9 replies to this topic

[HOUTAN]

good day all

 

im working on a mini design project where i was asked to design a heat exchanger and to find its area, my question is how o calculate the overall heat transfer coefficient U to find the area using the following equation:

 

q= UA deltaTlmtd

 

how do i find U? 

 

im heating up hexanol from 50 Celsius to 100 Celsius using LPS

 

thank you 

[breizh]

Houtan ,

 

Either you assume a global heat transfer coefficient  ( link) or you calculate it ( Heat transfer textbook or link ) .

 

http://www.engineeri...ng/thermal.html

 

good luck.

 

Breizh

Edited by breizh, 25 November 2014 - 12:54 AM.

[HOUTAN]

thank you dear sir , 

 

i have another question , lets say i have lps steam to heat the hexanol from 50 to 100 celsius .

 

i know the inlet temperature of steam , how can i calculate the out let temperature of steam to get the LMTD? 

[ahmadikh]

 HOUTAN,

 

As temperature remains the same during a phase, the outlet LP Condensate temperature is the same as the inlet LP Steam temperature, unless you consider a pressure drop in your design, then for determining the outlet temperature of condensate, you have to deduct the pressure drop from the inlet LP Steam saturation pressure and at that pressure you will have to find the saturation temperature as per steam tables. However, actually the condensate will be slightly colder (subcooled) in reality...

 

Hope this helps...

 

Best,

Milad

[shan]

The outlet condensate temperature is the steam saturate temperature.  If your steam is de-superheated prior to entering the heat exchanger, the steam inlet temperature and the condensate outlet temperature are identical because latent heat is desired to be transferred.

[srfish]

Houtan,

 

ahmadikh gave you a good answer.

[srichem]

Can we use Superheated Steam at 300 Deg.C and 2 Bar pressure to Heat air from 30 Deg.C to 180 Deg.C?

We are trying to utilise the waste SH Steam in the plant to heat theAir which is required for one of the processes. Air flow is about 6000 cfm.

[srfish]

You can use the steam but you can't get  180 C outlet air temperature.Since the saturation temperature of the steam is  only 120 C. 

[shan]

There is no problem to just recover the steam sensitive heat from 300 C to 190 C with a co-current or counter-current heat exchanger. Also you may use a counter-current heat exchanger with hot end temperature difference between 300 C and 180 C and cold end temperature difference 120 C and 30 C for heating your air to 180 C.  However, the steam sensitive heat content (Cal/kg) is much less than its latent heat (Cal/kg).  Therefore, the more sensitive heat you recover and the less latent heat you transfer,  the higher steam flow rate (kg/hr) will be required and the larger heat transfer area  will be needed.

Edited by shan, 02 December 2014 - 01:29 PM.

[srfish]

Plot this out on a plot of temperature vs heat load and you can see that you can not heat the Hexanol above approximately 125 C. It cannot reach 180 C.