The thermodynamic behavior of ammonia is very different from halogen refrigerants. When a refrigeration pump turns on, the ammonia temperature rapidly changes from 100°F to -70° or below. So components used in ammonia-based systems must be able to withstand these changes without failure.
For example, pressure sensors are a common component used as part of control systems in refrigeration pumps and compressors. With the move to ammonia in refrigeration pump and compressors, manufacturers must specify a pressure sensor that can withstand dramatic temperature changes without degrading performance.
Often, O-ring sealed pressure sensors cannot withstand rapid thermal changes and, over time, the internal O-ring can fill. Fluid-filled sensors suffer from freezing effects.
This causes a rupture of the thin diaphragm membrane that is welded in place to hold in the silicon oil fill. Thin-film sensors (based on 15-5 and 17-4 PH stainless steels) that have their diaphragms welded to a pressure port will undergo thermal stresses that can eventually crack the steel.
One method of dealing with this situation is to use a pressure sensor built to withstand the temperature effects of ammonia cooling systems. American Sensor Technologies, Mt. Olive, N.J., says it has developed such a device. The Model HVAC/R pressure sensor is said to reduce the effects of thermal flash transients when the media is ammonia.
The device’s one-piece, stainless steel sensing element is composed of 316LSS wetted materials. “This design also provides excellent overrange and burst protection (5x rated line pressure or 2,000 psi, whichever is less) ratings for system safety in the most extreme conditions,” according to company officials.
The pressure sensor does not contain internal O-rings or fluid-filled cavities, eliminating the chance of permeation or system contamination. A nonclogging port design minimizes the effects of extreme temperature change. The sensor will recognize a uniform temperature so as to maintain system stability.
The sensor offers a cyclical life of approximately 100 million full cycles. The company also offers sensors with an SAE4 (7/16-20 UNF) female port for the refrigeration market that contains an internal Schraeder depressor pin. This port allows the sensor to be removed or installed when in service, eliminating the need for adaptors.
For more information, visit www.astsensors.com for more information on American Sensor Technologies.
Withstanding Pressure in Ammonia Systems
March 12, 2007