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A variable evaporator control system and method in a gas dryer for maximizing the cooling which can be accomplished for a given length heat exchanger by adjusting the evaporator refrigerant approach temperature responsive to changes in the gas load on the system. Pressure and/or temperature sensors

A variable evaporator control system and method in a gas dryer for maximizing the cooling which can be accomplished for a given length heat exchanger by adjusting the evaporator refrigerant approach temperature responsive to changes in the gas load on the system. Pressure and/or temperature sensors positioned at particular locations in the system provide feedback for controlling adjustments in the approach temperature depending on the gas load.

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What is claimed is: 1. A refrigerated compressed gas dryer variable evaporator control method comprising: a. introducing compressed gas at a first temperature through a first flow path in a heat exchanger; b. circulating a refrigerant through a second flow path in said heat exchanger, said refriger

What is claimed is: 1. A refrigerated compressed gas dryer variable evaporator control method comprising: a. introducing compressed gas at a first temperature through a first flow path in a heat exchanger; b. circulating a refrigerant through a second flow path in said heat exchanger, said refrigerant having a second temperature measured at a refrigerant inlet of said heat exchanger, said second temperature being less than said first temperature; c. cooling said compressed gas to a third temperature as it flows through said heat exchanger; d. controlling said second temperature responsive to changes in said third temperature to generally maintain said third temperature at a desired value; e. sensing a fourth temperature at a single location in said heat exchanger at which said fourth temperature is representative of said third temperature when a compressed gas load is above a certain level, and representative of said second temperature when said compressed gas load is below said certain level; and f. controlling said second temperature responsive to changes in said fourth temperature to generally maintain said fourth temperature at said desired value. 2. The method of claim 1 wherein said refrigerant is circulated under pressure, said method further comprising: a. maintaining said refrigerant at a predetermined pressure during circulation, said second temperature having a correlation to said predetermined pressure; and b. controlling said predetermined pressure responsive to changes in said third temperature such that said second temperature is correspondingly controlled to generally maintain said third temperature at said desired value. 3. The method of claim 2 further comprising: controlling said predetermined pressure responsive to changes in said fourth temperature such that said second temperature is correspondingly controlled to generally maintain said fourth temperature at said desired value. 4. A variable evaporator control refrigerated compressed gas dryer comprising: a. an evaporator heat exchanger having a first flow path for compressed gas at a first temperature; b. said evaporator heat exchanger having a second flow path for a refrigerant at a second temperature which is lower than said first temperature to cool said compressed gas to a third temperature; c. a compressor to circulate said refrigerant at a predetermined pressure, said predetermined pressure having a correlation to said second temperature; d. a first temperature sensor to sense said third temperature at an outlet of said first flow path; e. a second temperature sensor to sense a fourth temperature at a single location in said evaporator heat exchanger at which said fourth temperature is representative of said third temperature when a compressed gas load is above a certain level, and representative of said second temperature when said compressed gas load is below said certain level; f. a valve intermediate said second flow path and a source of refrigerant, said valve controllable to admit refrigerant into circulation to adjust said predetermined pressure; and g. a controller connected to receive feedback from said first temperature sensor indicative of said third temperature and said second temperature sensor indicative of said fourth temperature, said controller controlling said valve to adjust said predetermined pressure and thus adjust said second temperature to generally maintain said third temperature and said fourth temperature at desired values. 5. The compressed gas dryer of claim 4 further comprising a third temperature sensor to sense said second temperature at an inlet of said second flow path. 6. The compressed gas dryer of claim 4 further comprising a pressure sensor to sense said predetermined pressure in said second flow path and to provide feedback indicative of said predetermined pressure to said controller to control said valve to adjust said predetermined pressure and thus said second temperature to generally maintain said third temperature at said desired value. 7. A variable evaporator control method maximizing cooling of a gas passed through an evaporator heat exchanger of a given length, wherein gas at a first temperature flows through said evaporator heat exchanger in a first flow path and a refrigerant at a second temperature lower than said first temperature is circulated through said evaporator heat exchanger in a second flow path to cool said gas to a third temperature, the difference between said first and second temperatures being an approach temperature, said control method comprising: a. increasing said approach temperature as a gas load through said evaporator heat exchanger increases; b. decreasing said approach temperature as a gas load through said evaporator heat exchanger decreases; c. controlling said second temperature to implement said increasing or decreasing of said approach temperature; d. wherein said third temperature is thus generally maintained at a desired value irrespective of said gas load; e. sensing a fourth temperature at a single location in said evaporator heat exchanger at which said fourth temperature is representative of said third temperature when said gas load is above a certain level, and representative of said second temperature when said gas load is below said certain level; and f. controlling said second temperature responsive to said fourth temperature such that said fourth temperature is generally maintained at a desired value irrespective of said gas load. 8. A refrigerated compressed gas dryer variable evaporator control method comprising: a. introducing compressed gas at a first temperature through a first flow path in a heat exchanger; b. circulating a refrigerant through a second flow path in said heat exchanger, said refrigerant having a second temperature measured at a refrigerant inlet of said heat exchanger, said second temperature being less than said first temperature; c. cooling said compressed gas to a third temperature as it flows through said heat exchanger; d. controlling said second temperature responsive to changes in said third temperature to generally maintain said third temperature at a desired value; e. maintaining said refrigerant at a predetermined pressure during circulation, said second temperature having a correlation to said predetermined pressure; f. controlling said predetermined pressure responsive to changes in said third temperature such that said second temperature is correspondingly controlled to generally maintain said third temperature at said desired value; g. sensing a fourth temperature at a single location in said heat exchanger at which said fourth temperature is representative of said third temperature when a compressed gas load is above a certain level, and representative of said second temperature when said compressed gas load is below a certain level; and h. controlling said predetermined pressure responsive to changes in said fourth temperature such that said second temperature is correspondingly controlled to generally maintain said fourth temperature at said desired value.