## 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.

## 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.

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

Send me an email
Find us on the Web

## 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:

Conduction

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.

Convection

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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## Not a Fan of Fans Because Rising Air Temp Will Kill Your Electronics

Using a fan is a popular method for machine builders to provide cooling for an electrical enclosure.  The electrical panel stays cool for machine acceptance at the factory, and possibly for even the first 6-8 months of operation and then one day, there is a problem, and the machine shuts down due to an over heated component within the panel. This leads to opening up the panel, possibly placing an external fan, and operation of the machine in an unsafe condition, to meet the daily production needs.  What has led to this situation?  Summertime!

To better understand the situation, let’s review the heat formula.  The total heat content of air consists of the sensible and latent heat factors. Latent heat is the heat that is required to change the state of a material, say from liquid to solid.  Water to ice is an easy way to understand this type of heat.  When heat is removed from water at 32°F it turns to ice at 32°F.  There is no temperature change, but heat has been removed. Sensible heat is dry heat, it is a result in change of temperature, but not change in state or moisture.  For fan cooling, the air and moisture only change temperature and not state, we can focus on the sensible heat portion.

In English units:  Q = Cp x ρ x q x ΔT x 60 min/hr

And for air:

Q –  is the sensible heat flow in BTU/hr

Cp – is the specific heat in BTU/lb °F – 0.2388 BTU/lb °F

ρ – is the air density at standard conditions – 0.075 lb/ft3

q – is measured air flow in ft3/min – CFM

ΔT – is the temperature difference in °F – Final Air Temperature – Starting Air Temperature

Plugging in the constant values, gives us:

Q = 1.0746 x CFM x ΔT

It is common to chart the above formula for various ΔT values, plotting Q vs. CFM values on a dual logarithmic scale, as shown below-

As an example, for an internal heat load of 1300 BTU/hr, to ensure that the temperature rise (from ambient) in the cabinet does not exceed 20°F, 60.5 CFM of air flow is required (the red line above).  A fan with this CFM rating is specified and installed in the panel.

This works  when the ambient temperature is a comfortable 75°F, in a climate controlled factory, or the cooler months of the year.  The problem occurs when the ambient temperature increases to 95°, 100°, or even 105°F,  not uncommon in the summer, and in plants that create large amount of heat, like metal production, and near boiler systems and furnaces.  Under these conditions, the fan will still maintain the 20°F difference, but the internal temperature of the cabinet will rise to 115°-125°F, temperatures where electrical components start to fail or shut down.  The solution to this issue?  Lower the Starting Air Temperature.

The EXAIR Cabinet Cooler Systems use our Vortex Tube technology to take compressed air and provide a cold flow of air that enters the enclosure at 5o°F less than the compressed air temperature.  With a compressed air temperature of 70°F, common for industrial compressed air systems, the Cabinet Cooler will deliver cold air at 20°F.  Again using the chart above, flowing just 20 SCFM of this air will absorb the 1300 BTU/hr of heat (the green line), and result in an internal air temperature 80°F no matter the ambient air temperature.  The electronics in this enclosure will run trouble free, for a long time. This ambient air temperature problem is also true of air-to-air heat exchangers, as the ambient air temperature rises the ability to remove heat diminishes.

Another consideration, the fan system is bringing in air from the surroundings, which is hot and dirty, passing it through a filter (which gets clogged, reduces air flow, and needs to be replaced.) The Cabinet Cooler System, includes an automatic drain filter separator, which filters the compressed air to be free of dirt, dust and moisture. The air entering the enclosure is cool, dry and fee of dust and debris.

To discuss your application and how the EXAIR Cabinet Cooler System can be a benefit at your facility, feel free to contact EXAIR and myself or one of our other Application Engineers can help you determine the best solution.

