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Heat transfer and pressure

Doug_7 Member Posts: 247
System pressure has no effect on heat transfer. Water in an incompressible liquid. The thermal properties of water do not change with pressure - still 1 Btu / pound / *F.

The only things that really effect heat transfer are the temperature difference between the water and the room air and the heat transfer area. (Thinking radiators here.)

Water Flow rate has a small effect because increasing flow will reduce the water delta T - which increases the outlet temperature of the water.

The other important factor is fouled or dirty heat transfer surfaces - on either the water side or air side, or anything that restricts the free flow of room air across the heat transfer surfaces.



  • Any relation ?

    We know that the higher the temperature water flowing through emitters , the higher the BTU output .

    But what about pressure ?

    Say you have baseboard heat . Would there be a greater transfer of heat if the pressure was 25 psi. , compared to say 5 psi. ?
  • bob_46
    bob_46 Member Posts: 813

    question. IMHO no. The higher pressure water does contain more energy however, and the molecules of water do bang into the walls of the pipe harder.
    That's why the gauge reads higher. The amount of additional energy though would be quite small. Think about how many strokes on your Silver King pump it would take to raise the pressure in the system. The amount of calories you burn minus the friction in the pump would be how much energy is added. Maybe Scrook or someone else with an education can elaborate on this.

    There was an error rendering this rich post.

  • Andrew Hagen_2
    Andrew Hagen_2 Member Posts: 236

    Water is essentially incompressible, so static pressure has very little effect. Gases are a different story.

    Flow rate has a double effect. It decreases the delta-t and therefore increases the average temperature. It also decreases the thickness of the boundary layer of fluid "stuck" to the inside of the pipe. That layer acts like an insulator. Boundary layer thickness increases with higher viscosity and with lower velocity.
  • Yep , I know

    water cannot be compressed . But water pressure can increase against the piping . I was thinking ........ more pressure exerted on the piping , more heat transfer ?
  • Doug_7
    Doug_7 Member Posts: 247
    Temperature not Pressure

    For heat transfer the driving force is temperature not Pressure.

    The rate of heat transfer is proportional to the delta T.

    The equation used to calculate the quantity of heat transferred is U x A x delta T. U is the heat transfer coefficient, A is the surface area and the delta T is the difference in temperature between the media.

    System pressure does not enter into this equation. Pressure does not effect the rate of heat transfer.

    If you increase the pressure high enough to expand the pipe diameter - that will increase the surface area which will increase the rate of heat transfer.

    Easier to explain why something does happen than why something doesn't happen. There just is no relationship - its physics and we can't change it.

  • Doug_7
    Doug_7 Member Posts: 247
    Molecular Level

    If you are trying to visualize this at the molecular level - each molecule of water has an equal pressure working on it from all directions. Because it is incompressible, the molecule really doesn't care what that pressure is. Higher pressure does not cause the molecule to energize or move in any direction. So pressure has no effect.

    Temperature on the other hand energizes the water molecule to move faster and release heat.

    This is an important question, because we all need to know what does and what does not effect heat transfer.

  • Fred Harwood_2
    Fred Harwood_2 Member Posts: 195

    The notion that more pressure equals better heat transfer may come from the fact that water expands as it warms. However, once expanded by heat, water under additional pressure does not return to the same space, until it has given up the original expanding heat. Absent phase chances, liquid water in intimate contact with a heat exchanger is insensible to pressure, as long as contact can be maintained. As stated above, velocity and boundary layers are another calculation.
  • hot_rod
    hot_rod Member Posts: 22,486
    pressure does

    raise the boiling temperature, however. There are some ways to leverage that to move more heat.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • scrook_3
    scrook_3 Member Posts: 66

This discussion has been closed.