Heat Exchangers and Stall

Stall is the reduction or the cessation of condensate flow from the heat exchanger, and occurs when the pressure in the heat exchanger is equal to, or less than, the total backpressure imposed on the steam trap.

Lower than expected pressure in a heat exchanger may occur as a result of any of the following circumstances:

  • The secondary fluid inlet temperature rising as a result of a falling heat load.
  • The secondary fluid flowrate falling as a result of a falling heat load.
  • The secondary fluid outlet temperature falling due to a lowering of the set point.

As the control valve reduces the steam pressure to meet a falling heat load, the lack of differential pressure across the steam trap causes condensate to waterlog the steam space, as shown in Figure 13.1.1.

теплообменники для конденсатаFig. 13.1.1 An air heater battery suffering the effects of stall

Due to applied safety factors and because heat exchangers are sold in pre-determined sizes, they often have more heating area than required. This has the effect of increasing the heat transfer capability of the exchanger above that required. It also means that the operating steam pressure will be lower than in a comparable heat exchanger perfectly sized for the same duty. The result is that less steam pressure is available to push out the condensate than may be expected. The steam pressure in the heat exchanger is important because it influences the stall condition, which in turn affects trap selection.

Before any trap selection and sizing can take place, it is necessary to determine whether or not stall will occur, and if it does, to what degree. If this is not done, it is likely that the heat exchanger will suffer from waterlogging for some or all of its operating life. This, when it occurs, may not be immediately recognised by the observer or operator, as operating performance might not be reduced in an oversized heat exchanger. However, waterlogging can have severe financial consequences, short and long term, unless the heat exchanger is designed to operate this way.

Short-term problems

Consider an oversized heater battery operating as a frost coil and fitted with the wrong type (or size) of trap, as in Figure 13.1.1.

In this example, the frost coil is preheating chilled air before it passes on to the main heater battery. Though the frost coil is fulfilling its thermal expectations (because it is oversized for the duty), it will do so with the bottom half of its coils waterlogged. Incoming cold air approaching 0°C (typically flowing at 3 m/s) passing over the coils can easily cause the water in them to freeze. This results in having to repair or replace the heater battery, either causing inconvenience or unexpected outlay.

Waterlogging and freezing will not arise if the application is correctly designed.

Long-term problems

Traps that are undersized will sometimes show no immediate adverse effects on heater performance if the heater is oversized.

Ironically, the wrong type of trap fitted to a heat exchanger can often exaggerate a superficial improvement elsewhere in the condensate system. For instance, a thermostatic or fixed orifice fitted to any heat exchanger will hold back condensate so that it sub-cools below the steam saturation temperature. This will have the effect of reducing flash steam from any natural outlet such as a condensate receiver vent. The casual observer can interpret this as a way to save energy and can easily be tempted to fit these devices. Unfortunately, the situation is not as straightforward as it seems. The reality is that holding back condensate until it sub-cools implies waterlogging to some degree. Condensate that continually floods the steam space will cause corrosion with costly results. The service life of the heat exchanger is reduced, and the overall lifetime costs of the installation will increase.

The effects suffered by a waterlogged heat exchanger depend upon the circumstances of the particular installation.

The symptoms and effects of stall are itemised later in this Tutorial.