Saturday, August 10, 2013

How's It Flowing?

When air, water, refrigerant, or any fluid flows through a pipe, the flow can be characterized as laminar or fluid. I can hear you saying “what does this have to do with HVACR?” It applies to how refrigerant flows through piping, how air flows through ducts, how air flows across coils, and how refrigerant flows through those same coils. In short – it applies to pretty much everything in HVACR!

Laminar means in layers. Laminated furniture like you find in most schools has a thin layer, or laminate, on the surface to make it pretty and usually cardboard or glued together sawdust underneath to make it appear substantial. In laminar flow, the stuff on the top stays on the top, the stuff on the bottom stays on the bottom: there is no mixing. In turbulent flow, the fluid swirls all around mixing everything together and creating all sorts of eddies and whirlpools. If you turn on a water faucet that does not have an aerator on it, the stream of water is laminar. The water falls straight down without much mixing. After it hits the sink you have turbulent flow – swirling all around and mixing. Laminar flow is preferred in refrigerant piping, water piping, and ductwork. This is because the eddies, whirlpools, and swirling found in turbulent flow add a great deal of resistance to flow.

For example, take a bunch of people trying to leave a crowded room. If they file out in an orderly line moving in the same direction, they can all get out faster than if there is a lot of pushing and shoving with people going off in all directions and running into each other. To get fluid through a pipe or duct with the least amount of energy loss, laminar flow is the way to go.

High velocities tend to create turbulent flow while low velocities create laminar flow. That is why the resistance to flow increases so dramatically as pipes get smaller. Velocities increase and the flow goes from laminar to turbulent, increasing resistance to flow within the fluid. Its not just that the cross sectional area is smaller - the fluid is getting in its own way as it tires to go through the pipe.

In heat transfer coils such as evaporators and condensers, we need turbulent flow. In heat transfer, all the swirling becomes an asset. Turbulence makes more of the fluid contact the surface of the pipe and slows down the fluid, which helps it transfer more energy. This is why coils have features such as rifling, twisted tubing, or turbulators to encourage swirling and mixing.

2 comments:

  1. So basically a turbulent flow is vital as we need this in heat coils such as evaporators and condensers. This will help transfer more energy then. I wonder what happens without it?

    - RandolphAustin.com

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  2. Roger Hensley, CMS puts on an outstanding presentation of the effects of different fittings using the Lennox Flo Vu water table. It visually demonstrates the effects of fitting and diffuser placement. He puts these demonstrations on for RSES seminars. If you ever have the opportunity of going to his seminars I highly recommend as it will clear up any misunderstandings on air flow.

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