1、4742 (RP-1094) Coil Frosting and Defrosting Issues at Low Freezer Temperatures Near Saturation Conditions Pedro J. Mago, PhD Associafe Member ASHRAE ABSTRACT During the operation of industrial freezers, frost buildup contributes to coil heat transfer performance degradation due to the insulating efl
2、ect of the frost layer and the resulting coil blockage. While the frosting problem has been extensively researched, no study has investigated the problem when the freezer is operating near saturated or under supersaturated conditions. Field observations of industrial freezer operation indicate that
3、this is a common mode of operation and that the transition to supersaturated operation can easily occur if proper care is not exercised by either the refrigeration system designer or the freezer operator: Field observations also reveal that operating freezers under supersaturated conditions can sign
4、ificantly accelerate the occurrence of the negative aspects offrost formation vis-vis coil heat transferperformance. For this and other reasons, the study presented in this paper focuses on coil frosting and defrosting issues at low tempera- ture with the hope of developing tools andprotocols that h
5、elp refrigeration-system designers and freezer operators improve the performance offreezer coils that are particularly prone to the frosting problem. INTRODUCTION The frost formation problem is most acute in industrial and commercial freezers of the type found in food distribution warehouses and sup
6、ermarkets. Accumulation is typical around the freezer door, on the coil, and sometimes on the freezer floor and ceiling in extreme scenarios. This is espe- cially true in high-trafic freezers primarily because of the exchange of warm and humid air outside the freezer with cold and dry air inside the
7、 freezer space. The moisture introduced into the freezer space is likely to cause the air to become super- S.A. Sherif, PhD Fellow ASHRAE saturated at the prevailing air temperature. If that temperature is above the freezing point of water, moisture in the air would exist in the form of tiny liquid
8、droplets suspended in the airstream-a familiar condition known as fog. If the same scenario existed but with the air temperature below the freez- ing point, “ice fog” would form. Airborne ice crystals that normally constitute ice fog have a tendency to deposit on cold surfaces and are thus more like
9、ly to end up on the surface of the freezer coil. It is important to note that the term “supersaturated air” as used in this paper is a common term used in the industrial refrigeration community to refer to an equilibrium mixture of saturated air and suspended liquid water droplets or ice crys- tals
10、(depending on whether the air dry-bulb temperature is above or below the freezing point of water, respectively). This is not the metastable supersaturated state in the normal termi- nology of thermodynamics. We elected to use the terminology commonly employed in the refrigeration industry (instead o
11、f the more scientifically accurate description used in thermody- namics) to make the paper more relevant to the refrigeration community. The supersaturated state to which we refer in this paper is an equilibrium state and not a metastable state as is common in thermodynamics. While literature dealin
12、g with the frosting problem on different geometries is abundant, the present review will be confined to finned coils and cylindrical geometries as well as to studies dealing with frost property formulations. Among the investigators who reported frost formation on cylinders are Chung and Algren (1958
13、a, 1958b), Andrichak (1962), Parish (1970), Parish and Sepsy (1972), Padki et al. (1989), Raju and Sherif (1993), and Sengupta et al. (1998). Among the investi- gators who studied finned coils are Kondepudi and ONeal - Pedro J. Mago is an assistant professor in the Department of Mechanical Engineeri
14、ng, Mississippi State University. S.A. Sherif is a professor in the Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, Fla. 02005 ASHRAE. 3 (1988, 1989, 1991), Tao et al. (1994), andChenet al. (2000a, 2000b). Studies emphasizing empirical or theoretical frost pro
15、perty formulations were reported by Yonko and Sepsy (1967), Brian et al. (1969), Biguria and Wenzel (1970), Hayashi et al. (1977a, 1977b), and Marinyuk (1980), among others. Despite the large effort expended on investigating the problem of frost formation, very few studies looked into the formation
16、mechanism under supersaturated conditions while accounting for psychrometric effects. Thus, it appears that any new effort to study the frosting problem should try to address the aforementioned aspect of the issue. More recent investi- gations dealing with coil frosting inside freezers include those
17、 of Al-Mutawa et al. (1998a, 1998b, 1998c, 1998d), Al- Mutawa and Sherif (1998), and Sherif et al. (2001,2002). While there is a significant amount of information pertaining to coil frosting, studies dealing with coil defrosting are relatively few. Among the investigators who examined defrosting of
