Minimum pitch temperature
Minimum pitch temperature
(OP)
Dear readers,
I looked at the FAQ's and the historical discussions, as well as in some text books about heat exchangers, but I couldn't find a substantiated answer to my question. My question is the following:
I want to rate an existing heat exchanger for a somewhat different application. I have no dedicated software for this at my disposal. I use the e-NTU method for this.
My question is the following:
"What is the minimum pitch temperature that should be applied for a heat exchanger and on what grounds is the limitation of this minimum based?"
One textbook recommends 5 deg C. I would like to know though what the grounds are for this limitation? I mean a guide line in itself is no reason for a limitation.
I hope someone in this realm of heat transfer experts is able to lift a little corner of the veil.
Thanking you in advance for your attention and your effort I remain.
With best regards,
Karel Postulart
I looked at the FAQ's and the historical discussions, as well as in some text books about heat exchangers, but I couldn't find a substantiated answer to my question. My question is the following:
I want to rate an existing heat exchanger for a somewhat different application. I have no dedicated software for this at my disposal. I use the e-NTU method for this.
My question is the following:
"What is the minimum pitch temperature that should be applied for a heat exchanger and on what grounds is the limitation of this minimum based?"
One textbook recommends 5 deg C. I would like to know though what the grounds are for this limitation? I mean a guide line in itself is no reason for a limitation.
I hope someone in this realm of heat transfer experts is able to lift a little corner of the veil.
Thanking you in advance for your attention and your effort I remain.
With best regards,
Karel Postulart
Karel Postulart, The Netherlands
Nuon Power Generation





RE: Minimum pitch temperature
Pinch temperature depends on the exchanger design (type), heat transfer area, and geometry / flow pattern (approach to the ideal counter-current flow). For the same flows and inlet conditions of hot and cold streams, a 1-pass shell & tube exchanger will have different pinch temperature than a 2-pass or a 4-pass S&T exchanger. Spiral heat exchanger will have different performance than shell & tube exchanger, as well as any other exchanger type/design. For example, TEMA E-shell exchangers are typically designed with 15 degC temperature difference.
In design of heat exchanger networks, the "pinch" temperature has different meaning. There is a lot of information online.
Dejan IVANOVIC
Process Engineer, MSChE
RE: Minimum pitch temperature
TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers
RE: Minimum pitch temperature
The selection of temp approach must be weighed against other factors such as no of tube side passes, no of shells used, HX-TEMA type complexity, total surface area to be provided, cost of utilities, cost of the heat exchanger, and plot space - there may be others also.
In some cases, there may be a preference to use a co current flow type HX, while in most other cases, mix cocurrent and countercurrent may be acceptable.
In some 2 phase units with strange Q-sat temp profiles, there may be an internal temp approach which may be less than that at either terminus.
RE: Minimum pitch temperature
I realise I made a mistake with the title and in my text. Indeed it is "Pinch temperature" instead of "Pitch temperature". I do understand the general concept of the pinch temperature. Choosing a minimum pinch temperature in the sizing process results in an economical use of transfer area. Choosing it too small will lead to an unevenly large heat exchanger.
The heat exchanger in question is an existing shell tube heat exchanger and was originally sized with a pinch temperature of 5 deg C. I realise I didn't formulate my question properly. The question is the following:
When an existing heat exchanger is rated for a different situation it may turn out that it becomes so efficient that the resulting pinch temperature is much lower than the said 5 deg C. For example 1 deg C. The flows have become higher, so there's no case of possible switching from turbulent to laminary flow or vice versa. I am aware of the fact that a higher flow will result in a higher pressure drop over the heat exchanger. Are there any other reasons of physical nature (thermodynamically or hydraulically) that will limit the use of this existing heat exchanger under that circumstance ( i.e. with a much lower pinch temperature)?
I would appreciate your additional comment on this.
With best regards,
Karel Postulart
Karel Postulart, The Netherlands
Nuon Power Generation
RE: Minimum pitch temperature
This is all now very theoretical. Depending on what changes you make to the fluids (change type of fluid and/or their flows), the exchanger may slightly under-perform or over-perform compared to the base case. You may get higher performance (duty) of the exchanger but with higher pinch temperature, simply if you alter inlet temperature of one of the fluids. For example, if you increase hot fluid inlet temperature, LMTD will increase and you'll get more duty out of the exchanger but depending on the flowrates and exchanger design/geometry the pinch temperature difference might actually increase. The other way round is also possible under certain circumstances, by e.g. swapping from liquid-liquid to gas-gas exchanger where temperature approach is always higher due to lower heat transfer coefficients in gas service.
For a properly designed exchanger, I don't think it is possible to obtain substantially lower pinch temperature - unless the original design was way below the optimum. Much will always depend on the exchanger type/geometry (i.e. for TEMA E-shell with 2-pases, the cold fluid outlet temperature can never be higher than hot fluid outlet temperature, due to the element of cross-flow in these exchangers, making it impossible to cross this gap). On the other hand, with Plate & Frame exchanger or brazed aluminum exchanger you can get very tight temperature approach (less than 1 degree for BAHX).
You have two options - either to consult the exchanger supplier for re-rating of the equipment in new/altered service, or to have a heat transfer expert running simulations in specialized software (e.g. HTRI) to see what the performance prediction will be under new conditions.
Dejan IVANOVIC
Process Engineer, MSChE
RE: Minimum pitch temperature
TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers
RE: Minimum pitch temperature
There are other considerations like flow velocity being very low, resulting in fouling of the heat exchanger.
RE: Minimum pitch temperature
The only mechanical detail that I'd bring up would be for if this is a fixed tubesheet heat exchanger and you have significantly changed the flow conditions from the original design. You may need to take a closer look at the mean metal temperatures of your shell vs. tubes, and examine if you have any new issues related to differential thermal expansion.
It's impossible to tell from the information provided if this is a real concern, but I always bring it up when people talk about changing heat exchanger operating conditions.
RE: Minimum pitch temperature
In some cases where the process fluid temp must be limited to a certain range throughout the entire turndown range, a different design approach must be adopted. If you can disclose details of this application and why you have a concern with decreasing approach temperatures ( which can be translated to an LMTD F factor approaching 0.75 or so ), then we may be able to help.