Heat of Friction in Ductwork

Maybe, though the air is on the cool side for an industrial installation in temperate zones so it may be simply picking up heat along the way from other sources. After all, it is going to be pretty turbulent in large ducts, particularly at those sort of speeds, and turbulence enhances heat transfer.

No

A drop in pressure will cause it to cool.

The increase in temperature is related to heat transfer and retention in the ductwork.

On start-up, the ductwork absorbs heat from the air and cannot release the heat as fast as it is absorbing it. The ductwork gets hotter to the point where it is hotter then the air passing through it. The hotter the duct gets, the larger is the temperature difference with the surrounding air. The larger this difference is, the faster it will release heat. this continues until balance is achieved. The result is the ductwork operating at a temperature above the temperature of the input air where some of the heat is transferred back to the input air.

This is good, since the temperature of the air does not cool until it is out the ductwork. if it cooled in the ductwork, you would have condensation problems.

I think pressure drop due to frictional losses does cause a slight heating effect - where else does this energy go? From conservation of energy: loss of potential energy (pressure) must translate into something, that is, heat.

The pressure drop is caused mainly by equalization - open at the other end. The air moves from a high pressure towards a lower pressure.

The amount of drop due to friction is negligible unless there are allot of bends in the ductwork. Air is more likely to create turbulence which in fact decreases the friction much the way an avalanche picks up speed until it is faster then gravity.

If you compress air it creates heat, it takes energy to pressurize it. If you fill a high pressure tank it heats up. If you let all the air out it cools. The ambient temperature is directly related to the density + the energy of the molecules.

The energy is converted to velocity.

This sounds suspiciously like a dust collection system. I have always noticed a temperature rise in these systems but I always attributed it to the equipment and processes connected to the system. If a grinder, sander or cutting operation is vacuumed by the dust collection system, all the heat generated from the milling process is drawn into the duct as well as much of the machines motor, hydraulic and bearing heat.

A plant where I laid out a 100HP dust collector gets so warm they open the doors in the dead of the Midwestern winter to cool it down. In the summer it vents outside to create a nice 36,000 CFM exchange of air.

1. What is the quantity of air?

2. Is the duct insulated, as it should be?

3. What is the exact length of travel?

The air at 8.5 inch WC has potential energy, whch causes it to move towards a low pressure exit point. The potential energy overcomes friction with the walls of the duct, and all energy required to overcome this friction converts to heat. Energy will be conserved. A heat balance equation could be set up taking into account energy gain by the air due to friction + energy gain due to heat transfer into or out of the duct depending on external temperature.

1 The duct is 14" round.

2 It has 2" of insulation.

3 The distance between temp measurments is about 150 ft

Based on the above data, my calculations indicate a temperature rise as follows:

1. Due to friction in duct................................2.41 degF

2. Due to heat transfer into duct assuming a temp. of 90 deg F outside the duct..0.77F

3. This translates to a total rise of 3.18 deg F.

The differential between your measured temperature of 5 deg F and calculated temperature of 3.12 degF could be due to the following reasons:

• Outside air temperature greater than my assumed 90 deg F
• I have assumed the insulation to have a an R value of 4 per inch. this could be lower, and will increase heat transfer, and cause air temperature to increase
• Approximations or errors in measurement of duct pressures and temperatures. Quality of instruments and certification of accuracy is important.

Thank you this was the best answer I recieved and also the most logical. As an engineer I am always looking for conservation of energy and with such a pressure drop and high velocity your answer makes the most sense. If I give you a change of conditions could you give me another calculation or tell me what equations you used to arive at the answer. The numbers I gave were the original design. They have raised their flow rate and did not put a 14 " duct in but rather a 13" one. We are in the process of determining the condition of the insulation and the ambient conditions in the pipe chase in which the duct is located.

You said the air was saturated. Water is a great thermal conductor compared to air. Combine that with flow and you are probably picking up heat from the surrounding duct, especially if it's not insulated.

The pressure drop will have some affect although it should be minimal with the pressure you're talking about. When there is a decrease in pressure, the air is allowed to expand causing it to draw in heat (endothermic). The fact that it is saturated means there is less compressible gas per volume than dry air so the effect on individual air molecules will be greater, but the water molecules will not compress/decompress.