8 replies to this topic

[fallah]

Giman,

As far as i know:

-Sketch 1 is the normal practice for temperature control of cooling water cooler.
-Sketch 2 is to be used when there would be the possibility of the load change. In these cases the process time lag is being lowered for effective control by partially bypassing the process fluid.
-Advantage of the sketch 2 is: coolant isn't throttled which keeps the HTC up and minimizes fouling.
- The disadvantages of the sketch 2 are: higher pumping cost and lower temperature differences between cooling water supply/return.

Edited by fallah, 17 December 2012 - 03:51 AM.

[kkala]

I have seen only sketch 1 in refinery PIDs, it must be more common as pointed out by Fallah. An explanation is that process stream flow had better be more or less constant in most cases. On the other hand cooling water network is complex, so that variation in one cooling stream would not be "sensible". This is also valid for steam networks and other refinery utilities.
Even reduction of total CW return to cooling tower (within limits, not to cause e.g. channeling in tower or scales in the pipe) will not have an undesirable effect. For instance, consider hot water temp=100 oF, air wet bulb=70 oF. Perry's Fig 12-14 (7th edition, 1997, Mechanical Draft Towers) indicates cold water temp of 80 oF for "water concentration" = 2.5 gal/min/ft2, or 76 oF for 1.5 gal/min/ft2 (40% reduction in CW flow).

Edited by kkala, 18 December 2012 - 04:48 AM.

[mvp]

_{If the Cooling water is throttled, it may (depending on the skin temperatures, water quality, etc) increase the tendency to foul. It could be due to low velocities also.
The by-pass of process is a good option; assuming the process side is not fouling under low velocities.
There is a third option; provide a local re-circulation for the cooling water; typically done during winter weather conditions.
One will operate this only when needed; the velocities and heat transfer coefficients are maintained; and less chance of fouling due to high skin tempearture if any.
The down-side is a capital cost for a local pump system.}

[sybase666]

normally, sketch 1 is used for steam heating process (I have saw many cases like this)
and sketch 2 can be used.

[ElSid]

I have only used option 1. Fallah's comments bring up some interesting points.

[Anjaney Shukla]

1) Option is good when flow is steady across the exchanger.

2) Throttling the cooling water flow may cause fouling in cooling water exchanger, and also disturbance in cooling water network.