Why Does the Inside of Cars Get so Hot?

We’ve all heard the warnings: don’t leave your children or pets in your car (or truck) during this time of year.  The temperatures inside the passenger compartment can reach over 100 degrees easily and quickly causing heat stroke and death.  This sounds like common sense but, every year, during the summer, reports of a death of a child or animal left in a hot car, are not uncommon.  The reason that passenger compartments get so hot is because of the way heat is transferred into and out of the compartment.  First, in order for heat to flow, there has to be a temperature difference.  As long as the inside is cooler than the outside ambient temperature, heat will flow from outside to inside.  More specifically, heat transfer will occur by convection from the ambient air to the outer surfaces of the passenger compartment, including glass.  Heat will then be transferred by conduction through the roof, insulation and headliner.  Heat transfer by conduction will also occur through windows.  There is also a radiation component that serves to heat the interior.  The sun’s rays will heat the solid parts of the interior such as dashboards and steering wheels.  The heat absorbed by these objects is then radiated to the air inside the passenger compartment.  The real clincher to this process is that the heat entering the vehicle enters at a rate faster than it is dissipated.  As a result, the temperature inside the passenger compartment can only increase.  It is not until the rate of heat transfer entering the vehicle is reduced below the rate of heat transfer out of the vehicle that the temperature in the passenger compartment is reduced.  This point usually starts to occur at dusk. 

Remember, we still have approximately one and a half months of summer left.  Please be mindful of your young passengers and pets and don’t leave them locked in a death trap!

The photos below illustrate how hot a passenger compartment can get. The photographs were taken inside a Chevrolet extended cab pick up truck.

Temperature in cab after 7 minutes with windshield shaded
Temperature after 30 minutes, windshield uncovered

Temperature after 1 hour
Temperature after 2 hours

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Runaround Coil Heat Recovery

With all the talk about green house gases and oil dependency, energy efficiency and specifically, energy savings, is a top consideration these days.  One of the things that you don’t hear about too often is something called a “runaround” coil.  The system isn’t really limited to a coil but, instead describes a very simple  system for absorbing heat from one air stream and rejecting it to another.  Consider a large office building or hospital application where 100% outside air is conditioned, distributed to the individual spaces, and then completely exhausted.  A runaround system is nothing more than two heat transfer coils, one positioned in the inlet air stream ahead of the conditioning coils, and the other in the exhaust air stream.  The two coils are connected by piping in which water or a glycol solution is circulated by a centrifugal pump, hence the name “runaround”.  During the summer when outdoor air temperatures can reach into the 90s and 100s, the fluid in the runaround system absorbs heat from the outside air and in the process, cools it to a certain temperature.  The heat carried by the fluid is then circulated to the coil in the exhaust air stream where it is rejected to the air.  Heat rejection occurs because the exhaust air is at a temperature lower than that of  the fluid stream.  Significant energy savings occur when the cooling requirements of the building can be reduced.  For example, a building without a runaround system has a 100 ton load.  In order to meet the load, the cooling equipment has to cool outside air from 95 to 55 degrees.  Precooling of the outside air by 5 degrees (90 degrees) reduces the load on the cooling equipment to approximately 87 tons.  Dropping the cooling requirement by 13 tons can result in significant electrical energy cost savings.  Now, what happens if evaporative cooling is employed in the exhaust air stream? If an evaporative cooler is added upstream of the runaround coil, then the air stream can be cooled further resulting in an increased heat transfer rate from the fluid to the air stream, further reducing the cooling capacity requirements of the cooling equipment.   The use of a runaround coil is not a novel idea but one whose time is coming.

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