Method of dehumidifying an indoor space using outdoor air

Abstract

A method for dehumidifying air in an indoor space using outside air is comprised of the steps of: sensing whether there is a demand for dehumidification in the space; in response to a demand for dehumidification, determining whether outdoor air is suitable for dehumidification of the space; if the outdoor air is determined to be suitable, admitting outdoor air into the air supplied to the space; and selectively temperature conditioning the supply air in accordance with a predetermined temperature condition therefor. The method is disabled in response to a demand for heating or sensible cooling in the space. Further, the ratio of outdoor air to return air in the supply air is kept relatively constant, irrespective of whether the predetermined temperature condition is satisfied.

Description

TECHNICAL FIELD

This invention relates generally to space conditioning systems for conditioning the temperature and humidity of an indoor space and in particular to a method of dehumidifying the space using outdoor air.

BACKGROUND ART

Central air conditioning systems, including systems that use a vapor compression refrigerant to cool air circulated to an indoor space, often require relatively accurate control of humidity in the air within the space. In one such system, as described in U.S. Pat. No. 6,644,049, a reheat heat exchanger is positioned downstream of the system evaporator to reheat air that is cooled and dehumidified by the evaporator. Therefore, dehumidified air can be delivered to the space without appreciably cooling the space.

In the system described in U.S. Pat. No. 6,644,049, outdoor air is not admitted into the system in response to a demand for dehumidification in the space. In the air conditioning system described in U.S. Pat. No. 6,427,461, outdoor air is admitted into the supply air stream in response to a demand for dehumidification if the enthalpy of the outdoor air is suitable. Typically, if the enthalpy of the outdoor air is suitable for dehumidification, it is cooler than the desired temperature of the space. If as a result of introducing cooler outdoor air into the supply air stream, the supply air temperature drops below a predetermined setpoint, the amount of outdoor air admitted into the system is reduced to try to prevent overcooling of the space. However, there is no provision to heat the supply air in the absence of a demand for heating in the space.

SUMMARY OF THE INVENTION

In accordance with the present invention, a method is provided for dehumidifying an indoor space using outdoor air. The method is comprised of the steps of: sensing whether there is a demand for dehumidification in the space; in response to a demand for dehumidification, determining whether outdoor air is suitable for dehumidification of the space; if the outdoor air is determined to be suitable, admitting outdoor air into the space; and selectively temperature conditioning the outdoor air prior to admitting it into the space in accordance with a predetermined temperature condition for air supplied to the space.

In accordance with one embodiment of the invention, the method includes determining whether the temperature of the outdoor air satisfies a predetermined outdoor air temperature condition.

In accordance with another embodiment of the invention, the method includes determining whether the humidity of the outdoor air satisfies a predetermined outdoor air humidity condition.

In accordance with still another embodiment of the invention, the method includes sensing the temperature of the air supplied to the space and heating the air supplied to the space as needed to maintain the predetermined temperature condition for the supply air, even in the absence of a demand for heating in the space.

In accordance with yet another embodiment of the invention, the method includes sensing whether there is a demand for temperature conditioning of the air in the space. In response to a demand for temperature conditioning of the air in the space, the dehumidifying method is disabled.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic showing the circulation of air in an air conditioning system;

FIG. 2 is a block diagram showing the control for the air conditioning system of FIG. 1; and

FIG. 3 is a flow diagram depicting the dehumidification operation in accordance with the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The best mode for carrying out the invention will now be described with reference to the accompanying drawings. Like parts are marked in the specification and drawings with the same respective reference numbers. In some instances, proportions may have been exaggerated in order to depict certain features of the invention.

Referring now to FIG. 1, there is illustrated an air conditioning system in accordance with an embodiment of the present invention and generally designated by numeral 10. Air conditioning system 10 is shown, by way of example, as a so-called “rooftop” system having a generally rectangular box-like cabinet 12 adapted to be mounted on a generally horizontal surface such as a rooftop 14. Air conditioning system 10 is adapted to deliver conditioned air to an enclosed space 16 by way of an opening 18 and air within the enclosed space 16 is returned to system 10 by way of an opening 19 to cabinet 12 for conditioning by system 10.

System 10 preferably includes a vapor compression refrigeration unit 20, including a compressor (not shown) for circulating a conventional refrigerant fluid between indoor and outdoor heat exchanger coils. Indoor heat exchanger 32 is disposed within cabinet 12 and is arranged in the path of the air flow through cabinet 12, as shown by the arrows. When system 10 is operated in a cooling mode, indoor coil 32 functions as an evaporator to transfer heat from the air flowing through coil 32 on the outside thereof to the refrigerant flowing inside coil 32. Evaporator 32 is typically disposed downstream of a suitable air filter 33, which is also disposed in the air flow path between return opening 19 and supply opening 18 for space 16.

