Face and by-pass dampers

This is a method used to vary the output of a cooler coil in order to respond to changes in the sensible heat gain in a conditioned room, without using reheat and without using an automatic motorised valve in the chilled water supply to the coil.

Figure 8.6 illustrates the plant and shows the psychrometry for the simplifying assumption that 100 per cent fresh air is handled. Under design conditions the by-pass dampers are fully closed and all the airflow is over the face of the coil, the dampers of which are fully open. In the usual way, air is cooled and dehumidified from state O to state W. The apparatus dew point is A and the corresponding mean coil surface temperature is tsm. Fan power and duct heat gain cause the air temperature to rise from? w to ts. Air at this temperature is supplied to the room in order to maintain the design state R therein.

If the sensible heat gains in the room reduce, the room thermostat sends a signal to the motorised dampers on the cooler coil which causes the face dampers partly to close and the by-pass dampers partly to open. Less air flows over the coil. The two consequences of this are:

Q * by-pass









W — or M




Face and by-pass dampers

Design Reduced sensible sensible gain gain


подпись: (a)№)

Fig. 8.6 Plant and psychrometry for face and by-pass dampers. 100 per cent fresh air is shown to simplify the psychrometry. The system works equally well with recirculated air.

(i) The cooling load reduces and so the rise in temperature of the chilled water flowing inside the tubes of the coil is smaller. Hence the mean coil surface temperature falls from /sm to tsm>, in Figure 8.6(b), and the apparatus dew point, A, slides down the saturation curve to become A’.

(ii) With the reduced airflow rate the face velocity over the coil is less and hence the contact factor improves. See section 10.4.

The outcome of this is that the air leaving the face of the coil is drier under partial load conditions, with face and by-pass dampers, than it is under design conditions.

Air at state W, leaving the face of the coil, mixes with air at state O, by-passing the coil, to form a mixture state, M. Fan power and duct gain cause a temperature rise and the air is finally supplied to the room at a state S’. Figure 8.6 shows that state S’ has the correct temperature to deal with the reduced sensible heat gain without the use of reheat. The humidity in the room will probably rise above the design condition but this is seldom important for comfort conditioning. The fact that the air leaving the coil face is drier than under design operating conditions is helpful in countering the tendency for the room humidity to rise.


If a room suffers sensible gains of 11.70 kW and latent gains of 3.15 kW when 22°C dry — bulb with 50 per cent saturation is maintained therein during outside design conditions of 28°C dry-bulb with 19.5°C wet-bulb (sling), calculate the inside conditions if the sensible gain diminishes to 5 kW, the latent gain remaining at 3.15 kW and the outside condition staying at the design value.

The plant comprises air intake, pre-heater, cooler coil with face and by-pass dampers (used for thermostatic control of room temperature), supply fan handling 1.26 kg per s of fresh air, and the usual distribution ducting. Assume that the performance of the cooler is unaltered by variations in air flow across it and that the temperature rise due to fan power etc. is 3°C.


It can be calculated that the supply state under design conditions is 13°C dry-bulb with 7.352 g kg-1. The supply temperature for dealing with a sensible gain of 5 kW is approximately 18°C dry-bulb, and so the air temperature needed after the face and by-pass dampers must be 15°C at partial load.

The psychrometric changes involved are illustrated in Figure 8.7.

Assuming that the dry-bulb scale is linear, a good approximate answer is as follows:

15° — 10°

8w’ — 8vi 2g° _ |Q° ^ (8o ~ <?w)

= 7.352 + 4- x (10.65 — 7.352)


= 7.352 + 0.916 = 8.268 g kg’1 8r’ = gv/’ + moisture pick-up for the design latent load = 8.268 + (8.366 — 7.352)

= 9.282 g kg“1 The room relative humidity is about 56 per cent.

Face and by-pass dampers

Fig. 8.7 Psychrometry for example 8.4.

Posted in Engineering Fifth Edition