Booster Pump Sizing - closed loop chilled water

[BronYrAur]

I need some theoretical help on sizing a booster pump as I do not have all the information needed (at least not yet) to do it correctly.

I have an 800 ton chilled water system serving multiple air handling units. At some point in time, three new AHUs we added to the system and are served by a new 4" line that takes off from the supply and return mains. The 3 new units are served from the 800 ton plant during the winter, but not during the summer. The 4" pipes were cross-connected to another chilled water system after it was discovered that they could not get enough flow to these 3 new units during the summer. The 800 ton system is constant flow with 3-way valves; so, I'm sure they never had design flow but didn't need it during the winter.

They want to eliminate the other chilled water system and want to serve these 3 coils from the 800 ton plant. My problem is that I do not know the extent of the flow problem.

I have calculated that I need 328GPM @ 57' head to circulate water through the branch piping and 3 new coils. I'm not sure how to take it from here:

The 1st scenario would be that my central plant pump has enough flow capability, but just needs more head to reach these coils. But how much more? I don't have a flow or delta-P across the branch. If I size my booster pump for the full 55' head, that should be enough but probably overkill, right? I would use a triple-duty valve on the pump or balancing valve on the return line to just give me the 328GPM I need. Just a side note, which is better and why - Triple duty on the pump or circuit setter on the return line?

The second possibility is that the central pump is not large enough to give me the total GPM requirement. In that case, my booster pump head probably needs to account for its flow all the way back through the central pump, chiller, etc, correct? And will that cause issues since I will now be bringing more flow through the chillers?

I'm trying to obtain information on the central pump or better yet a balancing report of the system, but I don't know if they are available. I have been asked to select a booster pump to "fix the flow problem."

Are my lines of thinking correct? Any other ideas?

 

[KernOily]

Uh, I'm not an HVAC guy but I can tell you that the first step to solving a pumping problem is to generate a system curve. This is a plot of the head (TDH) vs. flowrate in your system. You calculate the friction loss for your range of rates needed, add the static head, and deduct the suction head. For a branching network of pipes and equipment, which it sounds like you have, you should use a nodal network analysis program like SINET or PIPEPHASE to calculate the TDH at the various flowrates.

Without a system curve you are throwing darts at the problem especially if you have variable loads. Is your main pump on variable-speed drive?

Next step is to overlay the pump curve on the system curve. The point where the two curves intersect is where your pump will operate. That will immediately tell you what the problem is.

A booster pump in series with your main pump will add head for each flowrate. If you have to go that way, make sure your booster pump's curve has a somewhat similar shape to the main pump. Hope this helps! Pete

 

[KernOily]

That's a cool handle by the way, are you a LZ fan? ;-)

 

[BronYrAur]

Thanks Pete for the info. Unfortunately I don't have info on the system pump yet other than it's constant volume. The job is "out of town" and I have yet to visit the site. I don't have software such as you mentioned; I should probably look into that. Thanks

By the way, yes I'm THE LZ fan. It begins and ends with LZ.

 

[quark]

The following method may give you some approximation.

The basic assumption is that pressure drop in the circuit including coils is predominantly dynamic. Check the discharge pressure (say P1) of the pump without bringing the 3 new AHUs into loop and check the flowrate (say Q1) from the performance curve. Now, open up the loop and check the discharge pressure (P2) and the corresponding flowrate(Q2).

Now, calculate the new pressure drop (P3) for the initial flow (Q1) by extrapolation (by the formula P3/P2 = (Q1/Q2²). The difference between P3 and P2 gives you the booster pump discharge pressure.

When you use a balancing valve in the return, your coil is exposed to greater pressure and you may want to avoid it. A triple duty valve in the booster pump discharge will absorb the excess pressure (or, more precisely, drops the pressure to suit the required flow condition)