Compressing a gas such as air creates heat because the mechanical process of pushing more air into less volume creates friction. The air molecules collide more frequently and exert more force on the pipes and containers holding the compressed air. The temperatures are high too. Where small household compressors may produce 120º skin surface temperatures, commercial air compressors with intercoolers and aftercoolers can have temperatures approaching 400º Fahrenheit. This diagram shows the air, oil, and water circuits on a typical water-cooled air compressor.
There are three basic types of air compressors are piston, screw, and centrifugal. Piston compressors usually have one or two cylinders and look somewhat like a large motorcycle engine. Centrifugal compressors have one or more circular turbine housings and look somewhat like a centrifugal chiller compressor. Screw compressors do not look like either of these, but they tend to make the most noise. When in doubt, consult the manufacturer. From our perspective, the hottest skin temperature is far more important than what type of mechanism compresses the air. Look for water piping to determine if the compressor is water-cooled- No water connections mean no water treatment concerns!
Heat exchangers are used to remove heat from compressed air to help the compressor operate more efficiently and extend equipment life. We are typically concerned with water-cooled heat exchangers, but finned air coolers are also common in smaller applications. An intercooler typically cools the air between stages in a multistage compressor. The term intercooler is often used to describe the heat exchanger between the compressor and the application, though technically that is an aftercooler since it is in line after the air has been compressed.
Oil in air compressors is used as a lubricant, a sealant, and a coolant for the compressor. Each of those functions involves heat when the compressor is operating, and that heat must be removed to keep the system operating efficiently. Oil temperature varies by design, manufacturer, and operating conditions, but typically runs up to 240º F. Since the water is typically inside the tubes in a shell and tube oil cooler, you can measure the oil temperature on the outer shell of the heat exchanger as long as it does not have a layer of insulation or other coatings.
The same way it does with process or comfort cooling systems. Higher temperatures mean less calcium and alkalinity can be in the water without forming scale. We use Saturation Index to determine how much calcium and alkalinity can be in the water without causing scale problems. Skin temperature is just as important as calcium or alkalinity, so make sure you know the temperature of the hottest point in your system. Parameters and settings that work great in comfort cooling often fail miserably in the extreme conditions of the air compressor.
Quality installation, routine service and sound preventative maintenance are what keep equipment running. Failures and short comings can occur in any of those areas. The best built equipment will not last long if poorly installed or improperly maintained. While there are plenty of air flow, power supply, and engineering variables governing performance, our main concern is water treatment. Even the best sized, highest performing air compressor will fall short of expectations if cooling water flow is inadequate, heat rejection capacity at the chiller or tower is inadequate, or scale or corrosion has impeded heat transfer.
These convenient and inexpensive thermometers can produce very accurate results if used properly. It is important to understand that the laser is merely an aiming feature and has nothing to do with measuring temperature. Each thermometer has a ratio that describes the area of temperature measurement. Ratios of 8:1 to 12:1 are common for inexpensive models. That means if you hold the front of a 10:1 thermometer within one inch of the point you are measuring, the instrument will display the average temperature of a spot 0.1 inches in diameter. The laser dot is about the same size at that distance. But if you aim your 10:1 thermometer at a 4″ water pipe 6 feet away (that is 72 inches), then the instrument displays the average temperature of a spot 7 inches in diameter. That means you are measuring not only the temperature of the 4″ pipe your laser dot is hitting, but also the ceiling or whatever is behind the 4″ pipe. And remember that the cone of measurement keeps getting bigger the further away it gets. The point is to hold the thermometer very close to whatever you are measuring for the most accurate results.
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