It is now necessary to find the heat load at which the system will stall. In order to do so, it is necessary to calculate the TDC for this heat exchanger from the design conditions.
At stall, the pressure in the steam space will equal the 0.5 bar g backpressure.
The saturation temperature of steam at 0.5 bar g is 111.6°C.
What is the heat load at stall?
From the heat transfer flowrate equation (Equation 2.6.5):
The selection of the trapping device will depend on whether the minimum heat load is higher or lower than the stall load.
The minimum load is quoted as being 60% of the full-load of 293 kW, therefore:
Minimum load = 0.6 x 293 kW = 176 kW
Stall load = 138 kW
As the minimum load is greater than the stall load, the system will never stall. It is therefore practical to fit a ball float steam trap, as there will always be a positive differential pressure across it.
However, the ball float steam trap has to be sized to carry both the full-load and the minimum load, and it is therefore necessary to calculate the steam flows and the corresponding steam space pressures at both conditions.
It is first necessary to calculate the secondary inlet temperature at the minimum load. This can be predicted by use of Equation 13.4.1:
The minimum load condition
From Equation 13.2.3:
This is the steam temperature at the minimum load of 176 kW, and is equivalent to a steam pressure of 1.0 bar g. The condensate pressure is 0.5 bar g. The differential pressure across the ball float steam trap at minimum load therefore equals 1.0 bar g - 0.5 bar g = 0.5 bar.
What is the steam flowrate (_{s(min)}) at the minimum heat load of 176 kW?
The minimum steam flowrate will depend upon the steam space pressure, which is 1.0 bar g with an enthalpy of evaporation of 2201.1 kJ/kg.
From Equation 2.8.1:
As it has been established that this system will not stall, a ball float steam trap is suitable. It is now necessary to size a ball float steam trap for operation up to the maximum system differential pressure of 3.5 bar and pass . . .
a) the full-load of 498 kg/h with a differential pressure of 3.4 bar g - 0.5 bar g = 2.9 bar g.
b) the minimum load of 288 kg/h with a differential pressure of 1.0 bar g - 0.5 bar g = 0.5 bar g.
It can be seen from the ball float steam trap sizing chart (Figure 13.4.1) that a DN25 (1”) FT14-4.5 will satisfy both of these conditions, and could be selected. However, if the minimum heat load were less than the stall load, then a pump-trap would have to be selected.
The methods of selecting trapping devices are further discussed in Module 13.8, ‘Practical methods of preventing stall’.