Cross-flow Heat Exchanger 2 phase calculation

In summary, if the pressure is 1 atm., the temperature of the liquid/gas mixture once it starts boiling is about 180 degrees Celsius.
  • #1
cheetahunter
3
0
Hi all!
I'm working on a project of heating water from exhaust gases of kamin's chimney.
I have a cross flow heat exchanger (see pdf).
we have done calculations of heat transfer rate and the outlet temerature of the water, and got 150C.
so it probably gets boiled somewhere inside the HX. I need to do further calculation which will consider 2 phases of the water and new outlet water temperautre and heat transfer rate.
Does anyone can give me a direction?
 

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  • #2
Tell us a little bit about how you did the analysis (assumptions and equations) when you assumed that the water wasn't boiling. Is your objective to heat the water or to cool the exhaust gas? Are you trying to transfer as much heat as possible? How accurate does your answer have to be?

Chet
 
  • #3
I have used the e-ntu method. I have assumed that the water remains liquid (single phase) but then I got that the outlet temperature of the water reaches 150C .
I am trying to heat the water, and to transfer as much heat as possible.
I need to get atleast a range of outlet water temp's, but accurate answer will be great also
 
  • #4
cheetahunter said:
I have used the e-ntu method. I have assumed that the water remains liquid (single phase) but then I got that the outlet temperature of the water reaches 150C .
I am trying to heat the water, and to transfer as much heat as possible.
I need to get atleast a range of outlet water temp's, but accurate answer will be great also
Is the pressure 1 atm (roughly)?

Chet
 
  • #5
yes.
 
  • #6
So, if the pressure is 1 atm., what is the temperature of the liquid/gas mixture once it starts boiling? So, recalculate the heat exchanger, for the part of the tubing up to this temperature, and find the corresponding length. Then recalculate the remainder of the heat exchanger holding the temperature fixed at this value. But, if all the water boils away before the end of the tube, you also need to consider a third section in which the water vapor is superheated.

Chet
 

1. How does a cross-flow heat exchanger work?

A cross-flow heat exchanger is a device that transfers thermal energy from one fluid to another, without the fluids coming into direct contact with each other. In a cross-flow heat exchanger, one fluid flows perpendicular to the other, allowing for efficient heat transfer through a barrier. This is typically achieved through a series of tubes or plates, with one fluid passing through while the other flows over or around them.

2. What is meant by 2 phase calculation in a cross-flow heat exchanger?

In the context of a cross-flow heat exchanger, 2 phase calculation refers to the consideration of both liquid and vapor phases of a fluid when calculating heat transfer. This is important because many heat exchangers deal with fluids that can exist in both phases, such as water and steam, and the heat transfer characteristics of each phase may differ. Therefore, a thorough calculation must take into account both phases for accurate results.

3. How do you calculate the heat transfer coefficient in a cross-flow heat exchanger?

The heat transfer coefficient in a cross-flow heat exchanger can be calculated using the Nusselt number, which is a dimensionless correlation that relates the heat transfer rate to the fluid flow properties. The Nusselt number can be calculated using the Reynolds number, Prandtl number, and the geometry of the heat exchanger. It is also important to note that the heat transfer coefficient may vary along the length of the heat exchanger, so an average value is typically used.

4. What factors affect the efficiency of a cross-flow heat exchanger?

The efficiency of a cross-flow heat exchanger can be affected by a number of factors, including the design of the heat exchanger, the properties of the fluids, the flow rates, and the temperature difference between the two fluids. Other factors that may impact efficiency include fouling, which is the buildup of deposits on the heat transfer surface, and pressure drop, which is the loss of pressure as the fluids pass through the heat exchanger.

5. What are some common applications of cross-flow heat exchangers?

Cross-flow heat exchangers have a wide range of applications in various industries. They are commonly used in heating, ventilation, and air conditioning (HVAC) systems to transfer heat between the air and a liquid, such as water or refrigerant. They are also used in power plants to cool steam and condense it back into water. Other applications include refrigeration systems, chemical processing, and oil and gas production.

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