Technicians who work on air conditioning know that, in most cases, the purpose is to create comfort cooling for people. The human body can cope with a relatively wide range of temperature extremes when the a/c is a bit out of whack; high-end computers are not so forgiving. Engineers constantly look for better ways keep the cooling of such equipment as stable as possible.

More than ever before, today’s computer cooling systems are using conventional refrigeration designs. One of the newest twists involves using the thermostatic expansion valve (TXV) and the compressor as control elements.

Major research from Yasin Makwana and Dereje Agonafer of the University of Texas, and Dan Manole of Tecumseh Products Co., experimented on a compressor supplied by Masterflux, a division of Tecumseh that makes variable-speed compressors for a variety of industries. In published reports, the researchers noted that “In the past, virtually all commercial computers were designed to operate at temperatures above the ambient.

“However, research has identified the advantages of operating electronics at low temperatures. The current research focuses on mainframes that use a conventional refrigeration system to maintain chip temperatures below that of comparable air-cooled systems, but well above cryogenic temperatures.

“Multivariable control of compressor speed, along with the TXV opening, can give better stability and performance.”

To measure all of this, the researchers first looked at the TXV being the only control element. Second, they had the TXV and compressor both acting as control elements.

In the first instance, “A TXV was the only control element adjusting the refrigerant quantity passing through the evaporator upon receiving the feedback about change in heat load while the typical compressor used was running at constant speed. … the Masterflux compressor was introduced in the system so as to have a multivariable control that will enable one to regulate the refrigerant quantity by not only varying the TXV opening, but also varying the compressor speed.”

THE SETUP

The compressor, condenser, TXV, evaporator, accumulator, and hot gas bypass valve were the main components in the setup. A sensor bulb was placed on the evaporator return line, and the temperature and pressure sensors were mounted at various places in the system, the researchers said. “A flow meter [was] also placed on the evaporator return line. A cold plate attached to a single plate of evaporator is made up of copper.”

A heater block was mounted on the copper cold plate. “This heater block simulates the multichip module and has resistive heaters embedded in it that provide a heat load of more than 1 kW,” said the researchers. “This heat load can be varied from zero to full-scale.” Thermal paste was applied where the heater block and evaporator cold plate interface, to minimize contact resistance and ensure proper heat transfer.

“The system is operated in two different configurations,” the researchers said. “The sensor bulb of the TXV senses the evaporative exit temperature. If this temperature is higher than the evaporating temperature, it will result in increasing the TXV opening, thereby allowing more refrigerant to pass through. If the evaporator exit temperature is lower than the evaporating temperature, the TXV opening will decrease, restricting the refrigerant flow.”

The researchers then looked at more actively utilizing the compressor with a controller unit. “A temperature sensor senses the evaporator exit temperature at the same location at the TXV sensor bulb,” the report said. “If the evaporator exit temperature is higher than the evaporating temperature, the signal from the temperature sensor will cause an increase in the compressor speed while the TXV sensor bulb signal causes a bigger opening, thus effectively increasing the refrigerant flow. An evaporator exit temperature lower than the evaporator temperature will result in a reduction of both the compressor speed and the TXV opening, thus reducing refrigerant flow.”

FINDINGS FAVOR FEEDBACKS

Based on data from the experiment, there was “a better system response and transient behavior when a two-feedback system is used compared to a single-feedback system.

“In a single-feedback system, the thermal resistance between the bulb and the evaporator return line can considerably affect system stability, and by increasing this thermal resistance, the stability can be further improved,” the researchers wrote. “Similarly, the sizes of the bulb and the two-phase heat transfer coefficient have an effect on system stability.

“By using a feedback signal to adjust the compressor speed, the effect of thermal resistance between the sensor bulb and evaporator return line can be minimized.

“Another big advantage for system architecture is that by using a compressor, a considerable reduction in volume of the cooling system is achieved.”

For more information, visit www.masterflux.com.

Publication date: 04/02/2007