7 replies to this topic

[James C]

Hello - fundamental thermo question which is puzzling me...

It concerns a multiple effect evaporator with condenser. Hot vapour is water, cooling medium is water.

My question stems from the following:
When the flow of cooling water to the condenser is increased, the pressure in the condenser (and thus the pressure profile of the entire evaporator) is reduced.

Here's how I understand it...

• Heat transfer area of condenser remains constant
• Overall heat transfer coefficient is increased slightly; given that the Reynolds number on cold side is increased
• Cooling water temperature out is reduced (assuming heat load is constant and inlet temperature is the same)

How does this in turn lead to a reduction in evaporator pressure, please explain in basic terms (what's actually physically happening rather than just formulae! ).

Thanks in advance for any replies

Edit: Also the mass flow rate of condensate will be the same for a given flow "in" of water and steam. Some thermodynamic activity is occuring here I just cannot work out what!

[Art Montemayor]

I think you forgot to mention that the increased cooling water flow increases the heat removal in the condenser.  It does this because of the heat balance.  If you do a heat balance under two cooling water scenarios, you will see how the heat removal increases.   This is due to increased turbulance by the cooling water (higher Reynolds Number) resulting in a higher heat transfer coefficient.

[James C]

So we have the equation Q = (U)(A)(LMTD) equation 1

 

And for the cooling water Q = (m)(Cp)(T2-T1) equation 2

 

For both scenarios and a given energy load, Q, if we increase (m) - then T2 will decrease, is this correct?

 

You are suggesting for equation 1 (U) will increase, I agree, but surely the energy (Q) removal would remain constant, as you are evaporating the same amount... So the LMTD would need to decrease - correct?

 

What I do not understand is why this temperature/pressure decrease physically occurs. Is it that more vapour is condensing? Please explain.

[James C]

I would also like to ask: is the LMTD (log mean temperature difference) valid for condensers? Quote from Wikipedia

 

 

A particular case where the LMTD is not applicable are condensers and reboilers, where the latent heat associated to phase change makes the hypothesis invalid.

[PingPong]

Wikipedia is not the most reliable source of information.

 

LMTD is valid for reboilers, and also for condensors as long as condensing enthalpy line is more or less straight.

If a superheated vapor enters a condensor then you need to view the condensor as if it consists of 2 parts: part 1 for desuperheating, and part 2 for condensing. For each part you can then calculate an LMTD, a Q, a U, and an A. Total required A is then A₁ + A₂

Edited by PingPong, 02 November 2013 - 01:21 PM.

[curious_cat]

Can you draw a sketch? I'm a bit confused as to what you are asking. 

[yjackz]

Hi James,

I have the opportunity working with 2 multiple effect evaporation plants. I hope my answer can be of help to you.

I think you are right in saying that the temperature/pressure decrease physically occur is due to more vapour being condensed. Ideal gas law PV=nRT may simply explain the temperature/pressure relationship. The more vapour particles condense, the more empty the space it leaves behind, hence decrease in pressure or increase in vacuum. Corresponding to the pressure, the temp will reduce accordingly.

[S.R.Shah]

Hello - fundamental thermo question which is puzzling me...

It concerns a multiple effect evaporator with condenser. Hot vapour is water, cooling medium is water.

My question stems from the following:
When the flow of cooling water to the condenser is increased, the pressure in the condenser (and thus the pressure profile of the entire evaporator) is reduced.

Here's how I understand it...

• Heat transfer area of condenser remains constant
• Overall heat transfer coefficient is increased slightly; given that the Reynolds number on cold side is increased
• Cooling water temperature out is reduced (assuming heat load is constant and inlet temperature is the same)

How does this in turn lead to a reduction in evaporator pressure, please explain in basic terms (what's actually physically happening rather than just formulae! ).

Thanks in advance for any replies

Edit: Also the mass flow rate of condensate will be the same for a given flow "in" of water and steam. Some thermodynamic activity is occuring here I just cannot work out what!

Hi James;

 

Please find following clarification

* Increase Cooling flow rate increases condensation

* In condensation ;Vapor condenses to liquid.Volume of vapor is quite higher than liquid.So,Continuous Volume contraction takes place.This results negative pressure over Heat transfer surface.In other words,Heat transfer surface acts like vacuum source.

*Increases in condensation causes increase in negative pressure,so reduction in total system pressure

 

Even if ,evaporation is under vacuum;There shall be increase in guage vacuum.

 

SRShah