Flowrate - room peak or simultaneous peak

[Jimbob73]

Hi, this should be an obvious one but am not thinking it through correctly....

sticking to cooling only for the question (decoupled primary/secondary system);

I have a 4 pipe fan coil system.

I know the FCUs will be sized on room peak which will give me a CHW flowrate through the FCU (lets say 1 l/s).

Say I have 10 rooms each with the same room peak = 10l/s sum of room peaks.

Say the building simultanous peak is calculated at 7l/s chw (say dT of 6 deg celsius) (diversified flowrate)

Do I size my pump for the building simultaneous load or the sum of room peaks? 3 port valves so I would think the pump flowrate is 10l/s. (I understand that the simultaneous dT is lower at the higher flowrate)

My question then is if the 'sum of room peaks' is giving 10l/s flowrate ( with low dT) but my chiller is sized for the simultaneous peak load (7l/s) - it does not add up

Typically I would match the secondary and primary flowrates. What am I missing here??

appreciate any help

 

[EnergyProfessional]

depending on how reliable your block load calculation is (7 l/s) you should size the pump a little larger and use a VFD to deal with the fact that much of the time all 10 FCU only use 25-50% of the flow on average. but at some time they might use more than 7 l/s in total since your block load is based on assumptions that can change over the next 30 years.

 

[Jimbob73]

Thanks, Maybe my question was not so clear......

How do I commission an individual FCU for the peak room load using a constant volume system which has a flowrate for teh simultaneous peak load?

For example I will not be able to commission each FCu for 1 l/s if the flowrate available from the pump (based on simultaneous load to match chiller duty) is only 0.7l/s (bypass must equal flowrate thru coil on 3 port system)

Cheer

 

[KiwiMace]

Why do you have all 3-way valves on a pri/sec distribution? The load calc might be struggling with this. Plant will always be sized on coincident load by software, and most of the time by people.

Normally the 2 way valves close down the sec flow, which ramps down the Vfd and your sec flow via pressure control. When you have 3 way valves, nothing happens on the secondary side, so it acts like a constant flow system, with low delta t at low load.

Back to your question, the load calc might be ignoring your pri/sec input based on the 3 way valves.

 

[Jimbob73]

its a refurb of an existing.

2 port valves, variable flow no problem I agree, but not what I have here.

Are you saying the pumped flow should be sized for the sum of room peaks?

appreciate the help

 

[quark]

The secondary pump flowrate should correspond to the sum of individual peak loads if the CHW flow control is by 3 way valves. In the absence of this, some of your FCUs will starve.

 

[Jimbob73]

Thanks Quark,

So if my chiller is sized for the simultaneous peak load, and corresponding flowrate at dT of 6 deg C the primary flowrate will be less than the secondary.

This will mean that the secondary return will mix with the secondary flow (via a low loss header that is installed) this because the secondary flowrate will be greater than the primary.

Can this be correct and is this normal for a 3 port system? (2 port will get around any of these problems)

Appreciate the help......

 

[quark]

That is correct. There are two issues with this arrangement. First one is that you see a low dT across the chiller during low load condition, which is inefficient. Secondly, as part of secondary return is mixed with secondary flow, chilled water temperature at FCU may be, sometimes, a bit higher than your design chilled water inlet temperature.

This was normal earlier and not now. With two way valves and variable secondary pumping, you will just provide enough flowrate to take care of instantaneous heat load at any moment.

Low dT still remains a problem at low loads even with variable secondary pumping.

You need not worry if your objective is to maintain conditions in the control space and energy optimization is not of a big concern.