Cooler coil construction
A cooler coil is not merely a heater battery fed with chilled water or into which cold, liquid refrigerant is pumped. There are two important points of difference: firstly, the temperature differences involved are very much less for a cooler coil than for a heater battery, and secondly, moisture is condensed from the air on to the cooler coil surface. With air heaters, water entering and leaving at 85°C and 65°C respectively may be used to raise the temperature of an airstream from 0° to 35°C, resulting in a log mean temperature difference of about 53°C for a contra-flow heat exchange. With a cooler coil, water may enter at 7°C and leave at 13°C in reducing the temperature of the airstream from 26°C to 11°C, a log mean temperature difference of only 7.6°C with contra-flow operation. The result is that much more heat transfer surface is required for cooler coils and, as will be seen in section 10.3, it is important that contra-flow heat exchange be obtained. The second point of difference, that dehumidification occurs, means that the heat transfer processes are more involved in cooler coils.
There are three forms of cooler coil: chilled water, direct expansion, and chilled brine. The first and third types make use of the heat absorbed by the chilled liquid as it is circulated inside the finned tubes of the coil to effect the necessary cooling and dehumidification of the airstream. The second form has liquid refrigerant boiling within the tubes, and so the heat absorbed from the airstream provides the latent heat of evaporation for the refrigerant.
Chilled water coils are usually constructed of externally finned, horizontal tubes, so arranged as to facilitate the drainage of condensed moisture from the fins. Tube diameters vary from 8 to 25 mm, and copper is the material commonly used, with copper or aluminium fins. Copper fins and copper tubes generally offer the best resistance to corrosion, particularly if the whole assembly is electro-tinned after manufacture. Fins are usually of the plate type, although spirally wound and circular fins are also used. Cross-flow heat exchange between the air and cooling fluid occurs for a particular row but, from row to row, contra — or parallel-flow of heat may take place, depending on the way in which the piping has been arranged. Figure 10.2(a) illustrates the case of a single serpentine tubing but a double serpentine, and other arrangements, are also used. In the double serpentine form shown in Figure 10.2(&) two pipes from the flow riser feed the first and third rows with two tubes from the second and fourth rows leading to the return header. Contra-flow connection is essential for chilled water coils in all cases. In direct expansion coils, since the refrigerant is boiling at a constant temperature the surface temperature is more uniform and there is no distinction between the parallel and contra-flow, the logarithmic mean temperature difference being the same. However, with direct-expansion cooler coils a good deal more trouble has to be taken with the piping in order to ensure that a uniform distribution of liquid refrigerant takes place across the face of the coil. This is achieved by having a ‘distributor’ after the expansion valve, the function of which is to divide the flow of liquid refrigerant into a number of equal streams. Pipes of equal resistance join the downstream side of the distributor to the coil so that the liquid is fed uniformly over the depth and height of the coil. It may be necessary to feed the coil from both sides if it is very wide. The limitations imposed by the need to secure effective distribution of the liquid refrigerant throughout the coil tend to discourage the use of very large direct-expansion cooler coils. Control problems exist.
All cooler coils should be divided into sections by horizontal, independently drained, condensate collection trays running across their full width and depth. Opinions seem to differ among manufacturers as to the maximum permissible vertical spacing between condensate drip trays. Clearly it depends on the sensible-total heat ratio (the smaller this is the greater the condensation rate), the spacing between the fins (the narrower the spacing the more difficult it is for the condensate to drain freely) and the face velocity (the faster the airflow the more probable the carryover of condensate). Fin spacings in common use are 316, 394 and 476 per metre (8,10 and 12 per inch) and the thicknesses used lie between 0.42 and 0.15 mm. (Thinner fins, incidentally, tend to grip the tube less tightly at their roots and perhaps give poorer heat transfer.) Fins may be corrugated or smooth, the former reducing the risk of carryover while improving the heat transfer by a small increase in the surface area of the fins. An analysis of manufacturers’ data suggests that for cooling coils having sensible-total heat ratios of not less than 0.65 the face velocities listed in Table 10.1 should not be exceeded without the provision of moisture eliminators.
