Abstract:
A control system for a cooling system for reducing humidity levels in a gas such as ambient air is provided. The cooling unit includes at least one of a compressor and a chiller, and cooling coils are connected to the cooling unit. A fan blows the gas onto the cooling coils. A fan speed controller is connected to the fan and variably controls the speed of the fan. A dry bulb temperature sensor is in thermal communication with the gas and connected to the controller, and a moisture sensor is in communication with the gas and connected to the controller. The speed of the fan is determined by the controller based on signals received from the dry bulb temperature sensor and the moisture sensor. By varying the speed of the fan, the moisture content of the gas can be modified much more precisely than by conventional air conditioning methods.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to air conditioning systems and more particularly to air conditioning systems that control humidity levels in the air. It is also applicable to removing or reducing the moisture content from other gases. 
     2. Description of Related Art 
     Air-conditioning, the cooling of the air of indoor spaces, has been known for decades. Residential or home air-conditioning is typically controlled by a thermostat that solely measures the dry bulb temperature or sensible heat of the air being conditioned. 
     However, one of the main objectives of air-conditioning is to reduce the moisture content or relative humidity of the air as well. Depending on the temperature and humidity, controlling an air-conditioning system based on dry bulb temperature alone may produce uncomfortable conditions and use energy inefficiently. A system that does not effectively dehumidify the air may leave the conditioned air cold but damp; for example, it is more comfortable to maintain a room at 75° F. at low humidity conditions than it is to cool a room to 68° F. but allow humidity levels to remain high. 
     One proposed solution has been to add a relative humidity sensor to the typical thermostat of an air-conditioning system and then control the air-conditioner to hold relative humidity within a predetermined range. There are several drawbacks to such a system. For one, humidity sensors that directly measure relative humidity are generally inaccurate and slow to respond to changes in ambient conditions. A problem may also result with such a system in that the relative humidity of the enclosed air will rise as it is cooled provided one remains above the dew point. Relative humidity is a function of both the amount of water vapor per volume of air and its dry-bulb temperature, specifically the ratio of the partial pressure of the water vapor in the air to the vapor pressure of saturated steam at that pressure. Since the vapor pressure of saturated steam drops rapidly with temperature, a small amount of water vapor at a lower temperature can result in 100% relative humidity. It is possible to have a situation where the humidity control calls for further dehumidification, the temperature drops, and the relative humidity rises. 
     Air-conditioning systems are sized to reduce both the sensible and latent heat at the design point. This is a point where the known maximum load may occur only 5-10% of the time during the cooling season based on local climatological weather data and utilization of the space. During periods when the sensible heat load is less than the design point and the relative humidity is high, a dry bulb thermostat will only need to run the air-conditioner for a short period of time before the space reaches the desired dry bulb set point and the thermostat is satisfied. This short run time does not allow the air-conditioner sufficient time to remove the required amount of moisture from the air and therefore the air will be cool and damp, an uncomfortable condition. 
     FIG. 1 shows the typical airflow in an air-conditioning system. The incoming air stream  1  is pulled through a cooling coil  2  by a fan  3  and leaves as cooler air  4 . Moisture will be removed from the incoming air stream  1  if the coil temperature  2  is at or below the point of condensation (dew point) of the incoming air  1  and will leave as a dehumidified air  4 . A thermostat  5  measures the dry bulb temperature by means of a temperature sensor  7  in the air of the conditioned space. Once the dry bulb air temperature is cooled to the desired point, the thermostat turns off the compressor or cooler  6  and stops the process of removing sensible heat and moisture. The occupant of the conditioned space may have an option depending on the model of thermostat as to whether or not the thermostat  5  will stop the fan  3  from moving air or continue to run the fan and circulate air in the conditioned space during the period when no cooling is required. By controlling the cooler  6  based on the dry bulb temperature sensor  7  connected to the thermostat  5 , there may be periods when the conditioned space is uncomfortable due to high levels of humidity for the given set point of the thermostat  5 . 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the invention to provide an air conditioning system that can efficiently reduce the humidity in the air in an enclosed area without overcooling the air. 
     It is another object of the invention to provide an air conditioning system that will provide comfortable ambient conditions in an energy efficient manner. 
