When a heat exchanger "stalls," condensate floods the steam space and causes a variety of problems within the exchanger:
Control hunting: As condensate backs up in the exchanger, the heat transfer rate to the process is significantly decreased. The control valve opens wide enough to allow flow into the exchanger. As condensate drains out, the steam space is now greater and the steam pressure increases. The process overheats, the control valve closes down, and the cycle repeats.
Temperature shock: Condensate backed up inside the steam space cools the tubes that carry the process fluid. When this sub-cooled condensate is unexpectedly replaced by hot steam due to poor steam trap operations, the expansion and contraction of the tubes stress the tube joints. Frequently repeating this cycle causes early failure.
Flooding - A flooded heat exchanger will allow the oxygen to dissolve, in addition to carbon dioxide and other gases found in the steam. Since the condensate is often sub-cooled due to the time it is in the exchanger, these gases are more quickly dissolved. Together the cool condensate and dissolved gases are exceedingly corrosive and will tend to decrease the effectiveness of the heat exchanger and decrease the heat transfer through the tubes.
Steam collapse - Under very low loads with the steam valve closed, the steam volume collapses to smaller volume condensate, inducing a vacuum. When the vacuum breaker opens, atmospheric air and condensate mix inside the exchanger, increasing the risk of corrosion of the tubes, shells, tube sheet and tube supports.
Freezing - Steam/air coils cannot afford poor condensate drainage, particularly if the coil experiences air below freezing temperature. Condensate backed up inside the coil will freeze, often within seconds, depending on the air temperature. A low temperature detection thermostat is suggested on the coil leaving side to sense freezing conditions.
As earlier explained, the only way to avoid "stall" is to eliminate back pressure on the steam trap. There are a number of options available for designing a system that greatly reduces the risk of "stall." The following are two such options:
o Set up the heat exchanger in a position so that the condensate freely drains by gravity to the condensate return line. In many cases this is not possible on account of existing piping around the area in which the heat exchanger is needed (e.g., the heat exchanger is installed at a level lower than the condensate return tank).
o Use an electric or pressure driven condensate pump package installed below the steam trap to pump condensate back to the boiler.
In actual practice, the first option may not be possible, and so the use of electric or pressure driven pumps to return condensate to the boiler room should be considered.