Expendable
refrigerants versus MRTP
Some functional tests on electronic
or passive devices require thermal cycling. Thermal cycling usually involves the
rapid and repeated heating and cooling of the Device Under Test (DUT). Heating
is a simple and straightforward process that usually involves passing an
electric current through a resistive device. The heat that is generated is then
applied to the DUT by several different methods usually involving convection or
conduction. Cooling a DUT though more difficult than heating can be achieved by
several different approaches. In order to cool a device, some form of
refrigeration is necessary. However, there are many different refrigeration
methods.
Expendable
refrigerants
Comparison between LN2 and LCO2
Satellite condensers
Benefits
Mechanical refrigeration
The importance of
efficient product testing
A re-think is necessary
Conclusion
The simplest method to cool a
DUT is to use
expendable refrigerants such as liquid nitrogen or liquid carbon dioxide.
Expendable refrigerants remove heat from the device by absorbing it and then are
exhausted into the atmosphere. These refrigerants have three main advantages:
The disadvantages of expendable
refrigerants
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Safety is also an issue. Expendable
refrigerants are usually exhausted directly into the work area and this can
result in very high levels of carbon dioxide or nitrogen in the air. Proper
ventilation and air quality monitoring are necessary to insure worker
safety. There are also inherent dangers in the handling of cryogenic fluids
and the associated heavy, bulky cylinders. The use of expendable
refrigerants can create ice and water condensation that can result in pools
of water on the floor. The presence of individual storage cylinders and
water puddles in the lab present a safety hazard and also make it very
difficult to keep the test area looking clean, neat and professional.
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Delivery of an adequate supply of expendable
refrigerant to the test area can also be very expensive. Delivery is usually
achieved by one of two methods. Bulk delivery systems utilize a big storage
tank outside of the building to hold a large quantity of expendable
refrigerant. The refrigerant is then distributed to the various test areas
by way of an expensive insulated delivery line. Another method is to use
separate storage cylinders of refrigerant to supply each test site
individually. The bulk method has a very high initial cost and is inflexible
in its application. The only saving grace is that it saves the trouble of
filling and handling the individual cylinders. Some companies claim that it
can cost as much as 500 dollars per foot to install the distribution lines
in a bulk system. Since the distribution lines in a bulk system are hard
plumbed, re-locating a test site is not flexible. The use of individual
cylinders at each test site also has its disadvantages. The need to
regularly refill the cylinders is labor intensive. However, it is more
flexible and has a lower initial cost than a bulk system.
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By using high-pressure liquid carbon dioxide
losses can be reduced because it can be stored at room temperature. However,
high-pressure liquid carbon dioxide has roughly half the heat removal
capacity of low-pressure liquid carbon dioxide. So, depending on the test
and delivery system, a lot more of the high-pressure liquid carbon dioxide
will probably be used to achieve the same results. Graph 1 shows the
theoretical heat removal capacities of expendable refrigerant. Things are
not always what they seem though. In the case of liquid nitrogen and
low-pressure liquid carbon dioxide, a lot of heat can be absorbed in the
process of delivering the refrigerant to the test site. This effectively
reduces the net heat removal capacity of the refrigerant. The degree of heat
removal capacity reduction is a function of how well the refrigerant lines
are insulated and how regularly the refrigerant is being used.
Graph1
Liquid nitrogen has a much lower
boiling point and under ideal conditions can remove a lot more heat and produce
much lower device temperatures than liquid carbon dioxide.
Due to its very low storage
temperature (as low as –195° Celsius) it often gains an enormous amount of
heat in the process of being delivered to the test sight and therefore its
higher BTU/pound capacity can be very installation dependent.
Liquid carbon dioxide does not
suffer to the same extent as liquid nitrogen does to a very low storage
temperature. That is because high-pressure liquid carbon dioxide is stored at
room temperature and low-pressure liquid carbon dioxide is stored at 0°
Fahrenheit.
When liquid carbon dioxide is
used as a refrigerant it changes from liquid to solid to vapor. The process of
sublimation occurs. As the pressure drops the carbon dioxide passes through a
solid phase. Solid carbon dioxide is called "dry ice". The "dry
ice" phase of carbon dioxide is very difficult to manage and requires
special expertise when using it as a refrigerant. High-pressure liquid carbon
dioxide has an additional undesirable trait. It is supplied in high-pressure
cylinders. Each cylinder is filled with 50 pounds of liquid carbon dioxide.
However, only about 35 of the 50 pounds can be drawn off as liquid and therefore
be usable as a refrigerant. The remaining 15 pounds exists only as a vapor and
therefore has no value as a refrigerant. In fact if the storage temperature of
the liquid carbon dioxide is above 87° Fahrenheit, its critical point, it
cannot exist as a liquid and therefore is useless as a refrigerant. Considering that high-pressure liquid carbon
dioxide only has about half the heat removal capacity of low-pressure liquid
carbon dioxide, and only about 70 percent of the high-pressure liquid carbon
dioxide is usable, it is clear how inefficient this approach can be.
