Multiple handles for cooling loop

This obviously depends on a lot, but any sort of general indication would help me here. I can't post any more detail.
Industrial cooling loop, water pumped round between a shell/tube exchanger
and a cooling tower. The exit temperature of the process side of the STE is to be controlled.
The process design we've been provided (which is locked in) has two handles for the cooling adjustment: (1) a bypass valve around the tower and (2) a control valve in the cooling water loop itself, adjusting the flow through the STE / tower. The process sizing looks reasonable, ie, not under or over. I'm wondering which of these two handles is likely to be the stronger one, and which should be used for the main control (or maybe both).
I recognise that reams could be written here, the two handles are certainly going to interact, will be nonlinear to a degree and will depend on many factors.
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Hi Bruce

If the size of the cooling tower is amply dimensioned and the bypass valve (1) is not used, the water entering the STE (shell/tube exchanger) should be of constant temperature. If I understand your system correctly, this temperature is the lowest you can reach with your system, because the bypass valve will raise the water temperature entering the STE.
Using the control valve (2) you change the volumetric flow and thus you change the amount of thermal energy you extract from the process. The difficulty with this control mechanism is that you can not fully close the valve as the pressure in front of the valve will rise and the reduced volumetric flow will set the pump into an operating condition that may not healthy.
Using the bypass valve (1) you keep the volumetric flow but change the temperature entering the STE. If the bypass is large enough hardly any water would pass the cooling tower. Thus, you can control the water temperature entering the STE from minimum possible to maximum (no cooling).
In my opinion the bypass would be the stronger one, as it gives you the full control range. Furthermore, you could enhance your control algorithms by predicting the amount of thermal energy transferred in STE. This seems simple to me when having a constant pressure and a known water temperature. I would have some trouble modeling the thermal energy transfer with different volumetric flows and pressures as it would be when controling (2).
Best regards,
pt
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wrote:

If the size of the cooling tower is amply dimensioned and the bypass valve (1) is not used, the water entering the STE (shell/tube exchanger) should be of constant temperature. If I understand your system correctly, this temperature is the lowest you can reach with your system, because the bypass valve will raise the water temperature entering the STE.
Using the control valve (2) you change the volumetric flow and thus you change the amount of thermal energy you extract from the process. The difficulty with this control mechanism is that you can not fully close the valve as the pressure in front of the valve will rise and the reduced volumetric flow will set the pump into an operating condition that may not healthy.
Using the bypass valve (1) you keep the volumetric flow but change the temperature entering the STE. If the bypass is large enough hardly any water would pass the cooling tower. Thus, you can control the water temperature entering the STE from minimum possible to maximum (no cooling).
In my opinion the bypass would be the stronger one, as it gives you the full control range. Furthermore, you could enhance your control algorithms by predicting the amount of thermal energy transferred in STE. This seems simple to me when having a constant pressure and a known water temperature. I would have some trouble modeling the thermal energy transfer with different volumetric flows and pressures as it would be when controling (2).
Best regards,
pt
Thanks. I'd just about given up hope for this newsgroup.
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Did the answer help?
pt
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wrote:

Yes it did, thanks. I agree with your analysis on which handle will be the stronger one. There is some preliminary input from the client regarding how the bypass should be used that I'm going to reraise.
Process design will occur before EOY, if I can I'll post some information on how this goes. There is a lot of focus on temperature control, as the target vessel is a bioreactor with narrow temperature limits.
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Hi Bruce

Do you already know how narrow the limits are? What sensor configuration do you have? You didn't mention any sensors at all.
pt
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