Mechanical Engineering Gas Question

[Shadow]

In a closed cooling system will running refrigerant through the system at faster speeds after it goes through the compressor cause it loose more heat than if it was traveling at slower speeds? Much like running cooler air over the pipe causes it to loose heat. Assume that in both cases the only difference in the system is the speed the refrigerant is traveling. It’s hard for me to understand the mannerisms of all the facts involved like pressure, velocity, temperature, and density.

 

[tsg]

The vast majority of heat removal is caused by the converting the refrigerant from liquid to gas. If it is moving too fast, the gas won't boil off and you lose a lot of capacity. You also run the risk of "slugging" (sending liquid refrigerant back to) the compressor. Too slow and the heat gain in the gas will be too much for the outdoor coil to reject to the outside and turn it back into a liquid.

 

[Frinkiak7]

Forgive me if this seems naive, but doesn't that also increase the overall pressure, and therefore increase the risk of a failure?

 

[Dilb]

Fluids, in general, will have a greater heat transfer coefficient as velocity increases, so a faster moving fluid in contact with a hotter surface will absorb heat faster than a slower moving fluid. However, it also spends less time at the surface.

There are 2 general types of fluid flow, laminar and turbulent. Laminar is smooth flow, so the fluid is somewhat like a solid: heat slowly sort-of-diffuses through it, and the fluid sets up a temperature drop from the edge of the flow to the middle of it. This transfers heat really slowly through it. Turbulent flow is mixed up, so there is much less of a temperature difference, and heat is transferred at the edge at a much higher rate. A 10 to 100 times increase in heat transfer rate is easily possible. Flows are naturally laminar at low velocities, and naturally become turbulent at high velocities, although it's extremely hard to predict with any accuracy.

In a straight circular pipe, heat transfer in laminar flow is roughly independent of velocity, and is pretty low anyway. According to Wolverine Tube Heat Transfer Data Book, heat transfer in turbulent flow is roughly proportional to velocity^0.8, and by the time a flow can be expected to be turbulent, will be much greater than a laminar flow. For a pipe, this means that you can run a fluid through faster, and increase the temperature it comes out at (if it's heating up). However, after drop caused by the flow becoming turbulent, the temperature will drop a bit as speed increases.

To answer your question: No. The condenser will almost certainly have turbulent flow, because otherwise it would be too slow or large, and faster turbulent flows would not cause the fluid to lose more energy per volume of fluid. I'm not sure about the evaporator, because boiling is apparently much harder to deal with. You might get a layer of vapour that insulates the fluid, for example, which I'd guess could be reduced with a greater velocity.

 

[burrahobbit]

Also, the gas in the condenser is Condensing. In that condition, the heat transfer coefficient is an order of magnitude larger than the HTC from the tube wall to the air blowing across (even including the effect of the fins). So the increase in gas velocity will not increase the rate of heat transfer.