Decisions about the number of coils, their locations in the refrigerated space, and the directions of air discharge are interrelated and influenced by room geometry, ceiling arrangement, and product placement. The sum of the refrigeration capacities of the coils must equal or exceed the design refrigeration load, but the number of coils into which the total capacity is divided is a compromise. A small number of large coils usually results in the lowest first cost (and lowest maintenance cost) because of the reduction in piping, valves, and controls for the coils. On the other hand, distribution of the total capacity into a large number of coils helps avoid pockets of high temperature in the space.
The goal of achieving uniform temperatures is dominant in the selection of the number of coils and their placement. Several guidelines frequently used by system designers include:
– select the coils and place them on the basis of a throw of 30 to 60 m (100 to 200 ft)
– blow the discharge air in the same orientation as the beams
– blow across doors and not away or toward them (Fig. 6.39)
– direct the discharge air down aisles
– direct the discharge air downward from ceiling coils in high-rise storage facilities
Sometimes, discharge air is ducted from the coil to convey it further, but most coils probably discharge directly into the space. There is a danger of ducts in low-temperature spaces becoming frosted without any automatic means of defrost. Coils are occasionally mounted near the floor for easy servicing (Fig. 6.40), and the air ducted to the ceiling. Dampers should be installed in the duct, and should close during defrost. Otherwise, the warm coil creates a chimney effect that retards the defrost rate. One of the situations where the floor-mounted unit would be considered is when coils are being installed in an existing facility where the roof structure may not be able to accommodate the weight of ceiling hung or rooftop units.
A concept that is gaining in popularity is the penthouse placement of coils, as illustrated in Fig. 6.41. The coils are clustered in a penthouse and the air directed downward and then in various horizontal directions using hort-elbow ducts. The penthouse installation of coils is likely to be the arrangement of highest first cost, but there are some operating advantages of this concept. Space that is otherwise available for the storage of product or processing operations is not occupied by coils. The arrangement yields’ a safer installation than ceilingmounted coils where the refrigerant piping is located in the refrigerated space where it is in danger of damage from collisions by fork-lift trucks. The valve groups needed for hot-gas defrost are located outside the penthouse where they can be serviced by technicians more comfortably, safely, and effectively. The refrigerant piping is outdoors above the roof. If there is a need for service personnel to work on the coils or the fans, they can stand on the metal grating of the floor of the penthouse. For such occasions it is possible to manually actuate the defrost to warm the penthouse above the space temperature for a short period of time. Perhaps 15% of industrial refrigeration air coils are now installed in penthouses.