Control of cooling tower circuit

[virk]

We are designing a cooling tower circuit. In total 22 similar consumers of cooling water will be connected in parallel. We will install one large supply and one return line, where all the 22 consumers are connected to. Each consumer is provided with a control valve in the outlet for regulation/control-purposes. We must assure the following:

- approx. constant inlet pressure at the cooling water supply line at about 3 bara. (This is because each consumer will be temperature controlled individually and should not be influenced by inlet pressure variations)

- approx. constant outlet pressure in the cooling water return line of about 2 bara. (This high pressure level is required due to (short-time) outlet temperatures of 120°C/250°F of the individual consumers.)

- Perhaps constant flow should run via the cooling tower.

I think to work with two or three control valves in the large cooling water lines, but I am not sure whether I found good or (best) solution.

Who of you has some hints for me? What should I think about? What problems didn't I consider?

Kind regards

virk

 

[katmar]

Hi virk,

On the supply side you basically have two options. Either you throttle the delivery of the pump to give a constant pressure downstream in the supply header, or you have a varying spill back from the end of the header to give a constant total flow. Throttling the pump discharge will result in the lowest pump motor power usage, but the spill back will use a smaller valve. Without knowing your absolute numbers it is impossible to make an economic selection between these two.

A possible complication will be the varying flowrate causing varying pressure drops between the consumer off-take points. If they are widely spread, controlling the pressure at any one point in the header will not mean that the pressure is constant at all points in the header. If the supply header is designed for low pressure drop this may not be a problem.

On the discharge side you have an unusual situation. I must admit that I have never seen cooling water from a cooling tower heated to 120 deg C. If it ever happens that several of your consumers are discharging very hot water, while other consumers are taking very little water, you could easily damage the packing in the cooling tower.

You could of course control the CW return header's back pressure with a valve. You should check for the possibility of flashing in this valve, with the risk of erosion. As an alternative to using a valve, if you have an existing high structure it might be an idea to rather take the return header up to a separator tank with an overflow/outlet 10 m above grade. This tank should be well vented and might help to do a bit of pre-cooling before you hit the tower. Any flash in this vessel would not be wasted water because the heat has to be removed by evaporation, either in this tank or in the cooling tower.

regards
katmar

 

[quark]

Like Katmar said, constant supply and return pressures have no meaning in real life, IMHO and to maintain them in a circuit is just impossible.

What best you can do is maintaining constant maximum flow through each user point. Reversed return piping gives you the advantage of similar pressure drops(atleast no short cycling) across all user points. If you want constant flow through cooling tower, go with 3 way valves. If you are energy conscious, you have to go with variable speed and 2 way control valve system.

You can get better ideas if you clear out your objective.

PS: Don't get misguided by constant pressures(for liquids), whatsoever.

 

[KenRad]

I did a similar system with multiple users a few years back. One thing to consider is maintaining minimum flow through your tower - in cold weather, reduced flow can lead to freezing in the tower fill. The tower manufacturer will have a minimum flow in mind.

I did a primary-secondary system, with the primary circulating the tower itself, which assures proper tower flow. This was a high flow, low head pump (relatively low HP). The secondary pumps had frequency drives, controlled to maintain a constant supply/return differential. In your case, you may also want to have a backpressure control valve at the end of the secondary circuit, to maintain a minimum return pressure. All of your users should be on two-way valves, except for one or two 3-ways to maintain a minimum secondary pump flow. As was said before, oversizing your headers would be a good idea if they are long, to keep constant d/p to all users.

---KenRad

 

[ChasBean1]

I'm wondering about the short term high temperature discharges of individual loads. How much for how long? If there are 22 loads and one guy squirts out hot stuff for a minute, the bulk water will quench and cool it so quickly to where your design pressure criteria might not be necessary. If you have an open cell cooling tower, I'm not sure how you'd meet these pressures anyway.

 

[virk]

Hello!

Herewith I want to clarify some aspects which perhaps remained unclear:

The cooling tower circuit works as cooling cycle for 22 sterilizers. These are heated up from ambient temperature up to above 120°C/250°F. Sterilization temperature is kept at this level for a couple of minutes. Then with "our" cooling water the sterilizers (with the product inside) are again cooled down to ambient temperature. So this high temperature only occurs for a short period. "Normally", because of the mixing of all cooling tower water, entrance temperatur into the cooling tower is about 50°C/122°F. I only want to keep high the pressure in the cycle to avoid evaporation/cavitation of water. I do not see other possibility for this. (Short before inlet into cooling water, there will be a (control) throttle valve reducing the pressure to cooling tower inlet pressure of about 0,2 barg.) Because of this high pressure in the outlet I need 1-2 bars more because of pressure drop of individual consumers (line, heat exchanger, control valve, strainers, etc.)
For design case it is assumed that 8 consumers are running simultaneously and are thus determining the maximum design conditions.
Some numbers: Approx. 400m3/h cooling water flowing through these 8 consumers. This results in peak load for cooling tower of about 8.000kW. CW temperatures are (wet bulb/supply/return) 21°C/25°C/42°C or 70°F/77°F/108°F.

