Exchanger parameters to verify Heat Tranfer Efficiency 2

[Vivaldi M. Smith]

Good morning,
I have a shell and tube Heat exchanger to heat gas to regenerate Propane Treaters. The exchanger is a 35"x20'; design duty is 7.129MMBTU/Hr. I suspect there is a problem with the exchanger because the Hot side ( Chemterm Oil) temperature is 565 F and the Cold side ( Chemterm Hot Oil) is 530; However, the Cold Side (Natural gas circa 85% Methane ) inlet Cold side is about 94 F and the Hot Side is about 192 F. 1. I am wondering what exchanger performance parameters I should check as I believe the Gas Hot side should have a much higher temperature than this.
2. What could be the parameters that prevent the hot side from reaching much higher temperatures?
3. P&ID says duty is 7.1 MMBTU/hr is this value based on the Hot Oil Q calculations?

Your Help is great,

 

[TBP]

I might suggest using terms like 1/ hot side inlet temp and hot side outlet temp and 2/ cold side inlet temp and cold side outlet temp.

What cold side outlet temperature was specified on the design?

 

[EdStainless]

Do you know any of the flow rates?
What was the design hot oil temp and cold gas temp?
You are heating the oil externally so what is controlling that?

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[georgeverghese]

Pls post the process / thermal TEMA datasheet for this HX

 

[pierreick]

Hi,
On top of design parameters make sure the hot oil is in good condition (degradation)!
Pierre

 

[shvet]

I am wondering what exchanger performance parameters I should check

Overdesign heat transfer area or ratio design:actual heat transfer resistance coefficient.
Measure fluids inlet/outlet composition/flowrate/pressure/temperature, model actual fluids properties, find HE datasheet and drawings, input to a modelling software (HTRI, Aspen EDR), model heat transfer and compare results with design. Easy.

 

[Christine74]

With heat exchangers there's absolutely no requirement for the temperature rise of the cold fluid to be equal to the temperature drop of the hot fluid, if that's what you're asking. For sensible heating/cooling, the difference in temperature changes across either side of the exchanger could be due to differing flow rates and/or different specific heat capacities of the fluids involved.

As far as I'm aware there is no consensus as to how to determine how "efficient" a particular heat exchanger is but I suppose it could be defined in terms of overall heat transfer coefficient, heat transfer rate per total installed cost, heat transfer rate per unit of pressure drop, heat transfer rate per unit of operating cost, heat transfer per unit of time between required cleanings, etc. And any of these will be lower if the fluids you're using happen to have a high viscosity, low thermal conductivity, etc. so easy way to make a comparison.

Most likely your exchanger is limited by the gas-side heat transfer coefficient, which in turn is limited by the available pressure drop on the gas side.


-Christine

 

[Vivaldi M. Smith]

Hello CHristine, thanks. Great inputs. I have checked the gas composition in and out of the exchanger and it has not changed much. I am supposing there must be some dirt in the exchangers since it has not been taken out of service for maintenance for almost ten years now.

 

[TBP]

In your initial question, you write: "... as I believe that the Gas Hot side should have a much higher temperature than this."

Why? Is there a downstream process issue arising from the HX outlet gas temp?

 

[Vivaldi M. Smith]

TBP, yes, the Hot gas outlet hot side is used to regenerate Propane Treaters, and the sequence cannot reach the design temperature limit of the Heating period.

 

[LittleInch]

Vivaldi,

You say nothing about mass flow rates or gas pressures.

Any change in any parameter, be it gas pressure, gas flow rate, oil flow rate could equally have a large impact on the heat flow.

Fouling could be an issue for sure, but first see if the heat in matches the rated power. All you need to know is oil flow rate and nheat capacity of the oil. This would be a start point.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.