The Stall Chart - Constant Flow Secondary - Varying Inlet Temperature - Constant Outlet Temperature

- Constant Flowrate - Constant Inlet Temperature - Varying Outlet Temperature The Stall Chart - Constant Flowrate/Varying Outlet Temperature All systems discussed up to this point assume that the secondary fluid outlet temperature remains constant. In some applications, the outlet temperature may change with time. This will also change the heat load and affect the stall point.

Such changes often occur in process applications, and also heating calorifiers that change their outlet temperature to compensate for changes in ambient conditions.

If the highest heat requirement occurs when the control temperature (the set point) is at a maximum, any reduction in the set point will cause a reduction in the heat load.

A reducing set point will tend to increase the stall load, as shown in the following calculations.

Once the design conditions are known, the effect of reducing the set point can either be calculated mathematically as shown below or illustrated on a stall chart by means of proportionality.

Example 13.7.1 Initially, secondary water at a rate of 1.5 L/s enters a heat exchanger at 20°C and leaves at 70°C. It is observed via a pressure gauge on the steam inlet that the pressure in the steam space under these conditions is 5.2 bar g (TS = 160°C). The condensate drains down to a vented receiver in a plant room below the installation. (T(back) = 100°C). If the set point is reduced to 60°C, what is the effect on the stall point and the steam load at stall? Calculating the effect of reducing the set point arithmetically It is first necessary to establish the heat exchanger TDC from the full-load operating conditions and by use of Equation 13.2.2: