Temperature control valve for closed loop cooling system 1

[dlynnbell]

I am doing preliminary design of a closed loop cooling system to retrofit an existing open loop cooling system. The new system will incorporate a recirc pump to provide a constant 3000gpm flow to the auxiliaries. A water to water heat exchanger will transfer the auxiliary loop heat to raw water. I need to economize use of raw water (secondary coolant). I intend to control the auxiliary loop outlet temp by varying the secondary coolant flow.

My question is, what is preferred method for piping the secondary coolant temperature control valve? A simple series loop, with a flow control valve on the secondary outlet, inversely proportional to auxilary temp... OR a three-way bypass flow loop on the secondary inlet, with a Robert Shaw type valve?

The bypass flow style provides a constant secondary cooling flow, I would prefer to economize use of raw water. I have never been given an Engineering explanation of the pros and cons of bypass or series temperature flow control.

 

[imok2]

"I need to economize use of raw water (secondary coolant)"

Have you given any thoughts of using 2 closed systems one for the secondary coolant using an evap condenser or an air cooled if you prefer. This will keep the condenser clean and conserve water and quite efficient It's done quite often in HVACR work.
2. If your gonna use a constant flow pump on the aux side I would use a 3 way valve you don't want to dead head the pump. You usually use 2 way valves with a system that has VSD pumping and a bypass line, the fluid needs somewhere to flow as long as the pump in running.

 

[CARF]

"I have never been given an Engineering explanation of the pros and cons of bypass or series temperature flow control."

1) Series flow control: you add hydraulic resistance with the valve and therefore choke the flow. With a constant speed pump you will lose energy (extra pressure drop).

2) By-pass flow control: the overall resistance will remain the same (or get lower) and the total flow will remain more or less constant. You do NOT lose enegy on extra resistance (pressure drop).

I would consider a variable speed drive with frequency inverter: you do not need valves and you save energy in case lower cooling flows are required.

http://www.cti.org/links.shtml

Good luck,
MVD

 

[patrickjmp]

dlynnbell,

Either way will work, series or bypass. The best approach is depending on your auxilliaries. How many are there and do they require the same flow, temp, and pressure all the time. Does your load vary and how much. Closing up at least part of the system will increase the life of your equipment and save water. If you are going to use a heat exchanger to close up part of the system and you will want to employ a filter or solids separator on the open side of the heat exchanger to keep you efficiency up and maintenance downtime to a minimum. A close circuit cooling tower or evap condenser with an internal heat exchanger is also a good alternative.

 

[dlynnbell]

Thank-you all for your replies. Here's some more info I left out.
Our raw water (secondary coolant) supply is gravity fed fresh water. (36" line @ 120psig) No pumps required. The Aux Cooling heat load varies from 5MMBtu/hr to 35MMBtu/hr depending on plant load. I want to keep 3000gpm flow constant to the Auxiliaries to maintain my flow velocities. I would like to play with a VFD, but need to keep flow constant in the aux loop.
Series flow saves me water ($) at low plant load conditions and that is one driver for this project... Does bypass flow have any control advantages I need to consider? i.e. is it more responsive?

 

[CARF]

By-pass flow is maybe a bit fater in system response (dynamics) so the control system reaction will be faster in case of disturbances. However since the disturbances (especially temperature) may show up very gradually you do not need fast control. In other words there's time enough to react for the (PID-)controller to correct and get back to set-point again.

So series control with VSD is prop. the best way to go,
MVD

 

[lilliput1]

What is the primary coolant? What is its flow, design entering temperature & leaving temperature? Indicate temperature range if variable. Is peak load during winter or summer?

At 3000 gpm maximum temperature rise on the secondary coolant (fresh water) is 23.33°F. What ir the range of entering secondary coolant temperture summer & winter. Once through systems usually are not allowed by Code. Consider using cooling tower to reject heat to the air by evaporation & cool back the secondary coolant.

 

[dlynnbell]

liliput1: The retrofit I speak of is to modify an existing open loop system that has been in service for over ten years. My proposed heat exchanger will add an additional boundary between the raw water and the existing process water loop. I propose a straightforward environmental benefit over our existing system and don't anticipate any regulatory issues.
I agree, that for new construction, heating the air is viewed as more PC (politically correct) than water. Adding additional cooling towers, or fin-fan coolers is cost prohibitive. What "Code" prohibiting open loop cooling are you referring to?
I mentioned in my first post that we will be using water. Deionized water with trace amounts of biocide and corrosion inhibitor in the closed loop, and fresh untreated water in the open raw water loop. 65deg F is the Summer temp for raw water and our peak load season. Aux cooling shall be below 104deg F.
Do you have a preference on plate heat exchanger brand?

 

[DLANDISSR]

Dlnnbell:
From what you said the aux water is constant flow and the raw water you wish to be variable flow. I assume you wish to use only the raw water needed to offset the temp rise in the aux water system.

In this case, the only rational choice is to throttle the raw water flow (since there is no pump in the raw water system). If there was a pump in the raw water sytem a VFD controled pump would also be a good choice.

A three way valve system allows by passing of the flow in response to temperature. For example, a water heating system using steam. The steam flow could be throttled to change the water temperature. The response would be slow due to the thermal inertia in the heat exchanger and water. If a three way valve were used, in the water line, to by pass the heat exchanger the response to would be much faster because the flow is diverted and the thermal inertia of the heat exchanger and water does not need to change to effect a change in temperature.

 

[davefitz]

If you want to minimize use of fresh water ( primary side) then you should also throttle the fresh water flow proportional to the secondary flow. Basically, throttle the primary side water flow so that its temperature gain is about 90% of the secondary fluid temperature gain. I am assuming the closed loop HX is counter flow, so the governing WCp remains the secondary side insofar as HX effectiveness is concerned.