By Trygve Behny
EDITOR’S NOTE: In January 2006, Minco launched the HVAC industry’s first combination temperature sensor, called Chill-Out. In this article, Trygve Behny, the Building Automation Industry Manager for Minco, compares the combination temperature sensor to the traditional freeze stat.
System integrators have been using conventional low temperature cut-outs, called “freeze stats,” in unit ventilators for decades. The sensors industry is now calling for a more sophisticated temperature sensor that not only provides easier and faster installation but also reduces life cycle costs and eliminates problems associated with the traditional freeze stat. This industry demand has led to the introduction of the “combination temperature sensor,” which combines a low temperature cut-out with an averaging sensor.
A Comparison of Functionality and Specifications
The freeze stat is a mechanical device consisting of a diaphragm that moves as the pressure inside a capillary changes with temperature and closes a switch at a specific temperature. The freeze stat typically shuts down the flow of outside air when in an alarm state. A separate averaging temperature sensor is also required to provide temperature input to the controller.
The combination temperature sensor contains all solid-state electronics. The low temperature cut-out senses a cold spot every 12" along its length and contains integrated circuit (IC) temperature sensors that output 0 or 5 volts. Under normal conditions, the sensors output 5 volts. When in an alarm state the voltage drops to 0 and the relay contacts change state. The alarm state can be displayed on a controller.
The freeze stat is quite fragile and has more installation requirements than the combination temperature sensor, which means that installing the freeze stat is more demanding and labor intensive. A percentage of freeze stats are typically damaged during installation and spare parts have to be installed before the system can be checked out.
The freeze stat can also exhibit these types of problems:
• Kinking—Kinks or sharp bends in the tubing cause poor flow in the sensing bulb and limit its effectiveness.
• Mounting—The sensing bulb must be mounted horizontally across the face of the coil with vertical portions kept to a minimum. Vertically mounted portions of the sensing bulb create undetected cold spots, and improper operation can occur.
• Diaphragm Case Location—The diaphragm can operate ineffectively when its case is mounted in the same ambient temperature as the sensing bulb. The diaphragm must be warmer than the set point for effective operation.
The combination temperature sensor was designed to simplify installation and prevent the problems associated with the conventional freeze stat’s installation. It addresses freeze stat problems in the following manner:
• Kinking—Element is unaffected by kinks and bends.
• Mounting—Orientation of the sensing element has no effect on its function. It installs directly on coils horizontally, vertically, or diagonally.
• Relay Case Location—Controls are integrated with the sensing element and are unaffected by ambient temperature.
Solid State Design Vs Gas-Filled Capillary Tubes
Some key aspects to keep in mind when comparing the combination temperature sensor and the freeze stat are:
• Solid state design of the combination temperature sensor replaces traditional gas-filled capillary tubes eliminating the risk of kinking during installation.
• Solid state design also prevents the occurrence of field failures due to leaking.
• Solid state design makes the combination temperature sensor sensitive to low temperatures in any direction up to six inches, as opposed to 12-18 inches for gas-filled capillary tubes.
• Unlike gas-filled capillary tubes that must be mounted horizontally, the combination temperature sensor can be mounted at any angle, which offers greater operational flexibility.
Summary of Benefits
A comparison of the freeze stat with the combination temperature sensor demonstrates that the new technology reduces the cost of installation, eliminates the problems associated with freeze stat installations, and offers facility owners a lower cost of ownership. The integration of a low temperature cut-out and an averaging sensor into a single device cuts installation labor in half, and the use of sensing technology that functions regardless of kinking eliminates the problem of rough handling during installation.
Trygve Behny can be reached by e-mail at [email protected]
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