GUIDANCE NOTES ON THE SIZING AND INSTALLATION OF COOLING COIL AND HUMIDIFIER DRAIN TRAPS
The necessity of fitting traps to air handling plant cooling coil and humidifier drain pans to enable the efficient removal of condensate water is generally appreciated by all engineers.
Problems can arise due to incorrect design and/or installation of traps. It is with this in mind that we set out in some detail the mechanics of an effective trapping arrangement together with the basic theory behind its design.
Too often the trapping is forgotten about until the installation of the plant is complete, by which time it is too late to incorporate a correctly designed drain system. The mounting height of the plant (i.e. the height of the builder’s work plinth or steelwork above the floor) should be sufficient to accommodate the depth of the trapping arrangement. If the trap cannot be fitted in the height available you have a major problem – the trap cannot be made shallower. In some cases it has been necessary to drop the drain pipe through the slab and put the trap at high level on the floor below also there are occasions when channels have to be cut into the slab.
Drain Tray Pressure
The first thing to consider is whether the condensate tray to be drained is situated upstream or downstream of the fan. The majority of air handling units installed are configured with the supply fan as the final element, the coil block is thus sited upstream and the drain tray will therefore be subjected to a negative pressure with respect to the atmosphere. Air handling units of the “blow-through” type, having the coil block downstream of the supply fan, will impose a positive pressure on the drain tray.
If no trap is fitted to a drain tray under negative pressure, the resulting inrush of atmospheric air through the drain connection will usually be sufficient to hold back the condensate entirely, resulting in the tray overflowing and flooding the plant.
Equally, if the drain tray is under positive pressure a jet of system air will issue from the drain connection at high velocity, together with the condensed water, resulting in (a) a waste of treated air and (b) a liberal soaking of everything (and everyone) around the point of discharge of the drain pipe. A correctly designed trap will eliminate these problems.
Having decided whether the tray will be subjected to positive or negative pressure, the magnitude of this pressure should now be determined. This is NOT the total pressure developed by the fan but depends on the configuration of the air handling unit and the resistance of any external ductwork connected to it.
It is advisable to try and ascertain the pressure from the manufacturer as he has the details of the pressure drop through the various elements of the plant and, in sizing his fan resistance, will have made allowance for the external resistance (supplied by you). If this information is not forthcoming it will be safe to use the fan suction pressure or fan discharge pressure as applicable. These should be TOTAL STATIC pressures, the velocity pressure will not affect the drainage system.
Recommendations for sizing of trap seal depths
To correctly size condensate traps the following information is required:
Total static fan pressure (Pascals) • Location of trap viz, upstream/downstream of fan • Unit drain size.
Detailed below are two examples based upon a total static fan pressure of 1500 and 200 pascals respectively.
Example – Positive Side
Unit total static fan pressure 1500 Pascals Seal calculated:
Example – Negative Side
Unit total static fan pressure 2000 Pascals Seal calculated:
“B” dim = Wherever possible should equal “A” If height restrictions, can accept 50% of “A” but not less than a minimum depths as detailed on folio 2
N.B. a) THE ABOVE CALCULATIONS APPLY TO ALL BORE SIZES