Brian Bergmann
Application Engineer

Send me an email
Find us on the Web

## Siphon Fed Atomizing Nozzle Improves Roll Forming Process

Last week I worked with a gutter manufacturer who was looking for a way to spray a light coating of vanishing oil on the rollers of a forming machine. Roll forming is commonly used when needing to maintain a constant and consistent shape or feature across the length of the part. In this particular case, a sheet of aluminum, used as a cover for the gutter, is fed into the machine where it passes over a series of dyes that bends “ribs” and punches small holes into the part to keep leaves or debris from settling on top, while allowing the rainwater to pass through the holes and into the gutter.

They were needing to apply the oil to the rollers because they were starting to see some irregularities in hole size as well as some deformities to the shape of the ribs due to heat being generated during the forming process. The customer was interested in using some type of atomizing spray nozzle in the hopes that providing an atomized mist of liquid may provide for a faster evaporation of the oil so there wasn’t much residue left on the part before packaging.

After further discussing the details, they advised that they were going to have the oil in a container about 12″ below the machine but didn’t have a way to pressurize or pump the liquid to the nozzle. Once again, EXAIR has the perfect solution with our 1/4 NPT Siphon Fed Atomizing Nozzles. These nozzles are the ideal solution where pressurized liquid isn’t available as they use the compressed air to the draw the liquid into the nozzle, up to 36″ of suction height, and mix it internally to produce a mist of atomized liquid spray. For this particular application, the Model # SR1010SS was a good solution as it provides a low flow rate of only 0.8 GPH and a tight spray pattern to focus right at the rollers to avoid any waste or overspray.

EXAIR offers an extensive range of Atomizing Nozzles that can be used for light coating applications, like above, or for wider coverage areas or higher flow rates. For help selecting the best option to fit your needs, contact one of our application engineers for assistance.

Justin Nicholl
Application Engineer
justinnicholl@exair.com
@EXAIR_JN

## Cooling Punch Points with a Super Air Knife

A stamping facility had a high speed perforating operation. The idea was to punch holes into a matrix at a fast feed rate.  In their operation, they started to see issues with the punched holes, and they also noticed that the punch points were prematurely failing.  With a tight punch-to-matrix clearance, heat was building up from the friction.  This effect was galling the material and affecting the hole appearance and dimensions.  They also noticed heat damage to the punch points.  They either had to slow their process down, or find a way to cool the punch points.  They contacted EXAIR to see if we could help.

To remove heat, you need to have a fluid moving across the material to carry the heat away. For this customer, the fluid would be air.  Just like a hot cup of coffee, you can cool it by blowing across the top of it.  In this instance, EXAIR can blow a lot of air with using very little amount of compressed air.  Because of the gap opening of the tool die was narrow, I suggested the Super Air Knife.  It has a compact design and can blow nicely between the upper and lower die.  With slight modifications, they were able to mount the Super Air Knives right into the base set.  Because the tool die was a “bowl” type design, I suggested that they should use two pieces of the model 110206 Super Air Knife.  They could mount one to each side to make sure to hit all the punch points.  (Reference the picture below).

EXAIR Super Air Knives are the most efficient compressed air knives in the market. It is designed to have a 40:1 amplification ratio.  That means for every one part of compressed air, it will entrain 40 parts of the free ambient air.  As with the coffee reference above, the more air that you can blow, the better the cooling effect.  With the Super Air Knife, we can reach a velocity of 11,800 feet per minute at 80 PSI.  After the customer installed the Super Air Knives, they were able to increase production by 10%.  Also, they found that the punch points were lasting twice as long.  They were so impressed with the effectiveness of the Super Air Knives, they mounted them to all their punch press machines.

If you find that heat is affecting your process, EXAIR could have a product to help you. We have a variety of efficient air movers to cool your parts.  As for this customer above, we were able to increase production and extend the life of their tools.

John Ball
Application Engineer
Email: johnball@exair.com

## Wearing Out Your Sole

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

## Let’s Cool Things Down With Heat Transfer Equations

When it comes to cooling products, we get many questions on what would be the best method. Generally with larger parts with heavy mass and large surface areas, we would recommend the Super Air Amplifiers, Super Air Knives or Super Air Nozzles. We have to look at many factors to determine the correct method, but if look at the mass of the part, the ambient conditions, the speed of the conveyor, and the change in temperature, we can get a good start in setting up an application.

