Reducing 2

[caluna]

Hi,
I have a project to remove a number of rooftop AC units, with their duct-mounted hot water coils, in order to replace with newer gas-fired units.

There is one heating water circuit serving these HW coils, and another circuit, with a much smaller head and flowrate, on same circulating pump.

Would it be best to replace the pump or could this situation be taken care of with a bypass across pump including a balancing valve?

thank,

caluna

 

[friartuck]

Depends on the various loads. We need to know what the loads are to each circuit and how the pipework is arranged.

Friar Tuck of Sherwood

 

[caluna]

Hi. Well, the original load on the pump was 160 gpm at 45' head. I think the reduced circuit (without the 11 heating coils) will have only about 50 gpm at 35' head. Pump curve is fairly flat.

there is yet another pump loop with its own inline pump-but none of that part will change. (That pump is 25 gpm 40' head)

Both pumps are fed off a a header which comes from a steam/water heat exchanger.

I know I will have to change something here, but was hoping to make do with bypass line or new pump (and just have reduced flow from heat exchanger). Am not too sure how reducing one of the (essentially "parallel") loop flows would affect the whole.

 

[friartuck]

The cheapest way I agree is to cut off the dead leg and add a by-pass. This would need a commissioning valve on it to control the flow rate.

Alternatively, isolate the dead leg and fit a VSD on the pump. The flow is about 12l/sec which is a fairly large flow rate...I guess you have pipework over 4" diameter and a fairly big motor rating (perhaps 5.5kW)

You really need to do a costing exercise and weigh the cost of either...

a. Simple bypass and an oversized pump running and wasting energy
b.Fitting an inverter drive (and isolating the dead leg.
c.Fitting a new reduced duty pump.

If initial costs are the driving force, then I would guess a is your choice.

Friar Tuck of Sherwood

 

[quark]

Try out a reduced impeller size with the same pump. Flow bypass will cost you morethan 60% excess energy consumption. Controlling the discharge can be an option for you but as you are reducing the flowrate near to 30%, there may be problems with heating up of pump. Check with the manufacturer for safe minimum flowrate.

 

[lilliput1]

Check the control sequence. It could be that the smaller pump is used for freeze protection, keeping the water circulating in the coil. On HW coils subject to below freezing temperatures, it is typical to put freeze protection pump at each coil. This pump is controlled to run continuosly when outdoor air temperature is 40°F and below.