Heat Transfer – 3 Types

When you have two objects and they are of different temperatures, we know from experience that the hotter object will warm up the cooler one, or conversely, the colder object will cool down the hotter one.  We see this everyday, such as ice cooling a drink, or a fan cooling a person on a hot day.

The Second Law of Thermodynamics says that heat (energy) transfers from an object of a higher temperature to an object of a lower temperature. The higher temperature object has atoms with higher energy levels and they will move toward the lower energy atoms in order to establish an equilibrium. This movement of heat and energy is called heat transfer. There are three common types of heat transfer.13580963114_f222b3cdd9_z

Heat Transfer by Conduction

When two materials are in direct contact, heat transfers by means of conduction. The atoms of higher energy vibrate against the adjacent atoms of lower energy, which transfers energy to the lower energy atoms, cooling the hotter object and warming the cooler object. Fluids and gases are less heat conductive than solids (metals are the best heat conductors) because there are larger distances between atoms.  Solids have atoms that are closer together.

Heat Transfer by Convection

Convection describes heat transfer between a surface and a liquid or gas in motion. The faster the fluid or gas travels, the more convective heat transfer that occurs. There are two types of convection:  natural convection and forced convection. In natural convection, the motion of the fluid results from the hot atoms in the fluid moving upwards and the cooler atoms in the air flowing down to replace it, with the fluid moving under the influence of gravity. Example, a radiator puts out warm air from the top, drawing in cool air through the bottom. In forced convection, the fluid, air or a liquid, is forced to travel over the surface by a fan or pump or some other external source. Larger amounts of heat transfer are possible utilizing forced convection.

Heat Transfer by Radiation

Radiation refers to the transfer of heat through empty space. This form of heat transfer does not require a material or even air to be between the two objects; radiation heat transfer works inside of and through a vacuum, such as space. Example, the radiation energy from the sun travels through the great distance through the vacuum of space until the transfer of heat warms the Earth.

EXAIR‘s engineered compressed air products are used every day to force air over hot surfaces to cool, as well as dry and/or blow off hot materials. Let us help you to understand and solve your heat transfer situations.

To discuss your application and how an EXAIR Intelligent Compressed Air Product can improve your process, feel free to contact EXAIR, myself, or one of our other Application Engineers. We can help you determine the best solution!

Brian Bergmann
Application Engineer

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The picture “Energy Transfer – Heat” by Siyavula Education is licensed under CC BY 2.0

Heat Recovery from an Air Compressor

On the whole most of us are quite aware of the considerable savings that can be accomplished by wise use and recovery of energy.   One way that a plant can save substantially is to capture the energy that an electric motor adds to the compressed air from the air compressor.  As much as 80% to 93% of the electrical energy used by an industrial air compressor is converted to heat.  A properly designed heat recovery system can capture anywhere between 50% to 90% of this energy and convert it to useful energy.

The heat recovered is sufficient in most cases to use in supplemental ways such as heating water and space heating, however generally there is not enough energy to produce steam directly.

Ingersoll Rand Rotary Screw Compressor


Packaged air cooled rotary screw compressor lend themselves easily to heat recovery, supplemental heating or other hot air uses very well due to their enclosed design.  Since ambient air is directed across the compressors aftercooler and lubricant cooler where the heat can be easily collected from both the compressed air and the lubricant.

Packaged coolers are normally enclosed cabinets that feature integral heat exchangers and fans.  This type of system only needs ducting and an additional fan to minimize back pressure on the air compressors cooling fan.  This arrangement can be controlled with a simple thermostat operated vent on a hinge and when the extra heat is not required it can be ducted outside the facility.

The recovered energy can be used for space heating, industrial drying, preheating aspirated air for oil burners or  other applications requiring warm air.  Typically there is approximately 50,000 Btu/Hr of energy available from each 100 SCFM of capacity (at full load).  The temperature differential is somewhere between 30°F – 40°F above the air inlet temperature and the recovery efficiency is commonly found to be 80% – 90%.

We all know the old saying there is “no free lunch” and that principle applies here.  If the supply air is not from outside the plant a drop in the static pressure could occur in the compressor cabinet thereby reducing the efficiency of the compressor.  If you choose to use outside air for makeup, you might need some return air to keep the air above freezing to avoid compressor damage.

Heat recovery is generally not utilized with water cooled compressors since an extra stage of heat exchange is required and the efficiency of recovering that heat is normally in the 50% – 60% range.

To calculate annual energy savings:

Energy Savings (Btu/Yr) = 0.80 * compressor bhp * 2,545 Btu/bhp-hour * hours of operation.

If we consider a 50 HP compressor:

.080 * 50bhp * 2,545 Btu/bhp-hour * 2080 hrs/year =  211,744,000 Btu/yr

Where 0.80 is the recoverable heat as a percentage of the units output, 2,545 is the conversion factor.

Cost savings in dollars per year = [(energy savings in Btu/yr)/Btu/fuel) x ($/unit fuel)]/primary heater efficiency.

If you would like to discuss saving money by reducing compressed air demand and/or any EXAIR product,  I would enjoy hearing from you…give me a call.