In determining a good estimate, we use a couple of heat equations to help. As with any customer, you want to make sure you have as much information to get a good platform to start. We use two equations to begin. The first equation is used for the product that needs to be cooled.

Equation 1

q = m * Cp * (T2 – T1)

Where:
q – heat (BTU) or (Kcal)
m – Mass (Lb) or (Kg)
Cp – specific heat (BTU/Lbm oF) or (Kcal/Kg oC)
ΔT – Temperature (oF) or (oC)

Once we have the amount of heat that we need to remove, then we can look at the product to cool it. With Super Air Amplifiers, we use an equation that is used in fan cooling. This is the second equation.

Equation 2

h = 1.08Qs(T2 – T1)       OR                h = 0.33Qs(T2 – T1)

Where:
h – heat rate (BTU/hr)                                   h – heat rate (Watts)
Qs – Flow (SCFM)                  OR                   Qs – Flow (NM^3/hr)
ΔT – Temperature (oF)                                  ΔT – Temperature (oC)

As an example, we have an aluminum part that came out of a baking oven at 400 oF (204 oC), and we want to cool the part down to 100 oF (38 oC) for handling.  If we give the part a mass of 20 lbs. (9.1 Kg), we can determine how long we would need to cool the part. The specific heat, Cp, of aluminum is 0.22 BTU/Lbm oF or Kcal/Kg oC. Applying this to Equation 1, we get the following:

q = 20 lbs * (0.22 BTU/lb/oF) *(400 oF – 100 oF)

q = 1,320 BTU

Or

q = 9.1 Kg * (0.22 Kcal/Kg/ oC) * (204 oC – 38 oC)

q = 332 Kcal

This tells us how much heat we would need to remove in order to handle. To keep going along with this example, we will use the 120021 Super Air Amplifier. With the large amplification level, it has a flow of 436 SCFM (740 NM^3/hr) at 6” away. To produce that volume, it only uses 8.1 scfm of compressed air at 80 psig. There are a couple of things that we should consider with our estimation. Ideally, we will want to be at a distance where the velocity will be between 1,700 to 2,500 fpm (8.6 to 12.7 mps). This gives us the best velocity for the maximum cooling rate. Depending on certain situations, you may have to add a little more time for cooling if the velocity is too high or too low. With the 120021 Super Air Amplifier, a distance of 18” from your target will give you a velocity close to 1,850 fpm (9.4 mps). The other consideration is the rate of heat loss. The bigger the temperature difference, the faster it will cool. As you get near the target temperature, the rate change becomes smaller. With that, I usually take the average temperature as an estimate. Using our example above, it would be (400 + 100)/2 = 250 oF, or (204 + 38)/2 = 121 oC. The other estimation will be the temperature of the ambient air. It will be cooler as it first hits the target and then heat up. If we add roughly 7 oF (4 oC) to the ambient air temperature because of the velocity, then the ambient temperature would become 68 + 7 = 75 oF (20 + 4 = 24 oC). With Equation 2, we will get the following:

h = 1.08 * 436 SCFM *(250 oF – 75 oF)

h = 80,404 BTU/hr

Or

h = 0.33 * 740 NM^3/hr * (121 oC – 24 oC)

h = 23,687 Watts or 20,372 Kcal/hr

With Equation 1 and Equation 2, we can estimate the amount of time needed to cool the part. In having to remove 1,320 BTU at a rate of 80,404 BTU/hr, the equation will give us 1,320 BTU/ (80,404 BTU/hr) = 0.016 hr or 1 minute.  In metric units, 332 Kcal/(20,372 Kcal/hr) = 0.016 hr or 1 minute. This means that we will have to keep the target in the air stream for about 1 minute. Depending on the geometry of the part, the angle of the amplifier, and the speed of the conveyor, we may need multiple Super Air Amplifiers. As I mentioned before, these are estimations, but it will help in getting an idea for your project. You can always contact the Application Engineers at EXAIR for any help.

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