The object of predicting steam pressures and their corresponding steam loads is to enable the selection of the correct trapping device for any application.
In this instance, the trapping device would be selected on the following information.
A ball float steam trap can be specified if it meets the following two criteria satisfying the initial brief in Example 13.7.1:-
1. It can pass the full-load condition, i.e. 543 kg / h at 5.2 bar differential pressure
2. It can pass the maximum stall load, i.e. 224 kg / h at the 60°C set point
Creating a static head above the ball float steam trap
At the stall condition, with the steam pressure inside the heat exchanger equalling the backpressure, a differential pressure would not exist to push the condensate through a ball float steam trap. Because of this, pressure has to be manufactured on the upstream side of the trap by means of a static head.
Static head must be available between the heat exchanger condensate outlet and the trap inlet to generate enough differential pressure to enable the trap to pass the stall load of 224 kg / h. In order to allow condensate to drain easily from the exchanger, a vacuum breaker is fitted to its steam inlet downstream of the control valve (Figure 13.7.8).
It can be seen in Figure 13.7.9 that a DN25 (1") FT10-10 ball float steam trap will accommodate these criteria. However, the trap requires a minimum of 4 metres head above the trap inlet to pass the stall load.
A 4 metre head might not be available in practice, and, if so, a larger trap would need to be specified. Refer to Figure 13.7.8.
For the purposes of Example 13.7.1, if the available head were only 200 mm then it can be seen from Figure 13.7.10 that a DN40 (1½") FT10-10 ball float steam trap would be required.
Should the backpressure have been greater than atmospheric pressure, due perhaps to a lift after the trap and/ or a pressurised condensate line, then the same sizing routine could be carried out. Depending upon the amount of backpressure, it may be that even the largest sized steam trap cannot pass the required amount of condensate at stall.
Under these circumstances, a ball float steam trap cannot be specified, as the heat exchanger will flood at part loads. Instead, a pump-trap must be used, which is able to clear the condensate from the heat exchanger into the condensate system at any heat load.