4 replies to this topic

[Light]

Hello to every one out here.

 

We all know that as the temperature of approach (i.e. the temperature difference between the two fluids exchanging heat in a heat exchanger) increases, the area of the heat exchanger required for a specific duty decreases. Then why is it that the temperature of approach in sub-ambient operations like air separation and natural gas liquefaction is kept as low as 1 or 2 degree Centigrade? What are the factors which limit the maximum temperature approach that can be used practically?

Edited by samkha, 15 April 2014 - 12:36 AM.

[Pilesar]

In a cryogenic system, the cost of incremental heat exchange surface area is small compared to the cost of additional refrigeration utility. In the real world, the heat exchange streams will not be exactly as the steady state design basis. So it is good to design some distance away from a zero temperature pinch for operability. The actual minimum internal temperature approach may be lower than design, but it is risky to depend on a lower MITA when designing the rest of the system.

[Bodhisatya]

In Air separation units,the Warm End Delta T,the temperature difference between hot Air stream and outgoing cold fluids,needs to be as low as possible,as this infers to energy transfer by hot fluid to Cold one..If this happens to be high,we are losing cold and will not be achieve the liquefaction temp of Air at that specified pressure.

 

Bodhisatya.

Edited by Bodhisatya, 17 April 2014 - 05:55 AM.

[Light]

Pilesar and Bodhisatya, thanks to both of you for replying.

 

the temperature difference between hot Air stream and outgoing cold fluids,needs to be as low as possible,as this infers to energy transfer to hot fluid by Cold one..If this happens to be high,we are losing cold and will not be achieve the liquefaction temp of Air at that specified pressure.

What do we mean by the energy transfer to hot fluid by cold fluid? And how do we lose cold when delta T is high? Please pardon my lack of knowledge.

[Art Montemayor]

Samkha:

 

I believe there is a misunderstanding of the fundamentals of process heat transfer going on – or perhaps a typo error.

 

The thermal energy in a hot fluid is transferred to a colder fluid.  It can never be transferred from cold to hot.  It is the hotter fluid that has the higher enthalpy and the enthalpy is the driving force that transfers the thermal energy.  The process is analogous to electrical flow: coulombs flow from a high voltage reference to a lower one.  So do Btus.  It can never be the other way around.  You are right in questioning the statement “energy transfer by hot fluid to Cold one”.

 

The temperature approach (the temperature difference between the incoming warm fluid and the outgoing cold fluid) to which you refer is kept as close as possible in a heat exchanger in order to maximize the amount of heat transfer.  That explains the reason for designing for a very close temperature approach in cryogenic processes.  Of course, the more heat that is transferred, the larger the required heat transfer area.  This is accordance with the basic equation: Q = U A (LMTD).  Therefore, the main factor that comes into play in selecting a close approach is economics - as Pilesar has explained.  The cost of refrigeration in a cryogenic process is the driving force that decides to what extent you can justify large heat transfer areas in order to maximize the heat transfer.