Further questions:

1) If I want to control supply pressure of the cooling water header, there are at least two possibilities, a) to control the pressure directly with a control valve which directly bypasses to the suction line, or b) to control the total volume flow through the pump with same control valve. Which solution is better, and why?

2) We already considered "KenRad's" system with primary and secondary circuit. At the moment we do not consider it to be necessary due to similar flows via cooling tower and consumers. What advantages would occurr, if we designed it this way (two circuits)?

Kind regards

virk

 

[ChasBean1]

Well, 0.2 barg is lower than 2 bara... I don't agree with your assessment of the pressure needed at cooling loop supply. If the individual chambers heat to 250°F on the primary side but the cooling loop effluent is significantly lower, you can accompish the task with a standard system that might have larger mains and higher flow to each chamber to reduce your loop cooling needs. Increase water volume/flow a bit to avoid excessive return temperatures and flashing/cavitation.

 

[quark]

I agree with CB on this. There are some wrong notions with this setup, IMHO.

All the towers used for conventional cooling, except that used for Generators, are designed with a range of 5⁰C. 17 ⁰C range is too high and you will have to pay the penalty on energy front. Bypass necessary quantity of cooling water to get a range of 5⁰C.

Deliberately dropping fluid pressure at the inlet to the cooling tower, by controlling a valve, is redundant and useless, to be frank.

Saturation pressure corresponding to 121⁰C is about 1.05 kg/cm²g. Further, the autoclave should have enough heat capacity to provide latent heat to the cooling water to evaporate it (only temperature is not enough).

If your setup is not yet ready, reversed return piping will ensure almost common return line pressure for each equipment. Provide supply at the bottom and return from the top of the autoclaves so that any superficial evaporation will be taken care at the free discharge into the cooling tower, or you can provide a vent line.

Just calculate the required flowrate and let your pump develop the required pressure.

Regards,

 

[virk]

Hello ChasBean1 and quark!

Normally you are right, but in this case increasing the cooling water flow through individual consumers to prevent high temperatures does not seem to be the solution: I must follow a given temperature/time curve to cool down the autoclave. If I began with more cooling water flow in the beginning, I would cool down the autoclave to quickly. Control equipment for this temperatur/time curve is a control valve in the cooling water outlet of the autoclave heat exchanger. I could decrease capacity of heat exchanger, but I need the heat transfer area for the end of the cooling process.

Best solution would be to control flow and inlet temperature into each autoclave heat exchanger; but I think this would be much more expensive.

Any further comments?

Kind regards

virk

 

[katmar]

To virk,

I like KenRad's idea of using a VSD on a second pump. Even if you do not have separate circuits and you put the fixed speed and variable speed pumps on the same circuit, this is likely to offer the best pumping energy efficiency. Use the variable speed drive to control the pressure at some representative position in the supply header.

To quark,

Please could you expand on why you do not like using a 17 deg C temperture range. If extra water is bypassed to bring the temp range down to 5 deg C it will mean pumping over 3x as much water as is actually needed. I have used ranges up to 12 deg C and dont see a problem with 42 deg C returns. In summer here in Durban we often get this sort of return temperature.

Please could you also explain what you mean by "reversed return piping". This is not a term I am familiar with. Thanks.

katmar

 

[quark]

There is no problem, absolutely, if the system is designed for the required range. The confusion may be due to my mixing up of the control parameter and the efficient running of the tower together.

Once we have the fixed L/G ratio and wbt, the change in the tower range effects performance(the NTU) of the tower). For example, WBT, water flowrate and tower entering water temperature fixed and assuming an L/G ratio of 0.73, for 5C you will get the NTU of 0.22, for 10C you will get about 0.75 and for the given range it is about 2.4479. This shows the increase in difficulty of cooling.

Reversed return may be a particular term being used in this geography. Normal return employs first in last out basis. As the inlet pressure for first equipment in the supply line is always higher and if the return from the first equipment goes into the return header last, there will be more flow due to the higher pressure differential. Reversed return employs first in first out condition. This will ensure more or less same pressure differential across all the equipment. (If you have equipment 1,2 and 3 then connect 1, 2 and 3 to the return blanking off the return header near the equipment 1 instead of blanking it off at equipment 3 as in a normal case)

 

[katmar]

quark,

You make a very good point about maintaining equal differential pressures by using "reversed return piping". Thanks for the explanation.

On the temperature range - I think that as the range changes the L/G ratio will have to change as well (assuming a fixed heat load), and the effect on the NTU will be much less than your numbers above show.

regards
katmar