Refrigerants thermoelectric cooling

The optical chilled-mirror dew point technique is a fundamental measurement, because the saturation temperature determines the saturation partial pressure of the water vapor. A temperature element is then placed in thermal contact with the mirror, and the mirror temperature is utilized directly as the dew point or saturation temperature. The cooling of the mirror surface in the past was done by acetone and dry ice, liquid C02, mechanical refrigeration, and later by thermoelectric heat pumps. The thermoelectrically cooled, optically observed dew point hygrometer probe is illustrated in Figure 3.26. 

The EMF measured by a readout device reflects the difference between the temperatures of the measuring junction and the reference junction thus, the temperature at the reference junction must be properly controlled. It is usually maintained at the freezing point of water (0°C) by using an ice bath (Fig. 16.23). The ice bath should be a mixture of crushed ice and pure water in a Dewar flask with considerably more ice than liquid. It can be very accurate but not too convenient in some applications, since frequent replacement of melted ice is required. With a thermoelectric refrigeration system [36], ice can be maintained by cooling the bath of water with thermoelectric cooling elements. 

Peltier effect A thermoelectric cooling effect used in small-scale refrigeration units. It involves a solid-state heat pump that transfers heat from one side of the device to the other through the consumption of electticity. The thermoelectric cooling effect is created between the junction of two different types of conducting materials. The device can also be used for heating. It is named after French physicist lean Charles Athanase Peltier (1785-1845) who discovered the effect in 1834. 

Shortly after the discovery of the thermoelectric effect by Seebeck, Peltier (1834) found the inverse phenomenon. Suppose all junctions shown in Fig. 5.11.3 are at ambient temperature. Then an external current source is connected to the terminals. This current causes Joule heating, i R, as expected, but, in addition, one junction experiences cooling while the other shows excess heating. The Peltier effect has been exploited to construct small refrigerators. Thermoelectric coolers, specifically designed for the operation of infrared detectors, are commercially available. They are able to cool a small detector up to 50 or even more degrees below ambient (Wolfe Zissis, 1978). 

The dewpoint hygrometer detects the dewpoint temperature of air by cooling a surface in contact with the air to the dewpoint temperature. There are several ways to achieve cooling and to observe the formation of condensate on the surface. The early dewpoint hygirometers were cooled simply by applying the vaporization of ether or some other suitable liquid. Condensate formation on the surface was determined visually. Other cooling methods are to use a refrigerant flow in direct or indirect contact with the back of the surface, or to use electricity with a (thermoelectric) Peltier element. 

Figure 3.12. The overall view of the goniostat of the Scintag XRD-7 powder diffractometer. This diffractometer has the horizontal goniometer axis, stationary x-ray source and synchronized rotations of both the detector arm and sample holder. The goniometer is equipped with a thermoelectric refrigerator-cooled solid-state detector, which enables the monochromatization of the diffracted beam by selecting a narrow energy window thus registering only characteristic energy photons. (Courtesy of Scintag, Inc.)...

Thermoelectric materials have been used extensively for thermal sensing, energy conversion (heat to electricity), and for cooling (Peltier effect). In general, thermoelectric devices consist of n-type and p-type semiconductor constituents connected electrically in series and thermally in parallel [151]. While energy conversion efficiencies for thermoelectric devices are not yet competitive with conventional refrigerator or power generation systems, it is possible to achieve enhanced efficiencies... 

Thermoelectric materials used for refrigeration utilise the Peltier effect. Two thermoelectric materials are coupled by metal plates, which act as the junctions. A current passed through the circuit in one direction will heat one plate and cool the other. For refrigeration to occur, the temperature of the hot junction is maintained constant by connection to a heat sink, which may simply be a fan. Continuous cooling will occur at the cold junction. These are widely used in food and drinks coolers powered by a car battery. 

Where cooling is required as well as heating, a closed loop refrigeration circuit, servoed in a similar manner, can be used. A more sophisticated temperature controller, which can both heat and cool, can be made from a Peltier thermoelectric device. In this device a voltage across a series of thermocouple junctions sets up a temperature... 

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