after effects of variable flow pump in chilled water system 1

[smvasakam]

hi,
we have a primary chilled water system, with a constant flow pump.
the head required is 27.5metres and 750 gallons.
but presently the pump delivers a flow of 900 gallons and 25metres head. chiller is of 300 tonne capacity.
so now i want to reduce the pump speed by installing a variable frequency drive. if i reduce the flow to 750 gallons the head will be reduced toaround 18 metres.
what i wouldlike to know is whether at this reduced head the water will be reaching all the places and will the variation in flow will affect the indoor room temperatures and to what extent.
help me

 

[quark]

smvasakam!

Generally a primary chilled water system consists of a pump, chiller and balancing tank and is in isolation with the end users. (Secondary chilled water system) Constant flow of the primary system is a must to maintain as the suppliers of chiller are hard to break shells

If you want only to maintain 750 gallons/(what time interval?) trim the impeller. (I can help you if you send me your pump performance curve)

If your system consists of single chilled water loop, reduction in flow rate does effect the room conditions to any extent. (When there is no flow room temperature will be equal to that of ambient, if I ingnore frictional heat)

Regards,

 

[smvasakam]

hi quark,

i wont be able to trim the impeller, as the same pump might be required for future(highcapacitychillers).
the option is installing a vfd.
incase if i reduced the flow and the head changed from 25 mtrs to 19mtrs will the flow reach the topmost part of the building.
the building height is 31mtrs.
rgds
smvasakam

 

[quark]

smvasakam!

Trim the impeller and install new one whenever requirement comes. This will be much cheaper for constant flow application rather than installing VFDs. (This I am saying by experience not as a free suggestion)

The liquid in either case doesn't reach 31m height with required flow rate. There are some fundamentals that should be answered by you.

1. If you have designed for a duty point of 27.5 meters head why you require 31 meters?
2. Is your system a primary and secondary looping system or single loop system?

Regards,

 

[smvasakam]

hi quark,

the design head is 27.5mtrs.
but the actual head we measured from the difference between pump discharge and suction pressure gauges is 25mtrs.
but what i said as the building height is 31 mtrs the chilled water flow was sufficient at the topmost level with 25 mtrs head.
in case if i reduced the speed of the pump, the head will also reduce to 20 mtrs. in that case whether the water can reach the topmost place.
and the system is a single loop one.
rgds

 

[quark]

smvasakam!

I understand that your chilled water tank is at a height of approx. 6 meters from the suction of the pump. If you decrease the speed of the pump to get flow rate of 900usgpm(I presume) to 750 usgpm, the pump head developing capacity will be 17.36 meters. As already you have static suction head of 6 meters the net head will be 23.36 meters.This means water never reaches the topmost point.

But pump always tries to overcome the resistance at the expense of flow rate. (performance point shifts towards left of the curve)

Better is to install a differential pressure transmitter at the end of the loop (across supply and return headers) and use it to control VFD. You can set the dp as per your requirement. This will optimize your pumping energy in all load conditions.

Try to contact Danfoss as it has good product range and technical know how too.

Regards,

 

[Islander]

how do you know you are outputting 900 gpm?
i bring it up cause you said you read the operating head with "gauges" (plural).
2 things wrong here..
1. use one gauge...or you'll be off a bit.
2. verify your impellar size by reading shutoff head..then
use pump curve to calculate gpm via operating head.

also...read chiller pressure drop...is usually more accurate than reading the pump gpm. the formula used is....new pd/design pd...square rooted...x design gpm = new gpm.


PC
Test/Balance

 

[quark]

Islander!

I prefer two gauges instead. The actual head delivered by the pump is the difference between the discharge head and static head.

Regards,

 

[PacificSteve]

smvasakam,

What is your basic objective leading you to want to make changes?

You are probably running a 25Hp or 30HP motor? Perhaps 163000 kWH per year. Say $13,000 per year based on .08$/kWH. Using affinty laws and a change from 900 to 720 (300 tons *2.4 GPM per ton) it would be calculated that you could cut your energy cost to 51% of the original (this is a cubed law or s2/s1^3) Reality is not quite as good as affinity, but it will be close. I have arbitrarily used an exponent of 2.5 or 2.7 with closer results to reality. This pays for a VFD very quickly. Maybe you won't be able to drop the speed low enough to get you to 720 without starving some point of the system, but the potential for savings is still dramatic. Using the delta P sensor(placed very carefully) would help avoid starving your hydraulically most remote zone, allowing you to push the envelope of speed reduction.

Islander had a good point that I would like to add my voice to...that being that it is very hard to know what flow you have. Accuracy of measurements are critical. Using the same gauge in 2 positions is a method I have used to avoid adding in gauge error. But I have seen great differences between accurate head measurements, pumps curves, and measurement using ultrasonic. Estimation of GPM by chiller coil charts is OK, when chiller is brand new, or known to have super clean coils, but this measurement is hard because the DP is relatively small, and measurement innaccuracies can then result in large GPM innaccuracies. You can also throw on an amp meter on each of 3 legs to give you another data point to plot on your pump curve to see if things are jiving. Better yet to use a true kW meter that will correct for power factors, amps alone doesn't tell you much since your power factor could be anywhere from .65 to .9, so calculation of HP based on amps is sketchy at best unless you have a firm handle on PF.

There are some cool inline flowmeters out there, they are removable for cleaning without shutting the system down. Trouble is you have to put it into the pipe, and hopefully in a fairly straight section, not too close to the pump, meaning a sizible shutdown and draindown is probably required.

Sorry, I got carried away. I should have stuck with the first question...what are you trying to accomplish?

Pacific Steve

 

[walkes]

smvasakam,

As far as I can tell from your description, the head that you are concerned about is the system head due to friction in the piping and through various devices. The static head due to the building height won't come into account since the system is closed (there are effectively two columns of water, one up and one down, they will cancel each other out and not affect pump performance during system operation). By decreasing flow through the system you are just decreasing the friction losses. The building height will only be a concern for system fill and expansion tank sizing.

 

[Islander]

why not simply adjust your pump to 750 gpm and go read the results at the coils as a test...a quicky test would be take one of those non contact pointer thermometers and aim it at the supply and return grills as you walk around(with all tstats set to 55 or so.)..check your outside air temp too. Take the prints with you and put your readings on them.

 

[ChasBean1]

smvasakam, there's been a lot of good advice for issues related to your question. But the true answer to your question is by Walkes two posts up. In a closed loop, don't worry about the building height. -CB

 

[quark]

smvasakam!

What is the discharge pressure gauge reading?

regards,

 

not sure if I read anywhere if this system utilized 3 way valves or 2 way valves at the coils.

Your VFD application will not work very good with 3 way valves. You will need to convert them or replace them.
The DP setpoint at the end of the line ( generally the farthest coil from the system ) should be equal to the coil circuit design flow pressure drop.
Make sure you have design flow at the pump and that you can confirm that all vales in the system are full open before you find this setpoint with confirmation of design flow at this final coil.
last but not least, it sound like your converting a primary to variable flow.
I would suggest that you consult your chiller MFGR for the minimum flow limitations and make sure that you set something up in your controls to limit the turn down at the pump or provide some type of bypass relief.
if your not sure, consult your local control expert.