High pressure drop between distribution pump and heat exchanger

[bctarala]

Good day,

Im having a problem on a chilled water distribution line going to a air handling unit cooling coil.

Data is this

Distribution pump capacity is 120m3/h and head of 85m and with a distribution pipe of 100mm
Chilled Water Pressure/Temperature in distribution pump is 9barg abd 3degC respectively
The two air handling unit has chilled water distribution and return pipe of 50mm
The two air handling unit is approx 12 meters above the distribution pump
Total length of distribution pipe is 100meter approx

Scenario:

IF return isolation valve of chilled water out of Air Handling Unit cooling coil, pressure is about 8 barg
IF return isolation valve of chilled water out of Air Handling Unit cooling coil, pressure is about 1.5 barg only

Does this effect the flow rate, heat transfer and velocity of chilled water passing thru the air handling unit?

 

[willard3]

Pressure and flow are related, flow and heat transfer are related.

Is this homework?

 

[bctarala]

Hi willard3,

I know that pressure and flow are related, and i know that flow and heat transfer are related. What I want to validate is why there is a large pressure drop between distribution pump and cooling coil of AHU? is this possible because pressure is dropped from 9barg to 1.5barg?

Additional info, chilled water buffer tank is an open tank.

 

[LittleInch]

To be honest this sounds like a system that hasn't been designed, but put together using random parts. You need to sketch this out so that we can understand your system. Is the 100m made up mainly of 100mm(OD? Steel? ) Where does the 50mm pipe come into it?

Where are these pressures measured? - Note you say the same IF thing twice identically - did you mean to put an "open" or "Closed" in those sentences?

There will be a large pressure drop because your pump is probably capable of supplying much more than 120m3/hr because the system design will let it do so.

If you supply 10% of the information anyone else needs to understand your system you will only get 10% of the answer...

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[bctarala]

Hi littleinch

Sorry, i will re-phrase my data

Data is this

Distribution pump capacity is 120m3/h and head of 85m and with a distribution pipe of 100mm
Chilled Water Pressure/Temperature in distribution pump is 9barg abd 3degC respectively
The two air handling unit has chilled water distribution and return pipe of 50mm tapped to 100mm distribution pipe header
The two air handling unit is approx 12 meters above the distribution pump
Total length of distribution pipe is 100meter approx

Scenario:

IF return isolation valve of chilled water out of Air Handling Unit cooling coil closed, pressure is about 8 barg read by gauge installed in in and out of AHU cooling coil
IF return isolation valve of chilled water out of Air Handling Unit cooling coil opened, pressure is about 1.5 barg only read by gauge installed in in and out of AHU cooling coil

Pipe is 100mm ID (pipes are measured on their ID not on their OD)
Pipe is Schedule 40
Pipe is Carbon Steel (is there a relation between pressure drop and pipe material used)??

thanks

 

[IRstuff]

The drop in pressure with the valve open is indicative of strong, high flow, isn't it?

TTFN
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[LittleInch]

Ok, Still not a huge amount of data, but what it tells me, like IR stuff indicates, is that you pump is probably running off or close to the end of the curve and if you measured its pressure when flowing my guess is that it would be about 5 barg discharge. Centrifugal pumps don't like running like that and in reality your pump head is too high for your system resistance. What happens in this sort of instance is that the flow increases until the system resistance, which rises as a square of the flowrate/velocity, matches the pump output, or the pump motor gives up and trips on high temperature, high amps or high vibration.

Hence I think you have a lot more chilled water running through our AHU's which means they can provide more cold air if the air fans can handle it or more likely they provide more or less the same amount of cold air, but your chilled water pump is consuming more power than it needs to and fluid velocities are too high.

BTW, steel pipes are normally sized as a nominal OD (100 or 4" in this case) as this doesn't change, but wall thickness and ID do for the same Nominal size pipe. Also PE and other plastic pipes are much smoother and have lower resistance to flow hence why I asked the question.

What exactly is your question??

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[bctarala]

Hi IRstuff,

Thats what i want to know, if that large pressure drop indicates a high flow of my chilled water into the cooling coil. We try to throttle the return valve of chilled water and inlet/outlet pressure begins to rise, from this throttling, do my flow rate increase versus from fully opened valve?

Thanks

 

[LittleInch]

Yes, a high pressure drop indicates high flow. If you throttle it the flow rate will go down.

However if your flow rate is higher than the AHU can actually handle, there should be no difference in the amount of cooling as the limitation would be the area of the coils and the amount of air that the AHU fan can push past it.

Think of this like a conveyor belt where a person takes one orange every 10 seconds to pack into a bag. He can't take any more any faster. If you speed up the conveyor belt so that there are 2 oranges going past him every 10 seconds, he can still only take one and the speeding up of the conveyor belt is not helping you and wasting a lot of oranges. Similarly a higher flow through your AHU than the AHU can handle has no impact on the cooling, but may be damaging your pump and also using more power to move more water for no benefit.

If you actually measure flows in these lines with a UT meter or some other device, you will then have some data to compare it to, but otherwise this is all conjecture and guesswork, enjoyable as it is.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[IRstuff]

I guess the bottom line is that you should be using flow meters to measure flow, not pressure meters. For example, let's say there a blockage 1 ft upstream of the meter that's reading low pressure. So, you could have little flow up to that obstruction, and high pressure up to that point, but once past the obstruction, the little that gets through really can't generate much in the way of pressure, and the downstream reading is low.

TTFN
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[MintJulep]

Closed valve = no flow = no pressure drop.

 

[willard3]

You apparently don't know that pressure and flow are related, flow and heat transfer are related.

You need to take a course in thermodynamics and fluid mechanics.

 

[LittleInch]

You said you were having "a problem". What sort of problem?

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[MintJulep]

His problem is that his pipes are too small.

 

[LittleInch]

On the contrary I think his pipes are too big, which is why his pump head is falling away so much - but who knows - certainly not us....

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[MintJulep]

120 cubic meters/hour through a 50 mm nominal diameter pipe gives a velocity near 35 m/s.

Just a tad high...

 

[LittleInch]

I think it must be Friday afternoon - I get closer to 17m/sec for one pipe and the OP indicates that the 50mm is when there are two sets of pipes so closer to 8-9 m/sec. Still pretty fast and I suspect he's flowing a bit more than 120 m3/hr...

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way