ASHRAE Journal - February 2009 - 33

Air Velocity (m/s) approach can reduce latent loads in temperature controlled spaces but the added cost of the desiccant system operation must be carefully evaluated to understand whether or not the total cost of operation will be lowered. Conclusions 4.00 3.50 3.00 2.50 2.00 1.50 1.00 0.50 0.00 0 250 500 750 1000 1250 1500 Time (min) 1750 2000 2250 2500 Region of Experiment Uncertainty Run #1 Run #2 Run #3 Run #4 Run #5 With 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 Figure 7: Average face velocity of air across the coil during frosting operation. –24 –25 Average Air Temperature (°C) –26 Average Inlet Air Temperature Run #2 Run #4 Run #3 Run #5 Model Prediction –11 –13 Average Air Temperature (°F) –15 1. 2006 ASHRAE Handbook—Refrigeration, Chapter –27 –17 42, “Forced-Circulation Air Coolers.” –28 2. Stoecker, W.F. 1957. “How frost formation on coils –19 affects refrigeration systems.” Refrigerating Engineer–29 –21 ing, 65(2):44 – 45. Average Outlet Air Temperature –30 3. Cleland, D.J. 2005. “Implications of coil frosting –23 –31 on system designs for low-temperature applications.” –25 ASHRAE Transactions, 111 (1):336 – 345. –32 Run #2 Run #4 4. Mago, P.J. and S.A. Sherif. 2005. “Coil frosting and –27 Run #3 Run #5 –33 defrosting issues at low freezer temperatures near saturaModel Prediction –34 –29 tion conditions.” ASHRAE Transactions, 111(1):3 – 17. 0 250 500 750 1000 1250 1500 1750 2000 2250 2500 5. Aljuwayhel, N.F., D.T. Reindl, S.A. Klein, G.F. Time (min) Nellis. 2008. “Experimental investigation of the performance of industrial evaporator coils operating under Figure 8: Average coil inlet and outlet temperatures during frosting operation. frosting conditions.” International Journal of Refrigera40 tion, 31(1):98 – 106. 6. Smith, G.R. 1989. “Theoretical cooling coil calcuRun #1 Run #3 lations at freezer temperatures to avoid unfavorable coil 126 35 Run #2 Run #4 frost.” ASHRAE Transactions, 95(2):1138 – 1148. Model Prediction 7. Smith, G.R. 1992. “Latent heat, equipment-related 108 30 load, and applied psychrometrics at freezer temperatures.” ASHRAE Transactions, 98(2):649 – 657. 90 8. Reindl, D.T. and T.B. Jekel. 2008. “Infiltration rate 25 determination for low temperature freezing systems.” ASHRAE Transactions, 114(2):264 – 272. 72 20 9. IRC. 2005. “Air balance issues in refrigerated facilities.” Cold Front. Industrial Refrigeration Consor54 15 tium, University of Wisconsin-Madison, 5(4):1 – 8. 10. Barrow, H. 1985. “A note on frosting of heat 250 500 750 1000 1250 1500 1750 2000 2250 2500 0 pump evaporator surfaces.” Heat Recovery Systems, Time (min) 5(3):195 – 201. 11. Seker, D., H. Karatas, and N. Egrican. 2004. Figure 9: Coil capacity (load) as the unit operates from clean to frosted condition. “Frost formation on fin-and-tube heat exchangers. Part I—modeling of frost formation on fin-and-tube heat exchangers.” 13. Machielsen, C.H. and H.G. Kerschbaumer. 1989. “Influence International Journal of Refrigeration, 27: 367 – 374. of frost formation and defrosting on the performance of air coolers: 12. Yao, Y., Y. Jiang, S. Deng, and Z. Ma. 2004. “A study on the standards and dimensionless coefficients for the system designer.” performance of the airside heat exchanger under frosting in an air International Journal of Refrigeration, 12(3):283 – 290. 14. 2006 ASHRAE Handbook—Refrigeration, Chapter 13, “Refrigsource heat pump water heater/chiller unit.” International Journal of eration Load.” Heat and Mass Transfer. Cooling Load (kW) February 2009 ASHRAE Journal 33 Cooling Load (Ton)
ASHRAE Journal - February 2009

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