Sensors for measuring the temperature of cold and hot water in HVAC systems

If the air temperature is the most important consumer characteristic, the end product of heating or air conditioning equipment, then the coolant temperature is a parameter that affects the result only indirectly, rather for “internal use”, control of the automation itself and the formation of diagnostic and emergency signals. Except for the temperature directly in the heating radiators, where, however, it is also only an indirect means of measuring such a consumer parameter as the heat output of the radiator, which depends far from only on the temperature of the coolant (then its difference at the inlet and outlet is more important).

In many HVAC systems, the heater/cooler is placed at a considerable distance from the room that needs to be kept at the desired temperature: it is both technically and economically unfeasible to place a chiller evaporator or an electric heater directly in the fan coil unit. In principle, it is possible to supply already heated / cooled air from a boiler or a remote chiller, but the air duct must have a considerable cross section, so it is always bulky and material-intensive, spoils the aesthetics, difficult to install, but most importantly, due to the large dissipation area, it incurs huge heat losses , and high-quality thermal insulation of such a volume is in itself a difficult and expensive procedure. At many facilities (especially those that already have a ventilation duct), these issues are resolved in this way. But it is much more rational to use liquid coolants.

The specific heat capacity of liquids is 3–5 times greater than that of gases, and the density is 3 orders of magnitude greater. That is, to transfer the same amount of heat at the same carrier speed, a hydraulic tube with a section several thousand times smaller than an air duct will be required (of course, due to viscosity, the speed is set less, but a hundredfold decrease in the section is guaranteed). Proportionately, losses, the cost of materials, installation are reduced, there are significantly fewer problems with aesthetics and thermal insulation (excellent results when laying simply in plastic boxes).

Depending on the conditions of the liquid, different ones can be used. In special cases - alcohols, oils and others. But the cheapest, most common and at the same time the most heat-intensive substance on Earth is ordinary water (of course, distilled water is better for hydraulic systems). It is also characterized by moderate corrosivity. Pipes and other parts made of stainless steel, any polymers and non-ferrous metals often used in such equipment can be in contact with water for an arbitrarily long time and without any additional coatings, and if the system is sealed (without oxygen from the surrounding air) and the water was before sealing degassed, then from ferrous metals.

The only drawback of such a coolant is the anomalous expansion during crystallization. Unlike other liquids, water can break the hydraulic system not only when it boils, but also when it freezes. Therefore, one of the functions of thermal sensors is to track the approach to both points of change in the state of aggregation.

The whole variety of heat sensors for water (and not only for water, and not only for liquids in general) is divided into:

•  According to the installation method - overhead and submersible (channel). The former should have a convenient and reliable fastening that provides good thermal contact with the outer surface of the pipe, radiator, expansion tank and other metal elements of the hydraulic system, the temperature in which is measured. For polymer containers and high measurement accuracy, only submersible ones are suitable - cable (without rigid fastening), threaded and flanged.
  
•  According to the type of temperature-sensitive element - a bimetallic mechanical sensor, a thermocouple, a thermistor (wire or metal film), a semiconductor thermistor.
•  By the presence / absence of signal processing in the sensor - active and passive.
•  According to the connection diagram (2-, 3- and 4-wire).
  
•  In terms of dimensions, weight and associated inertia.
  
•  By accuracy.
  
•  According to the material of the protective sleeve.
  
•  By temperature range.
  

Fancoil Motors offers to buy a wide variety of components for climatic equipment, including measuring instruments and sensors, for example, a submersible water temperature sensor, the price of which, thanks to our bulk purchases and discounts from old trusted suppliers, may not exceed the manufacturer's price.