18、freezer coils are Kerschbaumer (1 97 i), Niederer (1976), Stoecker et al. (1983), and Coley (1983), among others. While there is a reasonable amount of interest in coil defrosting studies, it is obvious that there are areas that can benefit from additional research. For example, there seems to be no
19、 quantitative information on the performance of freezer coils at low temperatures during the defrosting process near saturated freezer air conditions. Developing a reliable and extensive body of data for those types of scenarios should therefore prove a worthy cause. EXPERIMENTAL FACILITY The experi
20、mental facility utilized in this research is a laboratory-size freezer with an industrial-size freezer coil (having four fins per inch) located at the center and a water- vapor generator (WVG) facing the coil. Each of the freezer doors has good rubber seals to protect the freezer and the test result
21、s from the effect of air infiltration. The heat transfer rate of the test enclosure was determined experimentally while it was clean, dry, and empty. The finned-tube freezer coil is a liquid overfeed recircu- lating evaporator with an overfeed ratio of three. This coil has a refrigerating capacity o
22、f about two tons at a coil suction temperature of -40C (40F) and is part of a complete refrig- eration system discussed in detail in Al-Mutawa et al. (1998a, 1998b, 1998c, 1998d). The coil finned tubes are arranged in eight rows in the direction of airflow and in a staggered pattern of 38 x 33 mm (1
23、.5 x 1.3 in.), where the tube material is copper having 15.9 mm (0.63 in.) outside diameter and 0.46 mm (0.0 18 in.) thickness. The fins are made of aluminum and have a flat pattern with flat edges. The fins have a thickness of 0.25 mm (0.01 in.), and their spacing is four fins per inch. The coil ha
24、s a finned height of 533 mm (20.9 in.) and a finned length of 737 mm (29 in.), where its outside dimensions are 1016 x 610 x 627 mm (40 x 24 x 24.7 in.). The finned-tube freezer coil is classified as a draw-through unit since the fan draws the air against the refrigerant in a crossflow direction whe
25、re each fluid flows at a right angle to the other. However, the tubes are circuiting in such a manner that the two fluids will approach in a counterflow type heat exchanger arrangement. The freezer coil is employed with one fan that has a diam- eter of 508 mm (20 in.) and a speed of 18 reds (1075 rp
26、m) and is operated by a 186-W (0.25 hp) motor. The unit has a face area of 0.39 m2 (4.2 fi2) and a capability to change the coil face velocity from 1.02 m/s to 7.62 m/s (200 fpm to 1500 fpm). The artificial load generation system is probably one ofthe most critical systems in the experimental facili
27、ty. It is designed to provide the freezer with the required latent heat load in order to be able to manipulate the moisture content inside the freezer during the testing period. This latent heat load is generated by the water-vapor generator (WVG) located outside the freezer. City water is allowed t
28、o flow into the WVG through an elec- tronic diaphragm metering pump in order to control the mass flow rate of the steam to be injected inside the freezer. The metering pump capacity ranges from 0.01 to 1 ml/s (0.0006 to 0.061 i./s), where its maximum capacity per day is 0.091 m3 (3.21 fi3). The pump
29、?s maximum injection pressure is 758 kPa (1 1 O psi). This metering pump has an adjustable speed, which ranges from 5 to 100 strokes per minute and an adjustable stroke length that ranges from 0% to 100%. The metering pump can be controlled manually or by a computer. The WVG is a liquid-injection ty
30、pe of water-vapor generator equipped with three heating elements strapped to its side and bottom. The vaporizer is insulated with 2.54 cm (1 in.) thick high-temper- ature insulation enclosed in galvanized steel housing. The WVG is also equipped with a thermometer, a thermostat, and a pressure relief
31、 valve. The bi-metal dial thermometer is accu- rate to within 0.5?C (0.9?F) and has a reading range of 10C to 288C (50F to 550F). The thermostat adjusting screw was used to obtain the desired setpoint. The pressure relief valve has a cracking pressure range of O to 138 kPa (O to 20 psi). The crackin
32、g pressure can be adjusted to the desired setpoint using the valve?s adjustment screw. The water injected into the WVG is heated to the desired temperature inside the vaporizer and is allowed to leave the WVG as steam through a copper tube that passes through the freezer wall to the steam outlet ins
33、ide the artificial load generator. To prevent steam from freezing inside the copper tube, an electric heating cable is wrapped around the copper tube inside the freezer. The copper tube and the heating cable are both covered by 19 mm (0.748 in.) thick Armaflex insulation. A ?clean-up? strip heater o
