Heat Exchangers And Stall

The primary side of the heat exchanger will be referred to as the ‘steam space’, and the steam trapping device will be referred to as the ’trap’. The ‘trap’ can be a ‘steam trap’, a ‘pump trap’, or a ‘steam trap and pump’ fitted in combination.

On these installations, a control sensor monitors the temperature of the outgoing heated fluid in the secondary circuit. The control valve endeavours to maintain a temperature determined by the controller, regardless of variations in heat load. The valve achieves this by opening or closing to alter the flowrate of steam, thereby varying the steam space pressure.

The discharge from the steam trap may be subject to a lift and/or pressure in the condensate line, or may fall to an open end where it is subjected only to atmospheric pressure. This Block will refer to condensate pressure as ‘backpressure’.

The heat exchange equipment can be almost anything that meets the above criteria. Examples include:

• Shell and tube heat exchangers.
• Plate heat exchangers.
• Air heating coils or batteries in ductwork.
• Pipe runs or pipe coils in process equipment, tanks, vats etc. For brevity, this Block will refer to all such devices as ‘heat exchangers’ or ‘heaters’, and the passage of fluid being heated by the heat exchanger will be referred to as passing through the ‘secondary’ side of the heat exchanger.

The performance of steam heat exchangers is often reduced due to condensate flooding the steam space and waterlogging. The two main causes of waterlogging are:

• Fitting the wrong type of trap.
• Stall. Important note Some systems aim to achieve control of temperature by positively encouraging partial flooding of the steam space of the heat exchanger. In these cases, the modulating action of the control valve at the condensate outlet varies the condensate level in the steam space. This changes the area of heating surface exposed to steam, and the effect is to change the heat transfer rate so as to control the secondary outlet temperature.

With systems of this type, it is important that the heat exchangers be designed and manufactured specifically to withstand the effects of flooding. Where this is not done, the presence of condensate in the heat exchanger will have an adverse effect on operating performance and will reduce service life.

This method of control can have certain benefits if the system is designed correctly. One is that the condensate sub-cools in the heat exchanger before it is discharged. This can considerably reduce the amount of flash steam in the condensate pipework, which may improve the performance of the condensate system and also reduce heat losses.

The main operational disadvantage is that systems of this type are slow to respond to variations in heat load. What is meant by 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.