Expendable Refrigerants Versus Mechanical Refrigeration
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.
Comparison between LN2 and LCO2
The importance of efficient product testing
A re-think is necessary
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 equipment designed to use them usually has a lower initial cost due to a simpler design.
- The cooling rates can be very fast. However, faster cooling rates are not guaranteed because there are a number of factors that determine just how quickly expendable refrigerants can cool a device. These factors will be covered in the next section.
- Expendable refrigerants have the capacity to remove large amounts of heat quickly. This is helpful if the DUT dissipates a large amount of heat.
The Disadvantages of Expendable Refrigerants
- Overtime expendable refrigerants are very expensive to use due to visible and hidden costs. Since they are only used once and not recovered it is necessary to continually replenish the supply. Some companies claim that their expendable refrigerant budget can be several hundred thousand dollars per year. It is not unusual for a single test site to consume well over 100 dollars worth of expendable refrigerant per day.
- 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.
- 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.
- An additional disadvantage in the case of liquid nitrogen and low-pressure liquid carbon dioxide is that the refrigerant is continuously being used up in the process of keeping itself at storage temperature. An average of three percent per day can be lost in this process. This means that a completely full cylinder will be completely empty after a month even if is not used at all.
- 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.
Comparison Between LN2 and LCO2
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.
Benefits of Satellite Condensers
This type of installation yields four primary benefits.
Almost no refrigerant is lost maintaining storage temperature and distributing the refrigerant.
Inexpensive and flexible distribution lines can be used such as thin wall copper. This is because the refrigerant is in a vapor state and can be distributed at room temperature.
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.
A ready supply of liquid carbon dioxide is always available right at the test site making for fast cycle times. This is because any vapor bubbles will rise up to the satellite condenser and be re-condensed into a liquid. There is no wait for the vapor to purge from the supply line.
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.
The Importance of Efficient Product Testing
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.
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.
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.
There is no more need to connect, disconnect, transport, or fill portable storage cylinders.
There will be no more water ice balls on the piping, dripping water, and puddles on the floor.
Because the mechanical refrigeration system only requires electrical power to operate, the system can be rolled to anywhere in the facility that it is needed and simply plugged-in to an AC outlet. That kind of flexibility is very attractive.
A Re-Think Is Necessary
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.