Air conditioning system 10 further includes a reheat heat exchanger coil 38, through which heated refrigerant gas may be circulated directly from the system compressor. Reheat coil 38 extends over a portion (e.g., 50%) of the area of evaporator 32 and is downstream thereof. Reheat heat exchanger 38 is used to reheat air cooled and dehumidified by evaporator 32 when system 10 is operated in a mechanical reheat mode in response to a demand for dehumidification of space 16. The use of a hot refrigerant gas to reheat air cooled and dehumidified by the evaporator is known in the art and is described, for example, in U.S. Pat. No. 6,427,461, the relevant portions of which are incorporated by reference herein to further describe operation of reheat coil 38. A drain pan 53 is located beneath coil 32 for collecting condensate runoff from coil 32.

System 10 also includes a motor-driven blower 48 of conventional construction and a suitable electric drive motor 50. In a preferred embodiment, as shown, blower 48 is disposed within cabinet 12 in a space 52 downstream of evaporator 32 and reheat heat exchanger 38 for returning conditioned air to space 16 through opening 18. However, blower 48 may be disposed upstream of evaporator 32 and reheat exchanger 38 in an alternate embodiment. Blower 48 is shown mounted on a heater unit 51, whereby air returned to enclosed space 16 by way of opening 18 may be conditioned by heater unit 51, if desired. Heater unit 51 may be one of several types of conventional heater units, wherein air discharged from blower 48 passes over suitable heat exchange surfaces when heater unit 51 is operable. Heater unit 51 is mounted on a discharge or supply air duct part 48a, which is in communication with opening 18. Heater unit 51 preferably includes one or more electrically resistive heating elements, but may also include a combustible fuel (e.g., gas) heater.

A suitable sensor 54 is preferably interposed in duct part 48a or may, depending on the configuration of system 10, be disposed in a suitable air supply duct farther downstream from and connected to duct part 48a, but upstream of space 16. The location of sensor 54 and duct part 48a is exemplary. Sensor 54 is preferably a temperature sensor for measuring the temperature of the supply air.

Upstream of evaporator 32 is an enclosed space 55 within cabinet 12, through which air from space 16 may flow by way of opening 19 and spaces formed between a set of moveable louvers or dampers 56, which are mechanically linked to each other and to linkage 58, for example, connected to a suitable actuator or positioning motor 60. Positioning actuator or motor 60 is also connected via linkage 62 to a damper comprising a second set of moveable louvers or dampers 64 for controlling the entry of ambient outdoor air into space 55 and discharge into space 16. The apparatus comprising the aforementioned sets of louvers 56, 64 and actuator motor 60 connected thereto is also sometimes known in the art as an “economizer”. Outdoor ambient air (represented by arrow 65) may be admitted to space 55 by way of a rain shroud 66, a suitable filter screen 68 and an enclosed space 70, which is partitioned from a space 72 by a suitable transverse partition 74.

A sensor 76 is disposed in space 70. In one embodiment, sensor 76 is a temperature sensor for sensing the temperature of ambient outdoor air admitted to space 70. In an alternate embodiment, sensor 76 is a humidity sensor instead of a temperature sensor for sensing the humidity of outdoor air admitted to space 70. A suitable humidity sensor 78 is preferably disposed in return air space 72 or may, alternatively, be disposed within space 16, for sensing the humidity of air in space 16. A conventional air conditioning temperature sensor 80 (which may be a thermostat) is disposed in a suitable location within space 16 for sensing and controlling the temperature of the air in space 16 as determined by an adjustable setpoint of sensor 80.

A suitable pressure relief damper 73 in FIG. 2, for space 72 may be provided to minimize any pressure increase in space 16 if outdoor air is introduced into space 55 by blower 48 for circulation to space 16. In a preferred embodiment of the invention, when outdoor air is introduced into the system in response to a demand for dehumidification in space 16, the relative proportion of outdoor air to return air is relatively fixed (for example, 40% outdoor air to 60% return air) so that louvers 56, 64 are controlled to maintain the fixed proportion of outdoor air to return air.

Referring also to FIG. 2, system 10 is controlled by a microcomputer-based controller 90 in accordance with a preselected control program. Controller 90 is responsive to inputs from outdoor air sensor 76, space temperature sensor 80, space humidity sensor 78 and supply air temperature sensor 54 for controlling the operation of dampers 56, 64, blower 48 and heating unit 51.