Steve Harrison
Application Engineer
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Photo courtesy of CC BY 3.0, https://en.wikipedia.org/w/index.php?curid=32093890



Methods Of Heat Transfer

“Nothing happens until something moves.”
-Albert Einstein

These five words are the foundation on which the science of physics is built upon. This statement not only applies to the things we can see, but to those we can’t…like heat transfer.

OK; technically, we CAN visually observe the EFFECTS of heat transfer…that’s called “reading a thermometer.” But the actual mechanism of heat transfer takes place at a molecular level, and concerns the rate of motion of those molecules: the higher the rate of molecular motion, the higher the heat of the material. Hence, the higher the rate of CHANGE of that molecular motion, the higher the heat transfer rate is.

All you need for heat transfer to occur is a difference in temperature between two materials. Contact, or even proximity, helps…but not always. More on that in a minute. And keeping at least one of the materials in motion can help maintain the temperature differential. We’ll unpack that a little more too.

Let’s start with the three ways that heat is transferred…what they are, and how they work:


What it is: The transfer of heat between materials that are in physical contact with each other.

How it works: If you’ve ever touched a hot burner on a stove, you’ve successfully participated in the process of conduction heat transfer.


What it is: The transfer of heat through a fluid medium, enhanced by the motion of the fluid.

How it works: If you’ve ever boiled water in a pan on a hot stove burner, you’ve successfully participated, again, in the process of conduction heat transfer (as the burner heats the pan) AND convection (as the heated water in the bottom of the pan both transfers heat through its volume, and moves to the surface.)


What it is: Remember what I said earlier about how you don’t always need contact or proximity for heat transfer? Well, this is it…the transfer of heat through empty space, via electromagnetic waves.

How it works: If you didn’t actually TOUCH the hot stove burner, but felt your hand getting hot as it hovered, then you’ve successfully participated in the process of radiation heat transfer. OK; it’s a little convection too, since the air between the burner and your hand was also transferring some of that heat. The best example of STRICTLY radiation heat transfer I can think of is the sun’s rays…they literally pass through 93 million miles of empty space, and make it quite warm on a nice sunny day here on Earth.

Regardless of how material, or an object, or a system receives heat, engineered compressed air products can be used to efficiently and effectively remove that heat.  For the record, they employ the principles of both conduction and convection.  If you’d like to discuss a heat transfer application, and the way(s) that an EXAIR product can work in it, give me a call.

Russ Bowman
Application Engineer
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What Is A Btu?

A Btu, or British Thermal Unit, is a traditional unit of energy and is a measure of the heat content of fuels.

Originally, the Btu was the amount of energy needed to increase the temperature of 1 pound of liquid water by 1 degree Fahrenheit.  The term became common among engineers in the late 1800’s.

A single Btu is insignificant in terms of the amount of energy used by a single household or by an entire country. In 2013, the United States used about 98 quadrillion (written out, 1 quadrillion is a 1 followed by 15 zeros) Btu of energy.

One Btu is approximately equal to the energy released by burning a match.


Interesting Energy Conversion Factors

Energy source Physical units and Btu (averages,¹ 2012)
Electricity 1 kilowatt hour = 3,412 Btu
Natural gas 1 cubic foot = 1,025 Btu
Motor gasoline (10% ethanol) 1 gallon = 120,524 Btu
Diesel fuel 1 gallon = 138,690 Btu
Heating oil 1 gallon = 138,690 Btu
Propane 1 gallon = 91,333 Btu
Wood 1 cord = 20,000,000 Btu (Estimated)

1Weighted averages across different contexts of each fuel such as imports, exports, production, and consumption. Source:  www.eia.gov/EnergyExplained by the U.S . Energy Information Administration

EXAIR manufactures the Cabinet Cooler System.  The Cabinet Cooler System is a low cost, reliable way to cool and purge electronic control panels.  They incorporate a vortex tube to produce cold air from compressed air – with no moving parts! EXAIR Cabinet Cooler Systems are available for NEMA 12, 4, and 4X type enclosures.  For the most efficient way to operate Cabinet cooler, a thermostat control system would be utilized. The standard thermostat control systems include an adjustable thermostat factory set at 95F.  Also, available is the ETC Electronic Temperature Control, providing precise control with easy adjustability and a digital readout.

Cabinet Cooler Family
EXAIR Cabinet Cooler Systems

In the United States, the power of HVAC (Heating Ventilating and Air Conditioning) systems is often expressed in BTU/hr.

The EXAIR Cabinet Cooler Systems are available with cooling capacities ranging from 275 to 5,600 Btu/hr.  To cool the down the equivalent of 98 quadrillion Btu’s of energy used by the US in 2013, it would take 17.5 trillion of our largest Cabinet Cooler Systems!

If you would like to find out how many Btu’s of cooling your electrical cabinet needs, please fill out and send in the Cabinet Cooler Sizing Guide and we can let you know.

