Heat Transfer – How Energy Can Move

Heat. One word can bring to mind so many different things from cooking to sun tanning. But what is heat and how does it move. Heat is essentially a form of energy that flows in the form of changing temperatures; this form of energy will flow from high to low. When you describe something as being hot, you are actually describing that the item in question has a higher temperature than your hand thus the thermal (heat) energy is flowing from that object to your hand. This phenomenon is what is referred to as heat transfer. Heat transfer can be observed all the way down to the atomic scale with the property known as specific heat. Every molecule and atom can carry a set amount of energy which is denoted by specific heat; this value is the ration of energy (usually in Joules) divided by the mass multiplied by the temperature (J/g°C).

Energy moving through atoms in an object

But how does this heat move from object to object? On the atomic scale, the atoms are storing the energy which will cause electrons to enter into an excited state and rapidly switch between shells. When the electron returns back to a lower shell (closer to the nucleus) energy is released; the energy released is then absorbed by atoms at a lower energy state and will continue until the thermal energy is equal between the two objects. Heat has four fundamental modes of transferring energy from surface to surface and they are as follows:

Advection
Advection is the physical transport of a fluid from point A to point B, which includes all internal thermal energy stored inside. Advection can be seen as one of the simpler ways of heat transfer.

Conduction
Conduction can also be referred to as diffusion and is the transfer of energy between two objects that have made physical contact. When the two objects come into contact with each other thermal energy will flow from the object with the higher temp to the object with the lower temp. A good example of this is placing ice in a glass of water. The temperature is much lower than the room temperature therefore the thermal energy will flow from the water to the ice.

Convection
Convection is the transfer of thermal energy between an object and a fluid in motion. The faster the fluid moves the faster heat is transferred. This relies on the specific heat property of a molecule in order to determine the rate at which heat will be transferred. The low the specific heat of a molecule the faster and more volume of the fluid will need to move in order to get full affect of convection. Convection is used in modern ovens in order to get a more even heat through out the food while cooking.

Radiation
Radiation is the transfer of thermal energy through empty space and does require a material between the two objects. Going back to the how thermal energy is released from atoms; when the electron returns to a lower energy shell the energy is released in the form of light ranging from infrared light to UV light. Energy in the form of light can then be absorbed by an object in the form of heat. Everyone experiences radiation transfer every day when you walk outside; the light from the sun’s radiation is what keeps this planet habitable.

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.

If you have any questions about compressed air systems or want more information on any of EXAIR’s products, give us a call, we have a team of Application Engineers ready to answer your questions and recommend a solution for your applications.

Cody Biehle
Application Engineer
EXAIR Corporation
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The picture “Energy Transfer – Heat” by Siyavula Education is licensed under CC BY 2.0

EXAIR Vortex Tubes are for a Variety of Cooling Applications

Vortex tube
Cooling or Heating with the Vortex Tube

EXAIR Vortex Tubes are a low cost, reliable and maintenance free solution to a wide variety of industrial spot cooling problems.  They only requirement is a supply of compressed air as the power source.  Vortex Tubes have no moving parts and can produce temperatures that range from -50°F to +260°F (-46°C to +127°C).

Vortex Tubes produce two air streams one cold and one hot, the percentage of cold air flow from the inlet flow is referred to as the cold fraction.  The cold fraction is adjustable by the hot valve on the hot discharge side of the vortex tube.  Adjusting the hot valve results in both air temperature and air volume changes. The colder the air becomes, the volume of that cold air declines. So for very cold temperatures, a smaller volume of air is produced compared to a warmer air temperature.

For the vast majority of industrial cooling applications a larger volume of cool air will provide more efficient cooling than a lesser amount of very cold air.  Generally speaking the highest Btu/Hr values are in the 70-80% cold fraction range.

