulated that it occurred when the line repre- senting the temperature and humidity of the air p
13、assing through the coil (the air approach line) crosses the saturation Donald J. Cleland is a professor in the Institute of Technology and Engineering, Massey University, Palmerston North, New Zealand. 336 02005 ASHRAE. I I I I I I I l Ts Temperature Ton Figure 1 Psychrometric chart showing the conc
14、ept of favorable and unfavorable frost formation and the demarcation between frost types. line of the psychrometric chart (i.e., becomes supersaturated) as shown in Figure 1. Further, the mechanism of unfavorable frost formation was postulated to be the formation of airborne water droplets or ice-cr
15、ystals, which physically deposit on any surface they encounter, in addition to the normal frost forma- tion by difision of water from high partial pressure conditions in the airstream to low partial pressure conditions at the coil (or frost) surface. Unfavorable frost is snowlike (and of low density
16、) and as a result may be more insulating, have higher impact on airflow rate reduction through the coil, and be more difficult to defrost than frost formed under less extreme condi- tions (Le., subsaturated air cooling path), which by compari- son is “favorable“ (although still undesirable). Sherif
17、et al. (2001) endorsed the theory in experiments with a typical low-temperature coil operating with an air-on temperature of -8.3“C (17“F), refrigerant evaporation temper- ature of -40C (-40“F), and variable heat load sensible heat ratios (SHR) and air-on relative humidities (RH,). However, the air-
18、on temperature difference (TD,) used was 31.7“C (57“F), which represents a very extreme condition compared with normal coil design and operating practice. Other frosting studies on finned tube coils have either held airflow rate constant and/or were conducted at high SHR conditions where unfavorable
19、 frost formation is unlikely (ONeal and Tree 1985; Kondepudi and ONeal 1987, 1993a, 1993b). Cleland and OHagan (2003) measured coil performance data under frosting conditions with air-on temperatures of about 0C (32F) for a range of operating conditions likely to be experienced industrially (i.e., m
20、oderate TD, declining airflow rate as frost accumulates, wide range of SHR) includ- ing both favorable and unfavorable conditions. For the same total frost accumulation, the airflow and heat transfer perfor- mance decline was more rapid at low SHR (high air relative humidity, RH) and high heat load
21、(high air to refrigerant temperature difference, TD) in a manner consistent with the unfavorable frost theory. The following equation for the demarcation between frost types (critical coil SHR) was devel- saturation Line/ I i II II Unfavourable T,I Favourable T, T, Temperature To, Ton Figure 2 Psych
22、rometric chart showing the principle used to estimate the demarcation between favorable and unfavorable frost types. oped based on the tangent to the air saturation line (Figure 2) and the straight line approach from the air-on condition to saturation at the coil surface temperature (Stoecker 1988):
23、 SHRcrit = Ah. ep( air relative humidity (%) sensible heat ratio temperature (“C; OF) air-to-refrigerant temperature difference (“C; OF) sensible heat transfer capacity rating (W/OC; Btu/ h.“F or TWOF) heat load (W; Btu/h or TR) density (kg/m3; lb/) Subscripts a = air crit = criticailtransitional co
24、ndition e = refkigerant evaporation lut = latent of = air-off condition on = air-oncondition S = coilsurface sen = sensible t = total w = water saturation to the dimensionality of the constants. Equations 1 and 2 are only valid if SI units are used due REFERENCES Amos, N.D., D.J. Cleland, A.C. Clela
25、nd, and N.H. Banks. 1993. The effect of cool store design and operation on air relative humidity. Proceedings of the International Conference on Agricultural Machinely and Process Engineering, Seoul, Oct. 1993, pp. 433-442. ASHRAE. 2002. 2002 ASHRAE Handbook-Refrigeration, Chapter 12. Atlanta: Ameri
26、can Society of Heating, Refrigerating and Air-conditioning Engineers, Inc. Chen, P., D.J. Cleland, S.J. Lovatt, and M.R. Bassett. 2002. An empricial model for predicting air infiltration into refrigerated stores through doors. International Journal of Refrigeration 25:799-812. Cleland, D.J, and A.N.
27、 OHagan. 2003. Performance of an air-cooling coil under frosting conditions. ASHRAE Transactions 109(2). Atlanta: American Society of Heating, Refrigerating and Air-conditioning Engineers, Inc. Cleland, D.J, P. Chen, S.J. Lovatt, and M.R. Bassett. 2004. A modified model to predict air infiltration i
28、nto refriger- ated facilities through doorways. ASHRAE Transactions 1 lO(2). Atlanta: American Sociey of Heating, Refriger- ating and Air-conditioning Engineers, Inc. Cole, R.A. 1989. Refrigeration loads in a freezer due to hot gas defrost and their associated costs. ASHRAE Trans- actions 95(2):1149
29、-1154. Atlanta: American Society of Heating, Refrigerating and Air-conditioning Engineers, Inc. East, A.R., P.B. Jeffery, and D.J. Cleland 2003. Air infiltra- tion through loading dock truck-trailer doors. Proceed- ings of 21st International Congress of Refrigeration, Washington, OC, August 2003 (pa
30、per 384). Eagleton, D.G., and J.A. Marcondes. 1994. Moisture-sorp- tion isotherms for paper-based components of transport packaging for fresh produce. Tappi Journal 77(7):75-81. Kondepudi, S.N., and D.L. ONeal. 1987. The effects of frost growth on extended surface heat exchanger perfor- mance: a rev
31、iew. ASHRAE Transactions 93(2):258-274. Atlanta: American Society of Heating, Refrigerating and Air-conditioning Engineers, Inc. Kondepudi, S.N., and D.L. ONeal. 1993a. Performance of finned-tube heat exchangers under frosting conditions: I, Simulation model. International Journal of Refrigera- tion
32、 16:175-180. Kondepudi, S.N., and D.L. ONeal. 1993b. Performance of finned-tube heat exchangers under frosting conditions: II, Comparison of experimental data with model. Inter- national Journal of Refrigeration 16: 18 1- 184. ONeal, D.L., and D.R. Tree. 1985. A review of frost forma- tion in simple
33、 geometries. ASHRAE Transactions 9 1 (2A):267-28 1. Atlanta: American Society of Heating, Refrigerating and Air-conditioning Engineers, Inc. Sanders, C.Th. 1974. Frost formation-The influence of frost formation and defrosting on the performance of air coolers. PhD thesis, Delft University of Technol
34、ogy, The Netherlands. Sherif, S.A., P. Mago, N.K. Al-Mutawa, R.S. Theen, and K. Bilen. 2001. Psychrometrics in the supersaturated frost zone, ASHRAE Transactions 107(2). Atlanta: American Society of Heating, Refrigerating and Air-conditioning Engineers, Inc. Smith, G.R. 1989. Theoretical cooling coi
35、l calculations at freezer temperatures to avoid unfavourable coil frost. ASHRAE Transactions 95(2):1138-1148. Atlanta: American Sociey of Heating, Refrigerating and Air- conditioning Engineers, Inc. Smith, G.R. 1992. Latent heat, equipment-related load, and applied psychrometrics at freezer temperatures. ASHRAE Transactions 98(2):649-657. Atlanta: Ameri- can Society of Heating, Rehgerating and Air-Condi- tioning Engineers, Inc. Stoecker, W.F. 1988. Industrial Refrigeration. Business News Publishing Company. ASHRAE Transactions: Research 345