34、f 150 W (0.2 hp) is also attached to the bottom ofthe steam outlet in order to prevent the steam from freezing at the outlet, which will then stop the flow of steam to the freezer. The artificial load generation system is employed with a direct-drive blower that is used to distribute the steam insid
35、e the freezer. The blower has a capacity of 0.73 m3/s (26 ft3/s) at 0.025 m (1 in.) static pressure when its motor is operating at a speed of 17.5 reds (1050 rpm). Temperatures have been measured at 20 locations using copper-constantan (type-T) thermocouples having an uncer- tainty of k0.2?C (0.36?F
36、). All thermocouples have been cali- 4 ASHRAE Transactions: Research brated using a constant-temperature water bath (model Polyscience-80). The relative humidity (RH) of the air inside the freezer was measured both upstream and downstream of the test coil. Relative humidity measurements were perform
37、ed using a Mamac humidity transducer (HU-224) connected to a remote probe hung inside the freezer. The two humidity trans- ducers have a humidity range of 0% to 100% and are accurate to k2% of the full-scale. They utilize a DC power supply of 12- 28 volts to operate, while they send output signals o
38、f 4-20 mA. All measurements have been recorded using a personal computer equipped with a state-of-the-art data acquisition system. Numerous tests were performed for the complete rerig- eratioddefrost (WD) cycle of operation. Because of space limitations, however, only results of selected tests will
39、be presented in the section that follows. RESULTS AND DISCUSSION The results presented in this section will cover a host of areas pertaining to coil frosting and defrosting in industrial freezers. Broad areas covered include (I) snow-like accurnu- lations versus conventional frost formation and whet
40、her or not there is a demarcation line between these two types of forma- tions; (2) coil defrosting issues using the hot-gas refrigerant method; and (3) a discussion of some ways of dealing with the aforementioned issues in the interest of promoting energy conservation. Examples of those potential e
41、nergy-saving methods include the use ofheat-pipe-assisted freezer coils, the use of dampered coils, and operating freezer coils at exces- sively high coil face velocities. Operating at high velocities may prove advantageous for reasons pertaining to the manner in which the frost crystals form during
42、 the refrigeration mode of the WD cycle. Discussions of these and other issues follow in greater detail. Demarcation Line Issues One of the objectives of this experimental program was to demonstrate the existence of a demarcation line between the frost that forms under subsaturated conditions to fro
43、st that forms under supersaturated conditions. This was achieved performing several experiments in which the freezer opera- tion started with a predetermined coil entering air tempera- ture, a predetermined room (freezer) sensible heat ratio (SHR), and a predetermined coil surface temperature, and t
44、hen steadily decreasing the coil sensible heat ratio until the cooling process line has crossed the saturation curve on the psychrometric chart into the supersaturated zone. Decreasing the coil sensible heat ratio was achieved by injecting known amounts of steam into the freezer while keeping the co
45、il entering air temperature and the coil surface temperature constant. Freezer temperature and sensible heat ratio were controlled by either adding an artificial sensible load to the freezer or by increasing the cooling rate inside the freezer employing the refrigeration system (without changing the
46、 coil surface temperature). Typical results of these tests are shown in Figure 1. In this experiment the starting relative humidity of the coil entering air was 64%, the coil entering air temperature was -8.3“C (17“F), while the coil surface temperature was -18.3“C (-1F). During this experiment the
47、relative humidity of the coil entering air was increased until it reached 99%, while keeping constant the coil entering air temperature at -8.3“C (17F) and coil surface temperature at -18.3“C (-1F). Four different scenarios were tried as shown in Figure 1. In Scenario 1, the inlet dry-bulb temperatu
48、re (DBT) was kept at 43C (1 7F) and the inlet relative humidity (RH) was 64%, while the leaving air DBT was -16.2“C (2.8 OF) and the leaving air RH was 92%. The degree of saturation (pd) for this scenario was 91.9%. In Scenario 2, the inlet DBT was -8.3“C (17“F)andtheRHwas 84%, whilethe IeavingDBTwa
49、s-162C (23F) and the degree of saturation (pd,super) was 102%. In Scenario 3, the inletDBTwas-8.3“C (17F) andRH %O, while the leaving DBTwas -16.1“C (3F) and the degree of saturation pd,super was 105%. In Scenario 4, the inlet DBT was -8.3“C (17F) and RH was 99%, while the outlet DBT was -16.1 “C (3F) and the degree of saturation pd,superwas 109%. It is impor- tant to observe that the relative humidity is not defined in the supersaturated zone of the psychrometric chart. An alternative term that is equally representative of the amount of moisture p
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