chilled water variable secondary pumping & Controls

[navas72]

We are going to install 1200 TR chiller with Variable Secondary Pump(1140 cmh) & Fixed Primary Pump(452 cmh).
The variable secondary Pump will be suitable to Handle 3000TR.
The present Load is only 800 TR
Which is the best economic way to control the secondary pumps.
with 3000TR flow , when it"s feeding to 800TR with VFD, the pressure head will be sufficient to serve the end use point.
whether we can control the VFD of secondary loop upto 30% of the rated Flow.
Please guide me in details.

Navas

 

[ChrisConley]

Why such a big secondary pump? Is there a potential to use duplex or triplex smaller pumps? You'd get a much better control at the 800 and 1200 ton loads.

 

[navas72]

Because the total future load is 20000 Tons. There is no potential for the pumps.
The specified condition is to be followed. so i need a better guidance for the above.

 

[RossABQ]

Are you asking what to use to control the VFD? If so, you'd have to tell us a lot more about the load variability. There is not much room to turn down that VFD, you're about at the lowest % for continuous operation.

 

[quark]

There are lot of inconsistancies in the data given by you. On a general note, all the cooling equipment are designed with a temperature difference of 5C for the equipment to be economic.

If the chiller capacity is 1200TR then the primary pump capacity should atleast be 727 cmh. The maximum secondary pump capacity should be equal to that of primary pump capacity. Multiple primary pumps and one secondary pump is exactly opposite to what an optimum design should be. Review your design once and cover the loop holes.

As far as your question is concerned, 30% is not a very bad flowrate for the secondary pumps to run but not very good either. You pump speed control should be done not based on flowrate but on the DP across the secondary supply and return headers. This will ensure that necessary head is developed by the pump at all load conditions.

 

[navas72]

Thanks Quark,
the temperature difference is 8C & The secondary pump is of 62 m Head. i am worried if VFD works at 30% speed , then sufficient head will be developed or not.
Please guide me

 

[cry22]

Install the seconday pump size you need today, add more secondary pumps in the future. No sense installing pumping systems for 20,000 tons today. Just leave space for your future pumps.

 

[quark]

The DP setting across the supply and return headers takes care of the required dynamic head. The pump, infact, will run above 30% to match the pressure drop. Please read RossABQ's comment carefully. 30% is the flow requirement at design peak heatload and this may come down significantly during AHU operation. Anything below 20% (actually most engineers prefer 30%) design flowrate can cause damage to the pump.