Ring Main cooling system Help 2

[Zaheer567]

Good evening
I need some assistance with a cooling system layout that utilizes a single cooling tower to cool oil via a heat exchange as well as to the moulds. The system functions with a pump that pumps water through a 90mm ID HDPE pipe that rings around the plant. At each machine, there are 2 outlets that go to the machines heat exchanger and the second goes to the mould for cooling of the product. The "hot/warm" water returns from both the heat exhanger and mould and leads into a main return line (header), that travels back to the cooling tower where it is cooled. I'm having some trouble understanding the pressure and flow rates within each branch. Further more, at the the last machine, the outlet pipe from the cooling tower (carrying cold water) RETURNS to the pump creating a ring.

 

[Latexman]

Where are the moulds?

Good Luck,
Latexman

 

[LittleInch]

Errr, your flow is shown as at most 14 l/sec, but HE10 is 4 l/sec.

Ten units equals 40l/sec. that's a shortfall of 26l/sec??

So my responses are:
To answer the questions:
1. there are no gauges so i cannot determine the pump pressures at the start and end of the lines.
- So fit some. If you don't you will never work anything out in the field
2. Each HX has a ball valve to control the flow.
- Bad idea. Fit a globe valve to each one as ball valves are very hard to adjust for flow
3. The HX are different sizes.
- So list the sizes and the flows they need. What is the total flow needed?
4. Reason of doing this study is to determine if i add on more machinery;
a. will it affect the flow through the current lines of machines.
Yes. Now how badly will it affect it we can't tell
b. will the current pump be adequately rated for an additional head loss due to an increase in pipe length, loss due to bends, taps etc.
Don't know - what are the pump details ( head / flow etc) What is the pump curve? As flow increase head goes down. If you need more head for more flow buy a bigger pump
5. The offtakes are 1'' each for both the inlet to HX and outlet of HX back into the main return line.
So 25mm total area of offtakes for 5 units is 2500mm2. Area of 90mm pipe feeding it is 6300mm2. So there may not be a lot of pressure drop in that main header.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[TBP]

Many hot water heating systems have used this arrangement for - well - forever:

https://www.pumpsandsystems.com/what-reverse-return-system

 

[btrueblood]

Are the cooling loads at each mold tool constant, or does the flow required vary (need to be modulated)?
Do the molds need to have cooling shut off (e.g. during a heating cycle) and then turned on on a periodic basis?

Both types of variable load may point you towards a pressure independent flow control valve. Or at least, a "circuit setter" type valve that will limit flow rates at each mold to a design value. Both types of valve will let you control the flow rate to the tool more precisely, despite variations in pressure caused by different tools demanding flow, and pumps delivering varying head depending on total flow demanded. A ball valve ahead of the flow control valve would allow for shutting off the tool when needed, without affecting the setpoint of the flow control element, although a pressure independent valve with an opening stop could perform both functions.

 

[EdStainless]

If you are using valves to throttle flow through each HX how are these adjusted?
Guess work?
I agree with the globe valve comments, but I would put the ball valve shut off on the supply side and the globe valve for adjustment on the outlet side of the HX.
You need to put in a bunch of taps for pressure gages and some temperature sensors. After all the outlet temp from each HX is really what you are trying to control.
There are two ways to build ring headers. Yours supplies both ends and tries to keep the pressure uniform throughout the header.
The other option is to actually return to the pump inlet side and recirculate all of the flow. This is done when uniformity is critical (fluid is chemically treated, or heated/cooled a lot from ambient, needs to maintain high purity).
Your setup looks reasonable, and the header size is adequate.
The impact of expansion will depend on how you splice it into the current system.
The pump can only operate where the pump curve and system curve intersect. If you put flow through additional units and increase the total circulation flow then the supply pressure will have to drop and all of the existing units will see less flow unless they are readjusted. And if you open the valves a little more the pressure will drop further. And so on.
It isn't uncommon to see systems like this with very large headers (>2x total use area) and small supply lines to each use to limit their max flow (or some sort of flow restrictor). This is done to minimize their effect on each other.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[Zaheer567]

Thank you all for the detailed responses. I've been busy the last few days acquiring as much data as I can despite the lack thereof. I came to the decision to treat the entire system a new cooling system. I managed to get flow rate requirements for each outlet as per the heat exchanger manufacturing data.

Since machines vary in size, with the largest requiring a flow rate of 0.00417 m3/s. I then designed the system to ensure each outlet has an available flow of 0.00417 m3/s. However, if there a X amount of machines, each requiring 2 outlets. Will that mean that I would require a flow of Q = (2)(X no.of machines)(0.00417) in the main line?

If that is correct, I then continue with the system head equation by analyzing minor and major losses. I was advised to exclude the return like back into the cooling tower from the system head equation? I do not think that is correct and I would need to include the pipe lengths etc.

From there I would compare against a pump curve and make a decision thereafter.

I look forward to any input.
Thanks

 

[1503-44]

You have never made clear anything about the machines, except for their required flow. I myself do not understand what you mean by each machine has 2 outlets. Why does one machine have two outlets? Normally one would assume that the sum of outlet flows at each machine is equal to the sum of the inlet flows to each machine.

The flow in the common supply feeding all machines is the sum of the flows to all machines, or if all do not operate at the same time, then it's the sum of all flows for the maximum number of machines that operate simultaneously.

You do need to include all pipe from each machine and the return pipe back to the cooling tower. In addition, there is probably some pipe leading upwards to the top of the cooling tower to get the water lifted up to the top. That pipe and both its length and its elevation change to the top of the tower need to be included when summing line head or pressure losses.

I have assumed that all the machines and all the pipes are at the same elevations. Elevations of all pipe and machines and cooling tower inlets matter.

Now, what's going on with the 2 outlets at each machine? One, two ? Where do they all go?

 

[Zaheer567]

1503-44
My first post stated that each machine requires two inlets & outlets of cold water. One for the Oil cooler and the other for the mould.

 

[Zaheer567]

Thanks for the clarification on including the return header into the system head equation

 

[1503-44]

Yes, you told us about 2 outlets, but I think you never showed the second outlet on any diagram.
Do both outlets carry part of the machine's inlet water?
Do both outlets send water to the cooling tower?

 

[Zaheer567]

yes you are correct in saying the P & ID did not include two inlets per machine. this was done as i first needed to understand the flow movement through the network.
a. yes, both outlets ( one returning hot water from the mould and second returning hot water from the oil cooler), both connect via a saddle to the main return header that connects to the cooling tower.

i do have another question, when calculating the losses due to each manifold outlet, do you treat it as an contraction outlet to get the loss coefficients ?

thanks again

 

[1503-44]

If the connection looks like a "reducing fitting" with diameter gradually decreases; or an "expansion fitting" with diameter gradually increases. If outlets are from a manifold with a sharp edge reduction of diameter, no gradual transition, it could be more like a "tee", or a "reducing tee". That's probably what you want to use. Or maybe it could even be more like a exit loss coefficient, as through a sharp edge outlet from a tank.

 

[Zaheer567]

Thank you 1502-44. Appreciate the response and assistance. It is a reduction fitting via a saddle. So there is somewhat of a transition but I wouldn't say gradual. I will opt for a tee or reducing tee.
Thanks again