If it is a blow through air handler with a housed fan, you need room for the air to transition from the blower discharge to the coil.
If it is draw through, you most likely need space for the motor, then some working clearance between the motor and the coil like 6 inches. If the blower frame is internally isolated this may encroach a bit also.
upstream of a coil, I do not think you really need much clearance at all if the face area for a flat filter section, or the 'projected face area' (for lack of a better term) of an angle filter section are roughly the same as the coil(s).
Sometimes, if you want to plan ahead for good service on a big air handler, you allow an access section rather than having a tech remove filters and reach through the filter track to clean the coil surface. Access section will most likely add 2 feet to the length.
With heating and cooling coils in series, you may want to add access between the two coils.
Take the "V" out of HVAC and you are left with a HAC(k) job.
Thanks Abby for your response.It is a draw through syustem.The fan is DIDW and will be located approx centre of the coil.The coil stack is approx 4.2 m wide and 2.4 m high.
I am wondering if I need to keep a certain minimium distance between the fan and coil in order to minmise the possibility of preferential air flow through the middle portion of the coil.If that happens,it might render the coil periphery areas ineffective.
I was thinking of a perforated plate between the coil and the fan.The plate need not cover the full coil face but only the central area where preferential air flow can take place.What do you think?
With a draw through variations in localized coil velocities be more stable.Unless the wall to inlet clearance is equal to at least one inlet diameter, some allowance must be made for the system effect of the plenum walls.
Attention must also be given to the path that the airstream follows in approaching the fan inlets. If this is such that the airstream enters the inlet unevenly or if spin is generated, loss of performance will result. If it is not practical to alter the flow pattern ahead of the inlet, some improvement can usually be obtained by the use of baffles at the fan inlet.
For velocitie outlets to 2,500 fpm, the 100 percent effective duct length is 2-1/2 duct diameters (or diameters of equivalent rectangular area). As the blast area (area over the cutoff) decreases in proportion to the outlet area, the flow distortion increases, and potential system effect losses increase. The blast area normally has to be obtained from the manufacturer.
Its a large AHU and good practise would be to allow for access to both side of the coil (say 600mm between filters and coil), and allow space in the fan chamber to get past the fan for maintenance.
Rule of thumb is to allow at least 3 fan diameters from coil to centre line of fan. For a large build up AHU I like to allow at least 600mm between the condensate trays and edge of the fan/motor base.
