Proper Location of Air Separator and Expansion Tank? 1

[BradV]

Currently have a small (10-gal capacity) 20% propylene glycol/water closed loop system, including pump and two plate & frame heat exchangers, and a bladder-type expansion tank (piped near discharge of pump). Piping and filling methods trap air in top of two-pass heat exchanger.

1. Does it matter where the expansion tank is piped in relation to the pump / Should this be piped in the suction line of the pump?

2. Can I install an air separator in the suction line of the pump to remove the trapped air in the heat exchanger after system fill? Will this remove all my trapped air?

 

[DLANDISSR]

BradV:
Near the suction of the pump is the usual place for the location of the expansion tank. This is usually the lowest pressure in the system and is to most stable. It can actually be piped any where in the system.

An air separator in the suction line will remove air before it gets to the pump. This a good location. Air in the pump suction can cause a phenomenon similar to cavitation and cause damage to the pump.

An air separator will remove only the air which reaches it. An air separator can only remove air which has been carried along the piping to the separator. Air trapped at high points in the system may not be disloged by the liquid flow and will therefore remain trapped. To get rid of this trapped air you could install a manual valve to vent off the air or an automatic air eliminator, which is a float valve similar to a steam trap (the same people make both).

Be sure that the pressure in the expansion tank is sufficient to provide adequate NPSH for the pump and provide enough pressure so that there will be positive pressure at all times at the high points of the system. This is necessary so the air vents will work. If the pressure is negative at any vent location and you open the vent valve you will get more air into the system rather than venting it out.

A bladder type expansion tank will retain its charge of gas. A non bladder type expansion tank will eventually lose its charge of gas because the gas will slowly be absorbed into the liquid.

Be sure to provide a pressure gage near the expansion tank so you can be assured that the initial charge and system pressure is correct and you can monitor the pressure for maintenance and trouble shooting purposes.

 

[TBP]

The expansion tank is the point of no pressure change in the system. For the best removal of dissolved air in the system, the sequence should be: heat source (boiler or HX), expansion tank, air separator, pump suction pulling water or glycol mixture through the air separator. This means that the air separator is seeing the hottest water (just after the heat source), at the lowest pressure (between the expansion tank, and circ pump suction) in the system. This will be the closest the water will get to the boiling point in the system, and so will be the spot where any dissolved air will be prone to getting kicked out of solution.

It's not that other arrangements won't work - they will - but this one will provide the best results.