8 replies to this topic

[Sherif Morsi]

Hi,

 

In a HVAC system, we have a closed circuit loop supplying the cooling medium to different users. As per a colleague, we shouldn't account for the pressure drop due to difference in head since it's a closed circuit.

 

My question is how would that be possible and we have 20-25 meters of difference in level to reach the users from where the supply pumps are? I do understand that as soon as the fluid reaches the top, it will gain back the head loss, but this means the pump suction pressure will change.

 

Correct me if I am wrong here, I am really confused.

 

Regards,

Sherif

[Zauberberg]

Please post a sketch of the system. The picture says 1,000 words.

 

I wouldn't agree that presure drop is irrelevant in a closed circuit. Neglecting elevation difference can lead to undersizing of the distribution pump.

But again, without a sketch your question is incomplete and may lead to incorrect answers. So spare some effort and produce a meaningful flow diagram showing all relevant information.

[breizh]

Hi ,
Dejan is right , a simple sketch will help us as he will help you to explain the problem .

Bernouilli should be part of the answer .

 

Breizh 

Edited by breizh, 27 May 2015 - 06:51 AM.

[Sherif Morsi]

Kindly see attached sketch

Attached Files

•  Cooling Medium Circuit.png   6.17KB   11 downloads

[Zauberberg]

Obviously the pump will have to develop (as a minimum) total discharge pressure equal to elevation difference (25m) plus pressure drop through the HVAC users, plus any additional pressure drop in the circuit (controls, fittings etc.).

 

A good amount of pressure will recover at the pump suction (not at the top of the installation), and it will be equal to the outlet pressure from HVAC users, plus static head, minus flowing pressure drop for the piping section feeding the suction of the pump.

 

So, practically speaking, the fluid energy does recover partially (the amount of energy/head loss due to friction and fluid flow through equipment/fittings is lost irreversibly), so you need a pump just to provide that much incremental energy/head to the system. As compared to a system where a pump pushes the liquid 25m high and it does not recirculate back to the suction, this arrangement requires less energy. But this does not mean that pressure drop need not to be accounted for.

[Bobby Strain]

Maybe you didn't give us a sketch of the whole system. It lacks any detail.

 

Bobby

Edited by Bobby Strain, 27 May 2015 - 08:22 PM.

[ahmadikh]

Actually, in this case you should not worry about the static heads as they balance out themselves and not to be considered for determining the pump head. Head is only to provide the required flowrate to the destination point which means overcoming the friction pressure drop along with the delta-P considered for sizing the control valves. So, the pump head should not be much huge (almost < 2 Bar)

 

As Bobby said, the sketch is not complete.

[juche]

As i understand this problem,

Consider your suction vessel and designation vessel are at same pressure and at same elevation.

you calculate the suction pressure based on Hydro-static head minus frictional loss you will get your suction pressure for the pump.

and for discharge pressure calculate discharge head required for elevation plus the frictional loss.

 

the differential of two will nullify the effect of Hydro-static head and you will be left with frictional loss only.

[Sherif Morsi]

Thank you all for your replies.

 

The sketch actually shows the actual system we have here. The pump from the chiller system pushes the cooling water to the HVAC users. We have around 5 or 6 users in series then water returns back to the pump and the chiller system. 

 

Regards,

Sherif