Temperature switches (i.e., thermostats, aquastats or freezestats) are commonly used in direct digital control systems to provide a digital input when a monitored process temperature rises/falls to a preselected value. Equipment protection, for example, from the overcooling or icing of hydronic coils, cooling coils and liquid handling pipes, has long been recognized, with a variety of known temperature switches available, selection being based upon the specific application. Heretofore known temperature switches employ a variety of known operating principals, such as for instance, bimetallic, fluid thermal expansion, freezestat and electronic.
The freezestat is commonly used to prevent water or steam coils in air handling units from freezing. Functionally, a fluid, existing as a saturated vapor at the switch set point temperature, is confined within a long capillary tube, or what is sometime referred to as a bulb. The tube is installed in a serpentine fashion over the area of the air stream to be monitored. If any point along the tube falls below the saturation temperature of the capillary fluid, condensation commences, which in turn causes a rapid, almost instantaneous change in system pressure, which actuates a switch in response thereto.
Several significant drawbacks are inherent to freezestats (i.e., gas-filled capillary switching devices). First, the gas-filled capillaries must, in order to be functional, be mounted in a horizontal orientation (i.e., only sections having a substantially horizontal orientation are functional: bending of the capillary tube is limited, e.g., bends should not have a radius of less than about 1.5 inches). This is inherently problematic, and is especially so when a large area must be traversed: the labor associated with the installation of a 30 foot plus capillary, or a plurality of such devices is quite time consuming.
Second, the relay portion is characteristically mounted (i.e., permanently mounted) to the capillary. This necessitates that the capillary be completely uncoiled, and subsequently feed through a small mounting hole, and then serpentinely configured so as to traverse the area requiring monitoring.
Third, the capillary tubes, especially those of small diameter, are notoriously fragile. Any kinks, pinholes, etc. render such devices inoperable.
Fourth, freezestats have a limited vapor charge, typically having a liquid volume less than the volume associated with a twelve inch length of the capillary or bulb. Thus, such capillary tubes have a sensitivity of twelve inches (i.e., temperature detection in any twelve inch continuous length of bulb).
Finally, if the thermostat bellows is colder than the capillary, the charge accumulates in the bellows, and the case temperature controls the action of the thermostat rather than the capillary temperature. Thus, the relay portion of the switch assembly must be mounted in an environment warmer than the capillary to properly function.
In light of the foregoing real and/or perceived shortcomings in the art, it is commercially advantageous to provide a temperature detection and switching assembly having improved sensitivity, greatly reduced installation effort, greater operational flexibility (e.g., no limitation on sensor orientation, no restriction on controller placement, etc.), and improved durability.
USPTO Backgrounds
USPTO Backgrounds