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Control system for refrigeration-based compressed-gas dryersRelated Patent Categories: Refrigeration, Processes, Circulating External Gas, With Removing Of MoistureControl system for refrigeration-based compressed-gas dryers description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060201167, Control system for refrigeration-based compressed-gas dryers. Brief Patent Description - Full Patent Description - Patent Application Claims
[0001] The present invention refers to an improved control system for refrigeration-based compressed-gas dryers, i.e. machines that are usually employed for removing moisture contained in a stream of compressed gas, in particular compressed air. [0002] A control system for refrigeration-based compressed-gas dryers of the above-cited kind is described in the publication EP 1 293 243 of the same Applicant, where a description is given concisely of the various kinds of dryers that are currently used or can be currently found on the market. [0003] As generally known in the art, a typical refrigeration-based compressed-air dryer usually works to reduce the water-vapour content in the stream of compressed air by causing the same vapour do condense--as a result of the air entering the dryer being cooled down--and eventually letting off the condensed vapour. In other words, the compressed air is first cooled down in an air-to-air heat exchanger (also referred to as economizer in the art) by the stream of cold compressed air flowing in from the evaporator of the refrigerating circuit. The cold compressed air leaving the air-to-air heat exchanger flows into the evaporator of the refrigerating circuit, where it cools further down due to the evaporation of a refrigerant or coolant medium. At the evaporator outlet, the compressed air reaches its lowest temperature (dew point under pressure), to which there corresponds a relative humidity of 100%. Before this air flow is conveyed back into the air-to-air heat exchanger, the condensed water vapour is separated therefrom and let out. At the outlet of the air-to-air heat exchanger, the water vapour content remains unaltered (same dew point under pressure), but the relative humidity thereof decreases due to the heat-up effect. [0004] In other kinds of dryers, i.e. the so-called "thermal mass" dryers, use is made of an intermediate medium to cool down the compressed air, wherein this intermediate medium is in turn cooled down in another heat exchanger by the evaporating refrigerant medium. [0005] In the so-called "hot gas" dryers, the refrigeration compressor operates constantly, even if there is no flow of compressed air, and the refrigerating output or effect that is produced in excess by the refrigerating circuit is compensated for by the hot gas being pumped by the compressor, which is by-passed directly on the suction side by a control member designed to prevent the temperature of the air from decreasing to any value below zero (FIG. 1). This control member is typically a pressure-operated valve controlling the suction pressure of the compressor. In these hot-gas dryers, the dew point is practically constant. [0006] In "thermal-mass" dryers, the oscillations, i.e. periodical variations of the dew point are not negligible and the electric power input is variable, since it is roughly proportional to the thermal load (a combination of flow rate, temperature, pressure and relative humidity of the gas). The refrigeration compressor is controlled by a thermostat that controls the temperature of the thermal mass (typically a temperature that is correlated to the minimum temperature of the air in the evaporator). Such thermostat switches the compressor cyclically on and off in order to keep the temperature of the thermal mass within pre-established limits (FIG. 2). When the compressor is working, the refrigerating power that is produced in excess is stored in the thermal mass and is used to cool down the compressed air when the compressor is switched off. Dryers of this kind, however, have necessarily larger sizes and involve higher costs than the other dryers known in the art, and their power input under nominal conditions, i.e. their power input rating is higher than that of corresponding hot-gas dryers, owing to the twofold heat-exchange process that forces the compressor to work at lower suction pressures and, as a result, with a lower energy efficiency (COP). [0007] Unlike thermal-mass dryers, a hot-gas dryer uses heat exchangers (i.e. the air-to-air heat exchanger and the evaporator of the refrigerating circuit) that do not have any sufficient thermal mass to keep the periodical variations of the dew point within an adequately narrow range (e.g., 3.degree. C.) and to ensure a number of on/off cycles of the compressor that is lower than the highest allowable one prescribed by compressor