Why is your steam regulated heat exchanger ticking

  

You probably recognise it: the ticking of heat exchangers that use steam to heat a secondary medium such as water or air. During the off-season especially - i.e. when capacity demand is less than 100% - heat exchangers produce some disturbing noises for central heating. Moreover, the temperature also fluctuates to an intolerable extent increasing the risk of severe water hammer. There is also a risk that the steam-pipe could burst, which may result in an accident. The question is: why can even the most accurate temperature regulation not prevent this?

Reason: condensate

The ticking is caused by condensate on the primary side (steam side) of the heat exchanger. When the hot steam comes into contact with this relatively cold condensate, the steam immediately cools off and implodes. This is referred to as 'thermal water hammer'. Condensate remaining in the heat exchanger could be caused by several factors:

  • Incorrect steam trap

If the steam trap is too small it is only logical that it would not drain condensate sufficiently. If an incorrect type of steam trap is chosen - e.g. a thermal type - the condensate would supercool before it is discharged by the steam trap. In both situations condensate remains in the heat exchanger.

 
  • Differential pressure

If pressure in the heat exchanger is lower than in the condensate system (in that case the control valve must be closed on the secondary side due to a decreasing demand), steam pressure reduces in the heat exchanger. Often this effect is enhanced by the use of oversized heat exchangers. This is often due to an built-in margin when calculating the capacity required. Heat exchangers are only available in predetermined capacities and the selected capacity often exceeds the calculations as described above, which was already too high.

Let us consider what impact the above causes have on reality..

Figure 1: oversized heat exchanger suffering the effects of stall

Incorrect steam trap

The heat exchanger as shown in figure 1 is oversized and is drained by a steam trap that is too small or incorrect. Due to the excessive capacity the heat exchanger will still pre-heat air to the desired temperature, even when it is only half-filled with condensate. Condensate is often aggressive and will attack the material of the heat exchanger at the points where it remains, with negative consequences. In the situation as shown in figure 1, the condensate may even freeze as a result of the cold inflowing air (0°C) that flows at a velocity of 3 m/s along the heat exchanger. This may result in that the heat exchanger rupturing, which would require repairs or even replacement.

Lower pressure

The examples shown in figures 2 and 3 clearly illustrate how the pressure in the heat exchanger drops below the pressure in the condensate system. These examples are based on the following principles:

  • The steam pressure is 4 bar(g)
  • The temperature is approximately 152°C
  • The condensate system is depressurised
  • The system operates in the winter at an inlet temperature of 70°C and an outlet temperature of 90°C
  • In the off-season the inlet temperature is lowered to 45°C and the outlet temperature to 55°C. 

We will first draw  a line (AB) from the inlet temperature of the secondary medium to the desired outlet temperature, then a line (CB) from the  steam temperature at 4 bar(g) to the desired secondary outlet temperature, and finally a line (DE) that shows the saturation temperature of the steam for the counter-pressure in the condensate system. Without counter-pressure the saturation temperature is 100°C. When there is sufficient demand for heat the control valve is fully open and the steam condenses. There is also sufficient pressure present in the heat exchanger to force the condensate through the steam trap.


 

Figure 2: Winter situation ‚Äč

Figure 3: Off-season

When the demand for heat decreases the control valve pinches (closes) and the pressure in the heat exchanger falls. This can continue until the pressure in the heat exchanger is equal to the pressure behind the steam trap: the 'critical stress point'. From this point - where lines CB and DE intersect - condensate is no longer being drained. In this example a vacuum can even be created if the demand for heat drops even further. The heat exchanger will now fill with condensate. 

As soon as the demand for heat increases slightly fresh hot steam flows against the relatively cold condensate, which in turn creates the ticking sound and - in the worst case scenario - even water hammer in your heat exchanger. In the winter situation (figure 2) the critical point is at 16%. This means that with a load of <16% condensate drainage does not takes place. In the off-season (figure 3) the critical point is already at a load of 46%, however.