Referring also to FIG. 3, in accordance with an embodiment of the present invention, outdoor air may be used to help dehumidify space 16 if the outdoor air is suitable for dehumidification. The term “FAH” in FIG. 3 stands for “Fresh Air Heating” and refers to an operating mode of system 10 wherein fresh (outdoor) air is brought into system 10 in response to a demand for dehumidification in space 16 and the supply air may be heated as necessary to maintain the supply air temperature above a predetermined threshold. If the outdoor air is cooler than the indoor air, then the humidity of the outdoor air is usually lower than the humidity of the indoor air. In that case, the outdoor air may be used to provide “free dehumidification” without having to operate the system compressor. However, the cooler outdoor air may lower the supply air temperature below an acceptable limit, resulting in overcooling of space 16. Therefore, it may be necessary to add heat to the supply air.

Controller 90 executes an iterative process as shown in FIG. 3. If fresh air heating (FAH) is enabled at decision point 100 in response to a demand for dehumidification, dampers 56, 64 are opened to a preset value at step 101 to admit outdoor air into the supply air stream in a predetermined ratio of outdoor air to return air (for example, 40% outdoor air to 60% return air). The preset value may be selected by the system installer or may be determined by the default setting in the system control program. Controller 90 then determines at decision point 102 whether the supply air temperature is less than a predetermined temperature threshold (for example, 68° F.) in response to the input from supply air temperature sensor 54.

If the supply air temperature is less than the threshold, heating unit 51 (preferably, one or more electrical heating elements) is activated at step 104 to heat the supply air. However, if the supply air temperature is not less than the threshold, heating unit 51 is deactivated in accordance with step 106. In either case, the process then proceeds to decision point 108. However, if FAH has not been enabled at decision point 100, dampers 56, 64 are operated in accordance with the normal “economizer” mode, wherein dampers 56, 64 are controlled automatically, as represented by step 107. The process then proceeds to decision point 108. If FAH is not enabled, system 10 cannot be operated to dehumidify the space using fresh (outdoor) air.

At decision point 108, controller 90 determines whether there is a demand for heating or sensible cooling in space 16 in response to the input from space temperature sensor 80. In response to a demand for either heating or sensible cooling in space 16, FAH is disabled at step 110 and the process returns to decision point 100. However, if there is no demand for heating or sensible cooling in space 16 at decision point 108, controller 90 determines whether there is a demand for latent cooling (dehumidification) in space 16 at decision point 112. If there is no demand for dehumidification in space 16, all “Reheat” modes, including FAH and mechanical reheat modes using the system compressor, are disabled at step 114 and the process returns to decision point 100.

If there is a demand for latent cooling in space 16 at decision point 112, the process proceeds to decision point 116, where controller 90 determines whether the outdoor air is suitable for dehumidification. This determination is based on input from outdoor air sensor 78, which senses either temperature or humidity of the outdoor air. If the temperature or humidity, as the case may be, of the outdoor air satisfies a predetermined condition (for example, outdoor air temperature is 45° F. or less), the outdoor air is determined to be suitable for dehumidification and FAH is enabled (or remains enabled, if already enabled at step 100) at step 118. The process then returns to decision point 100.

If the outdoor air is determined to be unsuitable for dehumidification of the space at decision point 116, FAH is disabled at step 120 and a selected mechanical reheat mode is enabled at set 122 in response to the dehumidification demand in space 16. The mechanical reheat mode involves the use of the system compressor to first cool and dehumidify the supply air using the system evaporator 32 and then to reheat the dehumidified air by circulating heated refrigerant gas through reheat coil 38. Alternatively, the mechanical reheat mode may use a heat source other than reheat coil 38 such as a gas heating section or electrical heating elements to reheat the dehumidified supply air. After the selected mechanical reheat mode is enabled at step 122, the process returns to decision point 100.

One skilled in the art will recognize that in accordance with the present invention, fresh (outdoor) air may be used for dehumidifying an indoor space if the outdoor air is suitable from a temperature and/or humidity standpoint and in the absence of a demand for heating or sensible cooling in the space. To prevent overcooling the space from the introduction of cooler outdoor air into the supply air, the supply air temperature is monitored and is adjusted to maintain a predetermined supply air temperature condition.

