Pump Question 2

[Dymalica]

On like chiller or cooling towers, is it better to pump through or suction through them? I would think pumping through (chiller or CT downstream) to overcome the pressure loss through the unit. I can understand their concern with downstream units, that there is higher pressure at that point which puts more stress on the piping joints and flanges, but it should be able to handle it. I've heard people say it is better both ways. What do you think?

 

[Jabba007]

Convention, as I have been taught, is to locate a pump before a chiller, after a boiler or cooling tower. I think it has to do with trying to get a warmest/lowest pressure spot in the hydronic system for deaeration. The dissolved gasses in the water will come out of solution at the warmest, lowest pressure spot. If you can combine these, it gives you the optimum location for the air separator.

Jabba

 

[marcoh]

I always put pumps in front of a chiller and pumps after a cooling towers.

Need to consider the NPSH of the pump. For a cooling tower, the cooling tower basin will provide positive NPSH. For a chilled water system the system pressurisation tanks/expansion tanks should be located on the pump suction and is typically easier to accomplish by locating the pump before the chiller.

 

[ChrisConley]

Convention, as I have been taught, is to locate a pump before a chiller, after a boiler or cooling tower.

It isn't just convention to pump away from a boiler, it is good design and piping practice. A couple of the good reasons:

1) Doesn't expose you boiler to the discharge pressure of the pump, which may be higher than the capacity of the boiler. (Viessman condensing boilers, for instance, can have a pressure rating of only 35 psi).

2) Locates the air seperator at the lowest pressure, hottest temperature point in the loop; enhancing air removal.

3) Locates the expansion tank such that the entire system is positvely pressurized and at no point in the system where a manual or automatic air vent is located will the system pressure drop enough to entrain air.

 

[willard3]

If you pump through the boiler you take advantage of the heat from pumping and you raise the boiling point of the water, a good thing. Since all commercial systems use air separators, air removal is not too much of a problem no matter how you pump. Location of expansion tank is a calculated value and auto air vents are trouble with time.

If you pump from cooling equipment, reducing pressure at heat exchanger will increase cooling.

 

[cry22]

Willard,
You afre doing the opposite of what every one out there is doing.
Food for thought.

 

[willard3]

Both work ....more food for thought.

Conventional wisdom is just that: conventional.
Physics doesn't depend upon convention.

 

[macmet]

I have only seen pumps before boilers and not after them.

 

[macmet]

I didn't mean for my last post to sound like I've seen every pump and every boiler in the world. But the ones I have seen have always had the pump before the boiler.

 

[GMcD]

I've been in the building systems design business for close to 30 years and what ChrisConley says is how I was taught, and the basic hydronics physics and practical applications support it. Boiler/hot water generators - ALWAYS "pump away" from the boiler - Google John Siegenthaler - Modern Hydronics. Cooling towers and open systems - always pump away from the sump/tank with the pump located level or below the base of the tank/cooling tower sump. If I see a pump supplying water into a boiler, it's a steam boiler feedwater pump. If I see a pump pumping water into a hot water boiler I'll know the designer or installer was ignorant.

Chillers: There is no hard and fast rule as there are so many primary/secondary and variable flow configurations for the chilled water it would make your head spin. However, 90% of all the chilled water installations I've seen and designed had the chiller pump pushing water into the chiller. From thousands of tons of campus chiller plant to a 10 ton air cooled chiller. Don't forget in a closed loop system with the expansion tank on the suction side of the pump to set your system minimum pressure, the pump could thoeretically be pushing into the chiller or sucking out of it - it really doesn't care as long as your minimum system pressure is set up properly to insure adequate pump suction pressure - everything downstream of the pump discharge is just friction head no matter where it is in the closed system.

 

[KiwiMace]

OK. I was confused and it looks like I wasn't the only one.

Thanks GMcD for being the one to clarify that half the world thinks of steam and the other half hot water when the subject of boilers is mentioned. I was wondering how you were going to pump away from a (steam) boiler.