By Chad Edmondson (JMP) and Norman Hall (RLD)
One challenge to selecting a steam condensate return unit is the return temperature of the condensate after leaving the steam heat exchanger. This week we'll offer some suggestions to assist the engineer or designer.
There are many types of condensate return units. Pumped condensate return units vented to atmosphere such as Bell & Gossett's Domestic Model CC or CB are used the majority of the time. There are also applications where a pumpless units such as the Kadant Johnson Liqui-Mover® are used with heat exchangers.
Vented Condensate Return Temperature Units
Most condensate return units are vented. At 0 PSIG gauge pressure, water entering a receiver over 212°F will have some percentage flash into steam. This flashing could cause damage to the condensate receiver. A vented receiver will therefore have a maximum temperature of 212°F. Two possible concerns with this temperature are the pump seal temperature limitations and net positive suction head (NPSH).
Pump seal limitations for condensate pumps such as the Domestic CC, CB, and CBE style units are rated for 250°F. Since the receiver is vented, there is no concern about seal temperature for these Domestic models. The second concern is the pressure available at the pump suction to keep this high temperature water from flashing into steam. In general, the condensate pumps must have a low net positive suction head required (NPSHR) to handle the low pressure, higher temperature condensate.
We recommend you read the Bell & Gossett article about NPSH and pump seals for more details.
Condensate, Flash Steam and Air
When the condensate from the heat exchanger passes through the Float and Thermostatic (F&T) trap to a vented dry condensate return we know a percentage will flash to steam. The dry return pipe is filled with air prior to the steam trap opening. Once the trap opens there is a mixture of condensate, flash steam and air. The fact that there is a percentage of air mixing with the flash steam will result in a significant drop in temperature. There is also a BTUH loss through the pipe to atmosphere. Both of these act to reduce the water temperature before it enters the condensate unit.
The exact pump type required for the project may require specific information about the system; however, as a general rule, here are some:
- If the steam pressure to the heat exchanger is less than 10 PSIG, use a Bell & Gossett Domestic Series CC condensate unit rated for return temperatures between 200°F and 209°F.
- If the steam pressure to the heat exchanger is over 10 PSIG but less than 20 PSIG, use a Bell & Gossett Domestic Series CB condensate unit rated 2’ NPSH for 210°F condensate.
- If the steam pressure to the heat exchanger control valve is over 20 PSIG, use a Bell & Gossett Domestic Series CBE (Elevated) condensate return unit rated 2’ NPSH for 212°F condensate. This may also be an excellent application for a Kadant Johnson Liqui-Mover® pressure powered pumping unit, sometimes called a pumping trap.
Obviously steam traps must be in good working condition and not leaking steam into the condensate system. In all cases, remember if the condensate lifts into the receiver it must not lift above the heat exchanger steam trap.