The exception to this would be in labs or special cases where the coldest temperatures are desired.  Adjusting a Vortex Tube is easy, simply insert a thermometer/thermocouple in the cold air exhaust and set the temperature by adjusting the valve on the hot end of the Vortex Tube.  You will know when you reach max refrigeration (80% cold fraction) as the cold air temperature will be 50°F (28°C) lower than the compressed air supply temperature.

EXAIR Vortex Tubes are constructed from stainless steel.  This ensures excellent wear resistance, corrosion resistance and assures years of reliable operation.  They are offered in 3 different size ranges (small, medium & large).  There are generators located inside the tube (user serviceable) that will change the volumetric flow.  The generators are available in a plastic construction or brass construction for high temperature applications.  The ranges 2 SCFM – 8 SCFM are designated as small Vortex Tubes, 10 SCFM – 40 SCFM are medium and 50 SCFM – 150 SCFM are large.  This feature allows you to customize or change your Vortex Tube for greater flexibility in a wide range of applications.

vortex_generator
Vortex generator

Large Vortex Tubes are specified when a high flow of cold air is needed. There are 16 models to choose from in total.  Capable of providing 3,400 BTU/HR up to 10,200 BTU/HR of cooling power.  These have been used to cool high heat loads that are centrally located or to help cool samples of gases for testing.

Medium Vortex Tubes are the most popular – there are twenty to choose from, depending on the cold air flow rate and temperature you’re looking for. These can produce temperatures as cold as -40°F (-40°C) when set to a 20% Cold Fraction (which is the percentage of total supply air that’s directed to the cold end) and cold air flows as high as 32 SCFM when set to an 80% Cold Fraction, which will produce a cold air temperature of about 20°F (-7°C). Some common uses are cooling ultrasonic welds and brazed joints.

The Medium Vortex Tubes are so popular, in fact, that they’re incorporated into our Adjustable Spot Cooler and Cold Gun Systems. They come ready-to-go with mufflers, cold air hose kits, and magnetic bases, so they couldn’t be easier to use.

Adjustable Spot Cooler

Cold Gun Lineup

Small Vortex Tubes are great when low flows (less cooling power) will succeed, or if compressed air supply is limited.  There are 12 models in total to choose from. These are specified for much smaller applications, like cooling the needle of a sewing machine, small drill bits, etc. You can also get one with a cold air hose & magnetic base…that’s the Mini Cooler System.

Mini Cooler

If you would like to discuss Vortex Tubes, Spot Cooling, efficiency of your compressed air usage, quieter compressed air products and/or any EXAIR product,  I would enjoy hearing from you…give me a call.

Steve Harrison
Application Engineer
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Vortex Tubes Make Hot Air Too

Back in the spring, my good friend and co-worker Neal Raker wrote a great piece, titled “Can I Use A Vortex Tube For Heating?” which I will try my best not to borrow too much from or outright plagiarize in the blog to follow. I only mention it because I had the pleasure of helping a customer with one of the “usually few and far between” Vortex Tube heating applications recently.

Like Neal said, the conditions under which Vortex Tubes fit a heating application are fairly narrow, but certainly not unheard of. In this situation, a reciprocating air motor had been in place on a piece of factory machinery for years. A recent change in the part being manufactured meant that the motor had to be slowed down, which meant throttling down on the vent valve on the motor’s pneumatic exhaust. When they did this, the valve became prone to freezing up, meaning someone had to rig up a heat gun and climb up on top of the machine to the vent valve, directing hot air on the valve until it thawed. It got to be a real hindrance to the process when this happened several times a day.

The caller was familiar with our Vortex Tube products, having used Mini Coolers and Cold Guns in other parts of the plant. He knew that there was hot air coming out of the other end, and thought it could be used to thaw the vent valve, but was concerned, because it was such a low flow.

Mini Cooler (left) and Cold Gun (right).
Mini Cooler (left) and Cold Gun (right).  Cold air from one end; hot exhaust minimized on the other.