Figure 4: winter situation with counter-pressure

Counter-pressure effect

Since the condensate mostly returns to a condensate tank after the heat exchanger, counter-pressure will often be present in the condensate return system. The application of risers provides counter-pressure and a specific pressure in the condensate tank. When combined with pipe resistance this results in total counter-pressure. The effect on the critical point of the heat exchanger is clearly shown in figure 4, which shows the same heat exchanger under the same conditions as shown in figure 2. 

In this example a pressure of 1 bar(g) exists in the condensate return system, which corresponds to a saturation temperature of 120°C.

Due to the counter-pressure line DE ends up higher and it already intersects line CB at 48% instead of 16%, as shown in figure 2. Higher counter-pressure therefore has a negative effect on the discharge of condensate: stall is created in the heat exchanger and the efficiency of the heat exchanger reduces.

Other effects include:

  • Fluctuations in the outlet temperature of the secondary medium
  • Constant shuttling between open and closed control valve 
  • A 'cold' steam trap
  • Water hammer
  • Reduced capacity of the heat exchanger
  • Reduced product quality
  • Corroding heat exchangers (leakage)

Condensate discharge under vacuum load

All of the above factors have a negative impact on the performance of the installation and solutions must be sought to remove condensate under all circumstances. Three solutions are discussed below.
 

Method 1: Free outlet and vacuum breaker

In figure 5 the steam trap has a free outlet, which means that there is no counter-pressure present behind the steam trap. A vacuum breaker is placed in the steam supply pipe between the control valve and the heat exchanger. As soon as a vacuum - and therefore stall - occurs in the heat exchanger, the vacuum breaker vents the heat exchanger. This removes the vaccuum in the heat exchanger, causing the condensate to freely flow from the heat exchanger. The condensate is lost in this arrangement, but it can also be collected in an atmospheric vessel and discharged through a pump. 

 













Figure 5: Static head, free outlet and vacuum breaker












Figure 6: Auxiliary drain method in combination with vacuum breaker
 

An alternative to this arrangement is shown in figure 6, in which not all of the condensate is lost. As long as a higher pressure remains in the heat exchanger than in the condensate system, the condensate is discharged through the condensate return system. When stall occurs as a result of the  pressure in the heat exchanger becomes too low, the condensate will be discharged through an auxiliary steam trap which vents to atmospheric pressure.

Method 2: Steam driven, steam trap pump combination

In figure 7 the condensate is discharged into the condensate return system. As long as there is a positive pressure difference, the combined steam trap pump operates as a standard steam trap. If the pressure difference is negative the reservoir in the steam trap pump runs full and the float rises until a mechanism switches over after some time, thereby opening the steam valve. 


Figure 7: Steam-driven, Pump-trap method

The steam pressure is sufficient to push away the condensate in the condensate system. Advantage: the combined steam trap pump requires  little space and it is suitable for capacities up to 2,500 kg/h. For optimum operation the steam trap pump must be placed lower than the heat exchanger. Also during the pump stroke the condensate must flow from the heat exchanger into a tank, located between the heat exchanger and the steam trap pump.


Figure 8:  Combination pump and steam trap

Method 3: Steam-driven pump with separate steam trap

The method shown in figure 8 is in principle similar to Method 2. However, here the condensate flows to and from a tank to a steam-driven pump in which the same pressure prevails as in the heat exchanger. 

With a positive pressure difference the steam trap discharges the condensate. If there is a negative pressure difference, the pump fills and the float in the pump rises. When the pump tank is full the steam valve opens and the condensate is pushed away. The steam-driven pump is suitable for large capacities (from 2,500 kg/h). When atmospheric pressure is created in the tank, condensate can be fed from multiple locations.

Conclusion

It is important to prevent stall in the heat exchanger. By applying the correct dimensions during the design of the heat exchanger and by ensuring efficient dewatering and condensate discharge, the efficiency, reliability and longevity of your installation is guaranteed.


Eric Kanaar, Steam Product Group Manager
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