Brian Bergmann
Application Engineer
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Match Photo courtesy of Samuel M. Livingston via Creative Commons License

Wearing Out Your Sole

3925 Adjustable Spot Cooler
3925 Adjustable Spot Cooler

A shoe manufacturer had a special abrasion test that was required by his customer to test special rubber compounds. The set up was to run a small chain across the bottom of the rubber sole.  The chain was looped to continuously rub against the sole of the shoe.  As they began their wear testing, they noticed that the chain was getting hot from the friction.  The heat would get high enough to change the composition of the rubber and cause a premature failure.  To properly test for wear, they needed to cool the chain.

As they discussed their application with me, they required the chain to be at a specific temperature. I suggested the model 3925 Adjustable Spot Cooler System.  This system comes with a dual point hose kit, a magnetic base, a filter separator, and two additional generators.  The generators of the Adjustable Spot Cooler are a piece which controls the total volume of air through the cooler. They can be switched in and out to produce more or less cooling capacity of the Adjustable Spot Cooler. The main concern was to keep the chain temperature constant.  With a temperature control knob and the additional generators, they could dial in the cooling capacity to keep the chain at the desired temperature.  If the chain was too cold, the sole would not wear properly, and if the chain was too hot, it would change the composition of the rubber material.

They mounted the Adjustable Spot Cooler to the abrasion machine with the dual points blowing on each side of the chain. They quickly noticed that they could keep the chain cooler than the specified temperature.  As a trial, they replaced the generator to the 30 SCFM (850 SLPM) flow rate.  This increased the cooling capacity of the Spot Cooler.  With the higher cooling capacity, they could increase the speed of the abrasion machine to shorten the failure cycle.  This was a great benefit to have as they were testing different rubber compounds to determine the best product; a pronounced advantage in research and development.

If you find out that heat is causing problems in your application, you can contact an Application Engineer at EXAIR for help in finding the correct cooling product. In this instance, friction was the culprit and the Adjustable Spot Cooler was the solution.

John Ball
Application Engineer
Email: johnball@exair.com
Twitter: @EXAIR_jb

Video Blog: Cabinet Cooler Systems also Stabilize Relative Humidity

EXAIR Cabinet Cooler Systems are able to cool your electrical panels using only clean, dry compressed air. Other systems such as cooling fans or heat exchangers use ambient air full of dust and humidity. The temperature of ambient air also fluctuates with the seasons and will be very warm in the summer months, which degrades their ability to cool as the temperature rises. One of the myths about compressed air cooling is that humidity from the compressed air source will enter the cabinet. A water/dirt filter separator will prevent condensate from entering the cabinet and since relative humidity is carried away with the hot air exhaust, relative humidity will stabilize to 45%. This video shows how quickly EXAIR’s Cabinet Cooler Systems will have an effect on relative humidity.

Dave Woerner
Application Engineer

A Few Questions about Powering Cabinet Coolers

Dual CC outside
NEMA 4X Dual Cabinet Cooler

Not too long ago, I was contacted by one of our customers regarding the Cabinet Cooler Systems and the quality of the compressed air used to power them.

The specific questions were:

  1. What happens if the compressed air gets dirty with oil or other particles if sufficient filtration is not available at the facility where Cabinet Cooler is being used?
  2. Where does the oil particle go, into the cabinet or out through the hot exhaust or both?
  3. If it goes into the Cabinet Cooler, should one expect a spray or will it simply form small droplets?
  4. Is there a way to filter the cold air outlet?

Dirty, oil laden air would exhaust throughout the Cabinet cooler (both hot and cold flows) as well as into the inside of the attached cabinet if the air were contaminated and there was not any filter located up-stream of the Cabinet Cooler System. This is precisely why we always recommend the use of filter/separator and oil coalescing filters to clean up the compressed air before it goes into the Cabinet Cooler. In fact, we include a five micron, auto-drain, filter/separator with all our stock systems. If oil is a known contaminant in a customer’s system, we will also recommend use of an oil coalescing type filter which we can provide as well. Without a coalescing filter, you can expect any oil in the compressed air supply to be atomized into a vapor which then has possibility of settling on components inside the cabinet.

Filtering the compressed air while it is still in its compressed state and before it goes into the Cabinet Cooler is the only way to make sure that the air is properly cleaned before processing through the Cabinet Cooler System. Filtering the air after it has gone through the Cabinet Cooler System is not possible. Many filtration systems rely on the high velocity of the compressed air for their filtering capability. If it is no longer in its compressed state (a condition that exists at the cold outlet of the Cabinet Cooler), then the right conditions for proper treatment do not exist. Also, by the time the air exits the Cabinet Cooler, your primary need for it is going to be for cooling anyway. Attempting to add filtration to the cold air output will interfere with the cooling function, which negates the purpose for having the Cabinet Cooler.

As compressed air and the systems that produce it become more widely understood, filtering, drying and removing oil from the compressed air stream are tasks that are done on the production side of things.

The best way to proceed is to have the necessary filtration on the compressed air supply, at the point of use, even if the facility has filtered, clean, dry air. It would still be good to employ it just in case any up-stream equipment that is normally used to clean up the air, went down for some reason. I call it the belt and suspenders method. The redundancy is worth the investment.

Neal Raker, International Sales Manager