manufacturers (e.g., 12 cycles/hour). [0008] Therefore, it is a first purpose of the present invention to provide a novel control system for a refrigeration-based compressed-gas dryer that, although making use of heat exchangers that are typical of hot-gas dryers, is effective in enabling an energy saving effect to be obtained under varying conditions of the thermal load, which is rather similar to the one achievable in thermal-ass dryers. [0009] Another purpose of the present invention is to ensure a lowest possible dew point of the compressed gas, which is further subject to variations of a satisfactorily limited extent. [0010] Yet another purpose of the present invention is to provide a compressed-gas dryer whose costs are comparably low as the ones of hot-gas dryers, and is further compact in size and low in weight as a hot-gas dryer. [0011] According to the present invention, these and further aims are reached in a refrigeration-based compressed-gas dryer provided with a control system incorporating the characteristics and features as recited in the appended claims 1 et seq. [0012] Anyway, features and advantages of the present invention will be more readily understood from the detailed description that is given below by way of non-limiting example with reference to the accompanying drawings, in which: [0013] FIG. 1 is a schematical view of a compressed-gas dryer of the hot-gas kind with compressor pressure control, according to the prior art; [0014] FIG. 2 is a schematical view of a compressed-gas dryer of the thermal-mass kind with temperature control, according to the prior art; [0015] FIG. 3 is a schematical view of a refrigeration-based compressed-gas dryer incorporating a control system according to the present invention; [0016] FIG. 4 is a diagram plotting the electric power input versus the thermal load in a compressed-gas dryer according to the present invention as compared with a hot-gas dryer and a thermal-mass dryer according to the prior-art; [0017] FIG. 5 is a diagram plotting compressor operation versus the controlled temperature, according to the control system of the present invention. [0018] As already noted hereinbefore, and illustrated in FIGS. 1 to 3, a refrigeration-based compressed-gas dryer comprises a refrigerating circuit including a compressor 10, which pumps the refrigerant medium into a condenser 12, where the same medium is converted from the gaseous phase into the liquid one for flowing then in this state through a throttling member 14 and, finally, into and through an evaporator 16, before being drawn in again by the compressor. [0019] Condensate forming as the gas, i.e. the air is cooled down is removed therefrom in a condensate separator 18, properly collected and finally let out by a condensate drain device 20. [0020] In addition, both prior-art dryers and the dryer according to the present invention comprise a heat exchanger 22, or economizer, in which the compressed gas or air is pre-cooled by the same compressed gas or air undergoing post-heating. [0021] The hot-gas compressed-gas dryer (FIG. 1) also comprises a further throttling member 30, which is usually constituted by a pressure-controlled valve that controls the suction pressure of the compressor by by-passing the latter, so as to prevent the air temperature from sub-cooling, i.e. decreasing to a value below zero. [0022] As opposed thereto, the thermal-mass compressed-gas dryer (FIG. 2) makes use of an intermediate compressed-air/liquid heat exchanger 32, as arranged between the heat exchanger 22 and the evaporator 16, and a thermostat 34 that typically controls a temperature of the liquid or a temperature at a point of the heat exchanger 32 that correlates to the outlet temperature of the compressed air from the heat exchanger 32 so as to cycle the compressor 10 on and off in view of keeping the temperature of the thermal mass within pre-set limits. [0023] According to the present invention, the novel control system for a refrigeration-based compressed-gas dryer is based on the principle of letting the dryer operate selectively as a hot-gas dryer or a thermal-mass dryer, so as to optimize the performance and the efficiency thereof through the measurement and corresponding detection of the thermal load. [0024] The novel control system is first of all aimed at solving the basic problem of reconciling two mutually clashing technical requirements, i.e. (i) keeping the highest allowable number of on/off cycles per hour of the compressor within the limits prescribed by the dryer manufacturer (e.g., 12 cycles per hour), and (ii) keeping the oscillation or variation extent of the dew point within limits that are acceptable by the user (e.g., between 1.degree. C. and 5.degree. C.). Continue reading about Control system for refrigeration-based compressed-gas dryers... 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