Condensate Drainage - Recommended Piping

When using heaters with a modulating control valve, it is most important not to attempt to lift condensate from the float and thermostatic traps. Attempting to lift condensate leads to poor temperature control. Any condensate lifting will be from the bottom and the lift line will always be full of condensate. If the lift line is higher than the submerged coil, the coil will flood with condensate whenever the modulating control valves closes. This will result in temperature overshoot, as the hot condensate will transfer heat through the coil and over heat the water.

It is necessary for the designer/installer to make provisions to drain the condensate by gravity or into either an electric or pressure powered condensate pump. All Cemline heaters are
furnished with modulating control valves and as these valves close, the steam pressure in the coil is reduced to zero psig.

An example follows. A heater designed to operate on 15 psig steam is installed in an improperly designed system with 10 feet of condensate lift. A 10-foot column of water will have a pressure of 4.5 psig. The pressure drop across the valve will typically be 30%. This means the maximum pressure in the coil head is 10.5 psig. The steam condensing in the coil will further reduce the pressure in the coil to a pressure less than the 4.5 psig required to lift the condensate. The heater will stall until no more steam can enter because the back pressure from the 10-ft head of the condensate is greater than the available pressure in the coil. The valve, sensing the temperature in the heater is dropping, opens wider which allows more pressure into the coil and overcomes the 4.5 psig and lifts some of the condensate. This influx of steam will bring the heater to the set and as the control valve closes, the pressure in the coil goes to zero psig. As soon as the pressure drops below 4.5 psig, no more condensate is removed from the coil and the 10-ft head of condensate will then back feed through the trap and flood the coil with 250 degree F condensate. The coil transfers the heat from the 250 degree con condensate to the domestic water, resulting in an over temperature condition in the heater.

It is the responsibility of the designer/installer to properly drain the condensate. Cemline Corporation is represented by trained, expert sales agents in steam systems. Please contact the factory or one of our sales agents for assistance.

Remember, Do not attempt to lift the condensate from a water heater or Unfired Steam Generator.

SOURCE: www.cemline.com/liftcond.asp
posted by permission of Cemline