Variation in tube wall temperature as steam condenses to form liquid

In summary, the tube wall temperature in a simple circular condenser tube is determined by both the cooling medium on the tube exterior and the condensing vapor inside the tube. The temperature at the interface between the condensate and the vapor is determined by the pressure, as a result of vapor/liquid equilibrium. The temperature at the wall is lower than this and decreases as the condensate film becomes thicker along the length of the tube. However, the wall temperature is also affected by the cooling medium on the outside, and if the flow rate is high, the temperature difference across the condensate film may be constant.
  • #1
sanka
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I'm having a bit of difficulty conceptually understanding a problem. Hoping someone here can clear it up for me.
In a simple circular condenser tube (which is cooled by water or air) what happens the tube wall temperature as the steam inside is condensed? Intuitively I would have thought that as the steam is condensed inside the tube to form liquid condensate, the tube wall temperature would decrease along the length of the tube (from inlet to outlet) as the condensate film becomes thicker as more steam is condensed. Does that make sense? Or is the wall temperature governed by the cooling medium on the tube exterior and thus, remains constant along the length of the tube as dictated by the cooling medium - implying that it is independent of the condensate film thickness?

I know that the condensate film acts as a barrier to heat transfer and thus, is a thermal resistance but I am just trying to understand what happens to the wall temperature as a result of this. I suppose that the steam temperature remains relatively constant along the tube length (isothermal heat transfer for condensation) For the thermal resistance to increase along the length (due to the film), a temperature difference must be present and this must increase along the length. For the temperature difference (ΔT=Tsteam-Twall) to increase, the wall temperature must then decrease (as the steam temp remains constant).

Maybe I've answered my own question! But I would be interested to hear any other ideas on the matter.
 
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I'm sorry you are not generating any responses at the moment. Is there any additional information you can share with us? Any new findings?
 
  • #3
The tube wall temperature is governed by both the cooling medium outside the tube and the condensing vapor inside the tube. There is resistance to heat transfer on both sides of the wall. Inside the tube, where the vapor is condensing, the temperature at the interface between the condensate and the vapor is determined by the pressure, as a result of vapor//liquid equilibrium. (assuming the pressure is constant in the condenser). The temperature at the wall is lower than this, and heat is conducted through the condensate. As the condensate layer gets thicker, the rate of heat transfer (and condensation) decreases. Outside the tube wall, you have convective heat transfer occurring, determined by the outside heat transfer coefficient. The wall temperature is higher than the bulk water or gas temperature, and heat is conducted through the thermal boundary layer from the wall to the bulk. The rate of heat conducted through the outside boundary layer must match the rate of heat conducted through the inside condensate layer at each location.

Chet
 
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  • #4
sanka said:
...what happens the tube wall temperature as the steam inside is condensed? ...

Not that it really changes anything in the question or the replies, but in the condensers I'm familiar with (large steam power plants) the cooling water runs inside the tubes and the condensing steam is on the outside. One effect of this is that the condensed liquid can drip off the tubes; I think the condensate layer thickness is therefore pretty much constant throughout the unit.
 
  • #5
Chestermiller said:
the temperature at the interface between the condensate and the vapor is determined by the pressure, as a result of vapor//liquid equilibrium. (assuming the pressure is constant in the condenser). The temperature at the wall is lower than this, and heat is conducted through the condensate. As the condensate layer gets thicker, the rate of heat transfer (and condensation) decreases

Thanks a lot for your response Chet, I appreciate it.
For the most part I understand everything you said. However, one thing is still bothering me and I have highlighted it in the quotation above. You say the temperature at the wall is lower than the temperature at the interface, which of course must be the case for condensation to occur. I know that a temperature difference must be present at each point (locally) along the tube length or else no heat would flow. The issue I am having difficulty with is what exactly happens this temperature difference along the length of the tube? Specifically, as the condensate film grows in thickness (as more steam condenses, more liquid mass is added) along the tube length, does the resistance provided by the thickening film, lead to a reduction in the wall temperature? At least I would have thought that to be the case...
 
  • #6
sanka said:
Thanks a lot for your response Chet, I appreciate it.
For the most part I understand everything you said. However, one thing is still bothering me and I have highlighted it in the quotation above. You say the temperature at the wall is lower than the temperature at the interface, which of course must be the case for condensation to occur. I know that a temperature difference must be present at each point (locally) along the tube length or else no heat would flow. The issue I am having difficulty with is what exactly happens this temperature difference along the length of the tube? Specifically, as the condensate film grows in thickness (as more steam condenses, more liquid mass is added) along the tube length, does the resistance provided by the thickening film, lead to a reduction in the wall temperature? At least I would have thought that to be the case...
Well, as I suggested earlier, it also depends on what is happening on the other side of the wall. Both heat transfer resistances (inside and outside) need to be considered. If the fluid flow rate outside the tube is very high, its temperature won't change much. If this were the case, the temperature difference across the condensate film divided by the temperature difference between the wall and the bulk flowing coolant would be equal to the heat transfer coefficient on the coolant side divided by the heat transfer coefficient on the condensate side (effectively equal to the thermal conductivity of the condensate liquid divided by the condensate film thickness).

Chet
 

Related to Variation in tube wall temperature as steam condenses to form liquid

What causes variation in tube wall temperature during steam condensation?

The variation in tube wall temperature during steam condensation is primarily caused by the heat transfer process between the steam and the tube walls. As the steam condenses, it releases heat, which is transferred to the tube walls. The rate of heat transfer and the surface area of the tube walls can affect the variation in temperature.

How does the tube material impact the variation in temperature during steam condensation?

The material of the tube can greatly impact the variation in temperature during steam condensation. Different materials have different thermal conductivities, which can affect the rate of heat transfer and therefore the temperature variation. Materials with high thermal conductivity, such as copper or aluminum, can lead to lower temperature variations compared to materials with lower thermal conductivity, such as stainless steel or plastic.

What is the role of the condenser design in the variation of tube wall temperature during steam condensation?

The design of the condenser can also play a significant role in the variation of tube wall temperature during steam condensation. Factors such as the size and shape of the condenser, the placement of tubes, and the flow rate of the steam can impact the heat transfer process and ultimately affect the temperature variation.

How does the surrounding environment affect the variation in tube wall temperature during steam condensation?

The surrounding environment can influence the variation in temperature during steam condensation in several ways. For instance, the temperature and humidity of the air can affect the rate of heat transfer from the tube walls to the surroundings. Additionally, the presence of other heat sources or insulation can also impact the temperature variation.

Is there a way to minimize the variation in tube wall temperature during steam condensation?

Yes, there are several ways to minimize the variation in tube wall temperature during steam condensation. These include using materials with high thermal conductivity, optimizing the design of the condenser, controlling the surrounding environment, and adjusting the steam flow rate. Additionally, regularly cleaning and maintaining the condenser can also help minimize temperature variations.

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