Table 10.1 Fin spacing and air velocity
The maximum vertical distance between intermediate drain trays should desirably not exceed 900 mm and more than 394 fins per metre should not be used with coils having large latent loads when the sensible-total heat ratios are less than 0.80. For sensible-total ratios less than 0.65 and for sprayed coils more than 316 fins per metre should not be used. When the sensible-total ratio is between 0.8 and 0.95 it is possibly safe to have drain trays 1200 mm apart, provided the face velocity and finning conforms with the suggestions in Table 10.1. Coils with sensible-total ratios exceeding 0.98 are virtually doing sensible cooling only and the risk of condensate carryover is slight. Water velocities in use are between 0.6 and 2.4 m s-1, in which range the coils are self-purging of air. Water pressure drops are usually between 15 and 150 kPa and air pressure drops are dependent on the number of rows and the piping and finning arrangements. A coil that is doing no latent cooling offers about one-third less resistance to airflow. Typical air-side pressure drops for a four row coil with 2.25 m s-1 face velocity are 60 to 190 Pa, when wet with condensate.
Condensate trays should slope towards the drainage point and there must be adequate access for regular cleaning. It is important that the piping connection from the drainage point should be provided with a trap outside the coil. This must be deep enough to provide a water seal of condensate that will prevent air being sucked into the drainage tray in the case of draw-through coils, or blown out in the case of blow-through coils. Condensate will not drain freely away unless there is no airflow through the outlet point. The trap should feed condensate through an air gap into a tundish, prior to the condensate being piped to the drains. The air gap is necessary so that the presence of condensate drainage can be verified by observation and, for hygienic reasons, to ensure there is no direct connection between the main drains and the air conditioning system. See Figure 10.5.
After installation the aluminium fins on cooler coils do not give uniform condensation over their entire surface area until they have aged over about a year of use. ASHRAE (1996) mentions the development of a hydrophylic surface coating for aluminium fins that reduces the surface tension of the condensate and gives a more uniform distribution on the fins from the start.
Careless handling in manufacture, delivery to site and erection often causes damage to the coil faces, forming large areas of turned-back fin edges that disturb the airflow, collect dirt from the airstream and increase the air pressure drop. The fins in such damaged areas must be combed out after installation before the system is set to work.
Other materials are sometimes used for air cooler coils but ordinary steel coils should never be used because of the rapid corrosion likely. Stainless steel is sometimes used but it is expensive and, because its thermal conductivity is less than that of copper, more heat transfer surface is required.
Air cooler coils tend to be wide and short, rather than narrow and tall. This is because it is cheaper to make coils with this shape, there being fewer return bend connections to make (where tubes emerge from the coil casing). It is also because a short coil drains condensate away more easily: with a tall coil there is the likelihood of condensate building up between the fins at the bottom of the coil, inhibiting airflow and heat transfer and increasing the risk of condensate carry-over into the duct system.
A consequence of the wide shape of cooler coil faces is that airflow over them is likely to be uneven, the airstream tending to flow over the middle of the coil face. This is usually dealt with in air handling units by using multiple fans in parallel.
Blow-through coils are sometimes used but may give unsatisfactory results because the airflow discharged from a fan is very turbulent and, even if multiple fans in parallel are used, air distribution over the coil face will be uneven. The coil should be as far as possible from the fan outlet in order to give the turbulent airstream a chance to become smoother.
With draw-through coils, smooth holes having belled edges are normally provided in the casings where the tubes emerge to join the headers or return bends. The holes are slightly larger than the outer diameters of the tubes in order to provide a clearance space for thermal movement. It follows that, with blow-through coils, condensate will be blown out of such clearance spaces. This cannot be allowed and coils used for blow-through applications must have the clearance spaces sealed by the manufacturer.
Galvanised steel casings are often used for coils with copper tubes and copper or aluminium fins. This is a poor combination since copper and zinc in conjunction with slightly acidic condensate favour electrolytic corrosion. If possible, other materials should be used for cooler coil casings.
Drain cocks and air vents should always be provided for cooler coils using chilled water or brine.
Posted in Engineering Fifth Edition