The number of air-cooling coils in operation in industrial refrigeration plants far exceeds the number of liquid-chilling evaporators installed. Before explaining the performance of air coils, the physical features of the several types of industrial refrigeration coils will be presented. The major components of air coils are tubes, tube sheets, fins, and drain pan.
Tubes. The tubes are pipes that enclose the refrigerant. The most common materials used for tubes are carbon steel, copper, aluminum, and stainless steel. If ammonia is the refrigerant any of the four materials except copper may be used, and most halocarbon systems apply coils with copper tubes. The most common sizes of steel-tube coils for ammonia service are 3/4, 7/8 and 1 in, although 5/8 inch tubes are also used. For smaller halocarbon coils, 1/2-in copper tubes are sometimes used.
Tube sheets. At each end of the coil a heavy plate supports the tubes by having holes through which the tubes pass. The pattern of these holes defines whether the tubes are in-line or staggered. The coil elements in Fig. 6.14 depict a staggered pattern for the tubes. A coil with a staggered-fin pattern enjoys slightly improved heat transfer, but at the expense of a slight increase in air pressure drop.
Fins. Section 6.4 explained from a heat-transfer standpoint why extended surface, or fins, are a logical feature of air-cooling coils. These fins may be appllied by wrapping a strip of metal in a helical fashion around the tube and
then bonding it to the tube. Much more common, however, is the use of plate or flat fins, as appear in Fig. 6.14. The materials available for these fins are the same as for tubes, and typical combinations of tube/fin materials are:
– copper tube/aluminum fin for halocarbon air-cooling coils
– aluminum tube/aluminum fin for halocarbon or ammonia air-cooling coils
– carbon steel tube/carbon steel fin for air-cooling coils using ammonia, halocarbons, antifreezes or water in the tubes
– stainless steel tube/stainless steel fin when special cleaning provisions are required on the air side
Stainless steel is usually used only for extremely low temperature, where there is a corrosive atmosphere, or whenever periodic cleaning is necessary. The thermal conductivity is less than that of carbon steel which itself is about one-fourth that of aluminum. The cost of a stainless steel coil may be five or more times that of a steel coil of comparable size.
The application of the coil, particularly whether it will become frosted, determines to a large extent the spacing of fins. In air conditioning coils with thin aluminum fins, the spacing may be 470 per m (12 fins per inch, FPI), while industrial coils are usually built with 118 or 158 fins per m (3 or 4 FPI). Coils serving spaces where the air temperature is below freezing usually have a fin spacing of 118 per m (3 per inch).
Bonding of the fins to the tubes. The fin must form a good bond to the tube, otherwise there will be additional heat-transfer resistance through air gaps. A steel tube/steel fin coil will be galvanized, which is a process whereby the entire coil is dipped in molten zinc. The zinc provides a protective surface against corrosion and also gives an effective bond between the tube and fin. For non galvanized coils the tubes are usually expanded against the collar of the fin to yield a tight fit. The tube is usually expanded by forcing a hardened ball at the end of a rod through the tube after the fin plates are stacked on the tubes.
Circuiting of the coil. In direct-expansion coils using halocarbon refrigerants, the general direction of the flow of refrigerant through the circuits is downward, while in order for flooded coils to function properly the general direction of the refrigerant flow must be upward. Forced liquid recirculation coils may be circuited either as upward flow (bottom feed) or as downward flow (top feed), as illustrated in Fig. 6.15. The coil designer chooses the length of the circuit such that the refrigerant when flowing with appropriate velocity receives enough heat in passing through the circuit to vaporize the desired fraction of refrigerant.
Orifices in liquid recirculation coils. One refrigerant circuit in the coils of Fig. 6.15 consists of six passes back and forth through the coil. There are a number of parallel passes, and the upper circuits in a coil are prone to receive an inadequate flow of liquid. In order to strive toward an equal distribution of refrigerant flow, orifices are placed at the entrance of each circuit, as shown by the cutaway in Fig. 6.15a. These orifices, as shown in Fig. 6.16, are thin metal discs with a hole. The holes are usually located eccentrically near the bottom so that oil which accumulates in the coil during refrigeration operation may more easily flow out of the coil during hot-gas defrost. Often the diameter of the orifices is greater for the upper circuits to achieve uniform distribution of refrigerant.
Drain pan for a low-temperature coil. All coils are equipped with drain pans, because in the normal operation of a cooling coil that operates above freezing temperature, some water vapor will be condensed from the air. This condensate is collected by the drain plan and drained to some convenient destination. Coils that operate below freezing temperatures must be defrosted periodically, and once again a drain pan is necessary to collect the melted frost to drain it away. The drain pan must be kept warm so that the melted frost does not refreeze, and when the method of defrost is by hot gas (see Sec. 6.22) a source of heat is available for the drain pan. Figure 6.17 shows that hot gas that reaches the coil for purposes of defrost is first passed through pipes embedded in the drain pan. Hot gas first comes to the drain pan and then flows on to the coil to defrost it.