The Basics of Calculating Heat Load for Cooling Electrical Cabinets

Is your electrical cabinet overheating and causing expensive shut downs? As spring and summer approach, did your enclosures have seasonal overheating problems last year? Is your electrical cabinets AC Unit failing and breaking down? Then it may be time to consider EXAIR Cabinet Coolers Systems. These systems are compressed air powered cooling units designed to keep your cabinet cool in hot environments. Major benefits include no moving parts to wear out, UL listed to maintain the NEMA integrity of your enclosure (also CE compliant), they are simple and quick to install and they reliably turn on and off as needed (perfect for solving seasonal overheating).

Just one question then; how do you pick which Cabinet Cooler is best for your application? It’s time to bust out ye ole trusty calculator and crunch some numbers. Keep in mind that the following calculations use baselines of an Inlet air pressure of 100 psig (6.9 bar), compressed air temperature of 70F (22C), and a desired internal temp of 95F (35C). Changes in these values will change the outcome, but rest assured a Cabinet Cooler system will generally operate just fine with changes to these baselines.

How the EXAIR Cabinet Cooler System Works


Before we dig right into the math, keep in mind you can submit the following parameters to EXAIR and we will do the math for you. You can use our online Cabinet Cooler Sizing Guide and receive a recommendation within 24 hours.

There are two areas where we want to find the amount of heat that is being generated in the environment; this would be the internal heat and the external heat. First, calculate the square feet exposed to the air while ignoring the top. This is just a simple surface are calculation that ignores one side.

(Height x Width x 2) + (Height x Depth x 2) + (Depth x Width) = Surface Area Exposed

Next, determine the maximum temperature differential between the maximum surrounding temperature (max external temp) and the desired Internal temperature. Majority of cases the industrial standard for optimal operation of electronics will work, this value is 95F (35C).


Max External Temp – Max Internal Temp Desired = Delta T of External Temp

Now that we have the difference between how hot the outside can get and the max, we want the inside to be, we can look at the Temperature Conversion Table which is below and also provided in EXAIR’s Cabinet Cooler System catalog section for you. If your Temperature Differential falls between two values on the table simply plug the values into the interpolation formula.

Once you have the conversion factor for either Btu/hr/ft2, multiply the Surface Area Exposed by the conversion factor to get the amount of heat being generated for the max external temperature. Keep this value as it will be used later.

Surface Area Exposed x Conversion Factor = External Heat Load

Now we will be looking at the heat generated by the internal components. If you already know the entire Watts lost for the internal components simply take the total sum and multiply by the conversion factor to get the heat generated. This conversion factor will be 3.41 which converts Watts to Btu/hr. If you do not know your watts lost simply use the current external temperature and the current internal temperature to find out. Calculating the Internal Heat Load is the same process as calculating your External Heat Load just using different numbers. Don’t forget if the value for your Delta T does not fall on the Temperature conversion chart use simple Interpolation.

Current Internal Temp – Current External Temp = Delta T of Internal Temperature
Surface Area Exposed x Conversion Factor = Internal Heat Load

Having determined both the Internal Heat Load and the External Heat Load simply add them together to get your Total Heat Load. At This point if fans are present or solar loading is present add in those cooling and heating values as well. Now, with the Total Heat Load match the value to the closet cooling capacity in the NEMA rating and kit that you want. If the external temperature is between 125F to 200F you will be looking at our High Temperature models denoted by an “HT” at the start of the part number.

From right to left: Small NEMA 12, Large NEMA 12, Large NEMA 4X

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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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

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.)

Radiation

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-

BTU-CFMGraph4

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.

ETC CC
NEMA 4 Cabinet Cooler System with Optional Electronic Temperature Control

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
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Twitter: @EXAIR_BB