He was right: the hot air exhaust of both the Mini Cooler and Cold Gun is a small fraction of the total air supply…that’s by design. Also, it’s passed through a noise reducing muffler which further spreads it out to make it nice & quiet…also by design.

That’s when a fuller explanation of Vortex Tube operation came into play: See, the Mini Coolers and Cold Guns are all set to a high Cold Fraction (the percentage of supply air that is directed to the cold end,) so, although the hot exhaust is indeed fairly hot, there’s just not a lot of it. By contrast, our 3400 Series Maximum Cold Temperature Vortex Tubes are adjustable for lower Cold Fractions (from 20-50%,) meaning that the hot exhaust flow can range from 50-80% of the supply air flow. Additionally, the hot end of the Vortex Tube has male NPT threads, for convenient porting & direction of the hot air flow.

The EXAIR Vortex Tube.  Cold air from one end; hot air from the other.  Fully adjustable.
The EXAIR Vortex Tube. Cold air from one end; hot air from the other. Fully adjustable.

Now, back to the conditions that made this a good fit for the Vortex Tube: the machinery already had an ample and easily accessible supply of compressed air…they were able to tap a line from the air motor’s supply. The closest outlet for their heat gun was on the other side of the walkway, which meant they had to stretch an extension cord across the walkway, creating a trip hazard. The vent valve is also small enough that they could use a Model 3402 Vortex Tube, which utilizes only 2 SCFM @100psig…a tiny fraction of what the air motor uses.

With the Vortex Tube mounted permanently in place, the vent valve now operates flawlessly, without the need for manually thawing with the incredibly inconvenient heat gun.

If you think you might have a decent fit for a Vortex Tube heating application, give us a call. You may be right.

Russ Bowman
Application Engineer
(513)671-3322 local
(800)923-9247 toll free
(513)671-3363 fax
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Vortex Tube Thaws Steam Connections on Bulk Liquid Sea Containers

Bulk container

Vortex Tube applications for cooling are many and a wide variety. Heating applications though, do tend to be a bit more elusive. That being said, I thought I would highlight a recent application where vortex tubes were used to thaw out steam lines that are used to heat up fluids in bulk sea containers.

We have all seen them running up and down the expressways, the large, bulk liquid containers that have the multi-modal capability to be on a ship, a train or a truck going down the road. I personally never thought about what the users of these tanks must do in order to get the liquid inside up to certain temperatures to allow the material inside to flow easily. I live in the Midwest area of the US, so we get really cold weather for only a few weeks during the year. In the Northern climates though, these bulk container users must have ways to thaw out the product before it can be used. To do this, these bulk containers are equipped with steam lines. Steam is used to heat the liquid inside to get it up to temperature. Once the steam is connected to the lines and circulating, all is well. But before they can get to that point, the steam connections on the tank are usually plugged with ice from condensate from previous use. The previous method was to simply snake a steam line up inside the heater lines to warm them up, but that presented a further problem. That same condensate ends up rolling out the pipe and dripping on the ground, re-freezing and creating a huge slip hazard.

 Steam connections

Above are the typical 1” BSP steam connections found on the bulk tanks.

In order to eliminate the slip hazard, the customer began looking for another method to supply a hot gas to these steam lines to thaw them out. In comes EXAIR with our Vortex Tube selection. The idea is to replace the mini steam line with the hot air output flow from a vortex tube to thaw out the connections. Since the customer has compressed air utility in plentiful supply on site, this makes for a very convenient way to warm up the pipes with a relatively “dry gas”. That being the dry compressed air supplied in the facility. The customer ended up using (2) model 3225 Vortex tubes with Cold Flow Mufflers, to provide the hot air for the steam connections. In fact, the diameter of the hot tube for the vortex tubes was the perfect size to simply slide up inside the steam pipes and hang there until the pipes were free of the problematic ice. There was still some small amount of liquid that re-froze from within the steam pipes, but it was certainly much more manageable than the mess the customer was dealing with previously.

Neal Raker, Application Engineer
nealraker@exair.com