Considering all the strengths
and weaknesses of expendable refrigerant it may appear that there is no best
choice. However, there is one approach that makes the most efficient and cost
effective use of expendable refrigerant. It is a hybrid system that takes
advantage of only the best characteristics of the high and low pressure liquid
carbon dioxide. It combines the high heat removal capacity of the low-pressure
liquid carbon dioxide with the economical storage and delivery characteristics
of the high-pressure liquid carbon dioxide. The device that makes this possible
is called a satellite condenser.
A satellite condenser is a
simple mechanical refrigeration system that converts carbon dioxide vapor into
liquid. It is usually located in close physical proximity to the test site. The
bulk liquid carbon dioxide is stored remotely at room temperature in
high-pressure vessels or at low pressure in low temperature vessels. Carbon
dioxide vapor is distributed to the test site by way of simple, inexpensive, and
non-insulated tubing. The carbon dioxide vapor is re-condensed into liquid by
the satellite condenser at the test site. Depending on its size, a satellite
condenser can serve a single or several test sites. A properly installed
satellite condenser can guarantee plenty of liquid carbon dioxide at the test
site anytime it is needed.
This type of installation yields
four primary benefits.
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Virtually 100 percent of the refrigerant that
is purchased is used. This is because the satellite condenser re-condenses
the vapor into a liquid. As long as there is sufficient vapor pressure in
the storage cylinder to force the vapor to the satellite condenser, then
liquid refrigerant will be produced.
If your company has already
invested heavily in a conventional liquid nitrogen or low-pressure liquid carbon
dioxide bulk system, the satellite condenser system may be a tough sell. In any case
it is still the most cost-effective use of expendable refrigerant. But there is
still a better way.
Mechanical refrigeration uses a
vapor compressor and a special refrigerant in a closed-loop system to cool the
DUT. The refrigerant is re-used over and over.
Expendable refrigerants such as
liquid nitrogen or liquid carbon dioxide required a great deal of energy to
produce in the first place. By adding to that the cost of storage, distribution,
and handling, it is easy to see why expendable refrigerants can be so costly to
use.
Expendable refrigerants are
used because the initial system cost is usually lower. Often only
the initial acquisition cost is looked at rather than the total cost of
ownership. But probably more important is the fact that a lot of environmental
stress screening equipment has been available for use only with expendable
refrigerant. This is no longer the case. A growing awareness of the high costs
involved when using expendable refrigerant has driven many companies to
re-examine its use as the cooling medium for thermal cycling of devices. When
all of the expenses involved in manufacturing a product are closely scrutinized,
including items such as electrical power consumption and expendable refrigerant
usage, it becomes obvious that there is tremendous potential for savings in this
area. If expenses such as electrical power consumption and expendable
refrigerant usage are merely considered the unavoidable fixed costs of doing
business or simply part of general overhead then there is little incentive to
initiate change.
It has been said that the
technology required to build electronic components and especially microwave
devices is uniformly distributed among the companies that are involved in this
endeavor. However, there is tremendous disparity in the approach and the
efficiency of the functional test process. How efficiently the devices that you build
are tested may to a great degree determine how competitive and
profitable the final product is. It was this realization by several product
managers and the subsequent communication with the manufacturers of
environmental stress screening equipment that has resulted in the development
and manufacture of thermal cycling equipment that uses closed-loop mechanical
refrigeration.
Mechanical refrigeration has
several advantages over expendable refrigerant as a means to cool electronic
devices in thermal cycling.
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Probably the greatest single advantage of
mechanical refrigeration is that it relies upon a fixed quantity of
refrigerant that is installed at the factory and never needs replenishment.
In many cases this single feature allows a mechanical refrigeration system
to completely pay for itself within the very first year of operation. See
the example on graph 2 below.
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The complete storage and distribution system
of expendable refrigerants is now eliminated. No more tanks, no more
insulated distribution lines, no more bulky storage cylinders in the test
area.
Graph 2
Mechanically refrigerated
systems require some re-thinking though. The systems must be properly sized to
the job that they are expected to perform. With expendable refrigerants there is
a virtually inexhaustible supply of available heat removal capacity. With
mechanical refrigeration you essentially make what you need as you go along.
Therefore it is very important that the system is capable of supplying the
necessary heat removal capacity, which can be determined by considering four factors. First, the total amount of heat that is dissipated
by the device that is being tested. Second the lowest temperature that the
device under test must be cooled to. Third, the rate of temperature change in
degrees per minute that will be required. And last, the total weight of your
device and the primary material that it is constructed of. With these four
pieces of information the mechanically refrigerated thermal cycling system can
be properly sized to the application.
By far the biggest reason to
consider mechanical refrigeration as the source of DUT cooling in a thermal
cycling system is cost. Consider graph 3 showing the comparative costs between
using mechanical refrigeration and expendable refrigerant over a ten-year
period. The graph shows a thermal cycling scenario and the potential cost savings.
In some cases, the cost savings returned by switching to mechanical
refrigeration from expendable refrigerant can pay for the entire system in less
than a year. The savings over ten years can be huge and the chart only
represents the savings from a single test site.
Graph 3
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offer to learn how to save money from the first day by purchasing an MRTP system
to replace a cryogenic thermal platform.
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