Operation Of Heat Exchanger Above The Overall Heat Transfer Coefficient 1

[Papps]

I recently carried out heat duty calculation for a couple of Heat Exchangers present in the process which are of Plate & Frame Heat Exchangers after the exchanger plates pack were replacement with new one's due to leakages.



From the calculations I have the below observations,



- Current flow rate through the heat exchanger is less than the design flow rate

- Current DT is almost double the times the design DT i.e Design DT is 8 deg C but attained DT is 15 deg C

15 deg C was intentional due to the process requirements.

- Confirmed that operating temperatures and pressure profile for the Hot & Cold side are within the design values.

- Calculated Heat Duty of the heat exchanger is less than the design heat duty. Heat duty was calculated based on the cold side flow rate which is known.

- Calculated Hot side flow rate based on the calculated heat duty as Qcold = QHot to verify if the Hot side medium flow rate is more than the design flow rate of hot side as DT is double the times of design DT but it was under the limit.

- Further, Calculated Overall Heat Transfer Coefficient with Heat Duty, LMTD, and Heat Transfer Area which was exceeding more than the design value.

What are the problems or negative implications which can be caused in the Heat Exchanger by operating the Heat Exchanger beyond the design Overall Heat Transfer Coefficient?

Is the design Overall Heat Transfer Coefficient value used for the thermal design/manufacturing of the Heat Exchanger parts such that operation of the heat exchanger beyond design Overall Heat Transfer Coefficient value could cause thermal stress or fatigue or material degradation thereby reducing the potential life span of the heat exchanger ?

 

[1503-44]

It's normally the target value of how much heat transfer has to be achieved, or provided, to maintain the design flow rate within a design temperature range.

You really don't use that value for anything but for and at design time. It is not something directly measured by process instrumentation, so it has no control significance. You would normally process-control the heat transfer rate (if you had to) based on inlet and outlet temperatures, actually controlling it by changing flow rate(s) via TI signal(s). Normally it is only necessary to monitor and control the outlet temperature of your process fluid, not to say that you may wish to monitor all temperatures.

OHTC will increase if you increase hot stream inlet temperature, or flow rate, reduce cool stream temperature, reduce ambient temperature, or reduce insulation (if it was OTHC of a pipe). The opposite effects happen taking the opposite actions.

Operation "out of OHTC" would indicate temperatures were higher, or perhaps lower, or both, than planned, thus those temperatures might affect stress adversely, or advantageously, depending on the resulting temperatures of various components in the system.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[IRstuff]

Is the design Overall Heat Transfer Coefficient value used for the thermal design/manufacturing of the Heat Exchanger parts such that operation of the heat exchanger beyond design Overall Heat Transfer Coefficient value could cause thermal stress or fatigue or material degradation thereby reducing the potential life span of the heat exchanger ?

I think you are confused about HTC, which is simply the ratio of heat transferred per unit degree of temperature difference and is consequence of the thermomechanical design of the heat exchanger, i.e., the heat transfer area, mass flow, etc. What is of concern is the ACTUAL temperature difference and mass flow rate, both of which were designed into the welds, pipe diameters/bend radii, etc. If you doubled the temperature difference, then the thermal stress is doubled; if the flow rate is doubled, then pipe erosion is worsened. Conversely, if the flow rate were halved, then fouling might increase, which would eventually decrease the operational HTC.

As a further note regarding fouling, it may be that the "design" HTC reflects end of life, to include fouling, while your measured HTC reflects beginning of life, knowing that HTC will degrade over the years of operation.

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[Desmond_Aubery]

PHE replacement plates may very well not have the same parameters as the original plates:
1. Pressing depth
2. Chevron angle
3. Chevron pitch

From your original information it looks as if your replacement plates may be more restrictive than the original plates - hence, the lower mass flowrate, and raised dT (Q = m'.Cp.dT). I'd expect changes in performance of both process streams.

Hope that helps.