Although the preferred embodiment of the invention has been described with reference to heating the supply air as necessary to maintain the predetermined temperature condition, the invention contemplates that in some cases the supply air may need to be cooled instead of heated. For example, if the suitability of the outside air for dehumidification is determined based on its humidity rather than on its temperature, in some climates, the outdoor air may be warmer and drier than the indoor air, so that if outdoor air is brought in to help dehumidify an indoor space, the supply air may need to be cooled instead of heated to maintain a predetermined temperature condition.

The best mode for carrying out the invention has now been described in detail. Since changes in and modifications to the above-described best mode may be made without departing from the nature, spirit and scope of the invention, the invention is not to be limited to the above-described best mode, but only by the appended claims and their equivalents.

Claims

1. A method of dehumidifying air in an indoor space, comprising the steps of:

sensing whether there is a demand for dehumidification in the space;

in response to said demand for dehumidification, determining whether outdoor air is suitable for dehumidification of the space;

if the outdoor air is determined to be suitable, admitting outdoor air into the space; and

selectively temperature conditioning the outdoor air prior to admitting it into the space in accordance with a predetermined temperature condition for air supplied to the space.

2. The method of claim 1 wherein said determining includes determining whether the temperature of the outdoor air satisfies a predetermined outdoor air temperature condition.

3. The method of claim 1 wherein said determining includes determining whether the humidity of the outdoor air satisfies a predetermined outdoor air humidity condition.

4. The method of claim 1 wherein said admitting includes controlling the amount of outside air admitted into the space.

5. The method of claim 1 wherein said selectively temperature conditioning includes sensing the temperature of the air supplied to the space and heating the air supplied to the space as needed to maintain said predetermined temperature condition, even in the absence of a demand for heating in the space.

6. The method of claim 1 wherein said admitting includes introducing a predetermined amount of outdoor air into the air supplied to the space.

7. The method of claim 6 wherein said introducing includes maintaining a relatively fixed proportion of outdoor air in the air supplied to the space.

8. The method of claim 1 further including sensing whether there is a demand for temperature conditioning of the air in the space and in response to said demand for temperature conditioning, disabling said method.

9. In a system for conditioning air in an indoor space, a method of dehumidifying the air in response to a demand for dehumidification in the space, comprising the steps of:

sensing whether there is a demand for heating or cooling in the space;

in the absence of a demand for heating or cooling in the space, determining whether outdoor air is suitable for dehumidification of the space;

if the outdoor air is determined to be suitable, admitting outdoor air into the system; and

selectively temperature conditioning the outdoor air prior to admitting it into the space in accordance with a predetermined temperature condition for air supplied to the space.

10. The method of claim 9 wherein said determining includes determining whether the temperature of the outdoor air satisfies a predetermined outdoor air temperature condition.

11. The method of claim 9 wherein said determining includes determining whether the humidity of the outdoor air satisfies a predetermined outdoor air humidity condition.

12. The method of claim 9 wherein said admitting includes controlling the amount of outdoor air admitted into the space.

13. The method of claim 9 wherein said selectively termperature conditioning includes sensing the temperature of the air supplied to the space and heating the air supplied to the space as needed to maintain said predetermined temperature condition, even in the absence of a demand for heating in the space.

14. The method of claim 9 wherein said admitting includes introducing a predetermined amount of outdoor air into the air supplied to the space.

15. The method of claim 14 wherein said introducing includes maintaining a relatively fixed proportion of outdoor in the air supplied to the space.

16. In an air conditioning system for supplying conditioned air to an indoor space, a method of dehumidifying supply air to the space, comprising the steps of:

sensing whether there is a demand for dehumidification in the space;

in response to said demand for dehumidification, determining whether outdoor air is suitable for dehumidification of the space;

if the outdoor air is determined to be suitable, introducing a predetermined amount of outdoor air into the supply air; and

selectively temperature conditioning the supply air in accordance with a predetermined temperature condition.

17. The method of claim 16 wherein said determining includes determining whether the temperature of the outdoor air satisfies a predetermined outdoor air temperature condition.

18. The method of claim 16 wherein said determining includes determining whether the humidity of the outdoor air satisfies a predetermined outdoor air humidity condition.

19. The method of claim 16 wherein said selectively temperature conditioning includes sensing the temperature of the supply air and heating the supply air as needed to maintain said predetermined temperature condition, even in the absence of a demand for heating in the space.

20. The method of claim 16 wherein said introducing includes maintaining a relatively constant proportion of outdoor air in the supply air, irrespective of whether said predetermined temperature condition is satisfied.

21. The method of claim 16 further including sensing whether there is a demand for temperature conditioning of the air in the space and in response to said demand for temperature conditioning, disabling said method.