Outlet temperature of HE at higher inlet temp. 2

[process1907]

Guys,

Appreciate your input / view on this.

How to calculate the new outlet temperature of a fin fan cooler if the inlet temperature is increased.

The inlet temperature of the fin fan cooler under normal operation is 130 oC. The outlet temperature is 50 oC.

There is one scenario that the inlet temperature to this cooler could be as high as 200 ºC max. How could I calculate the new outlet temp of this cooler.

Of coz, a simple solution is to compare the delta T between these 2 condition. i.e the new outlet temp is 120 ºC (delta T= 80ºK).

But there is a thought that at higher temperature, the efficiency of the cooler could be higher and hence the outlet temperature would probably lower than 120 ºC.

How could I confirm this by calculation?

My problem the design temperature of downstream equipment is lower than 120 ºC. Or else, I may need to change the equipment material to higher grade which is costly.

 

[LittleInch]

Ask the vendor.

Your heat transfer rate would increase but unless all your other conditions (flow rate, thermal capacity of the fluid etc) remain exactly the same as your normal conditions, the temperature out could vary a lot. You can calculate, but need to input a lot of data about your HX - Area, heat transfer coefficient, fouling co-efficient etc. Also if your HX isn't designed for it you could overstress your HX - these things tend to be designed pretty close to the limit. you really need to go back to the OEM, they would probably do this sort of check very quiclky

I would red flag this post and try in the mechanical engineers heat transfer post, but be ready to supply lots of information like I've just described above.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Drexl]

Hi,

If your problem is selection of design temperature, have you considered that the fan might trip and you blow 200°C through the heat exchanger?

 

[Krazylion]

Ome method you can use is to determine the effectiveness of the fin fan cooler.

First idenitfy the porduct of the mass flow rate and the mass heat capacity for each fluid, which would be call C min.
Then calculate the maximum (ideal) heat load that can be transfer for the HE as given by;

q max = (C) min *(Thot,in- Tcold,in)

After this you can obtain the effectiveness for the HE, according to, e= q /q max, where q is the actual heat load transfer between the fluids.

This way now you can calculate the new heat load transfered for the HE, if any or both T inlet change, according to the equation: q= e*(Cmin)*(Thot,in-Tcold,in).

Finally this new heat load (q) is transfered in the HE, and using the equation for basic design a HE:

q = m Cp (Delta) T for each fluid

This method is based the fact that for an existing HE, its efectiveness does not change if inlet temperature of fluids change. Very useful

 

[process1907]

Drexl,

Thanks for the reminder. This scenario (fan trip) was also discussed in our HAZOP. The HE tube is designed to withstand up to 220 ºC.
This uncondensed gas will then end up at flare. Design concern is more for the equipment on the liquid / condensed side.