     It is another object of the invention to provide an air conditioning system having an accurate inexpensive moisture sensor. 
     These and other objects are accomplished by the invention which includes a cooling system having a chiller/compressor/cooler, cooling coils, and a fan blower. The cooling system has a dry bulb temperature sensor and a moisture sensor. Both sensors are connected to a controller which controls the speed of the fan. The moisture sensor determines the level of moisture in the space to be conditioned and sends a signal to a controller. Based on the signals received from the dry-bulb temperature sensor and the moisture sensor, the controller controls the speed of the fan. 
     By varying the fan speed, the invention can vary the amount of moisture in the air as well as the temperature. At lower fan speeds, air passes over the cooling coils more slowly, there is more contact with the cooling coils, and thus there is more dehumidification and more cooling occurring; however, at lower speeds, a smaller volume of air is being conditioned. This results in a conditioned space having warmer dry bulb temperature but lower humidity than if the fan speed were faster. That is, at higher fan speeds, a greater volume of air is conditioned, however less moisture per volume is removed therefrom, resulting in a cooler but more humid space. 
     The invention further includes the method and apparatus by which moisture levels are detected. The invention uses a chilled mirror approach. A mirror is provided in thermal communication with the cooling coils of the air conditioner. Thus, if moisture is condensing on the cooling coils, then moisture will condense on the mirror as well. The formation of condensate means that the cooling coils (and mirror) are below the dew point of the room. A heater is provided in thermal communication with the mirror. When the heater is activated, the temperature of the mirror rises. At some point, the temperature of the mirror will rise above the dew point of the room, and the condensate will disappear from the mirror. The disappearance of the condensate can be detected by a photosensor, and the amount of heat needed to make the condensate disappear is easily determined. In this fashion, i.e., raising the mirror to the dew point of the room, the dew point can be determined, and the fan speed of the system can be altered accordingly. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic of a conventional air conditioning system. 
     FIG. 2 is a schematic of an air conditioning system according to the present invention. 
     FIG. 3 is a schematic of a moisture sensor according to the present invention. 
     FIG. 4 is a electrical schematic of the air conditioning system according to the present invention showing the controller in greater detail. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Increased levels of moisture removal may be accomplished during periods of reduced sensible heat load by reducing the volume of air passing over the cooling coil. The reduced flow of air across a cooling coil changes the ratio of sensible and latent heat removal with an increase in the capacity of the latent component. 
     Effective Temperature (ET*) combines temperature and humidity into a single index, so two environments with the same ET* should evoke the same comfort even thought they have different temperatures and humidities. A room at 78° F. with a low relative humidity will be just as comfortable as, a room at 70° F. with a high relative humidity. The low humidity in the 78° F. room allows a person&#39;s natural cooling system in the form of evaporation of moisture off the skin to work more efficiently. In addition, the amount of energy required in cooling a room to 78° F. is less than the energy required to cool a room to 70° F. In order to accomplish effective temperature control, both dry bulb temperature and the amount of moisture in the air must be known. 
     The invention will now be described with reference to the attached FIGS. 2-4. Although the invention is described as being an air conditioning system for use in conditioning indoor ambient air, the invention is also applicable for removing or reducing the moisture content of other gases in industrial applications. 
     As shown in FIG. 2, air conditioning system  50  addresses both the dry bulb temperature and the moisture content of the air in the conditioned space. During periods when the incoming air  1  has a low level of sensible heat but a high level of humidity it would be desirable to alter the cooling coils  2  sensible to latent heat capacity ratio. This can be accomplished by running the fan  3   b  at a reduced speed. Reducing the volume of air that moves across the coil  2  alters the latent heat to sensible heat ratio, increasing the capacity to remove moisture while reducing the capacity to lower the amount of sensible heat in the air. Air passage  1   a  delivers air from the conditioned space to a moisture sensor  10  that is shown in detail in FIG.  3 . The movement of air through the moisture sensor is caused by the low air pressure present at the back of the fan  3   b  on the outlet air passage  1   b  of the moisture sensor  10 . Passage  1   b  has two ends; one end  1   b   1  receives air from moisture sensor  10 , and the other end  1   b   2  is in communication with the rear of fan  3   b . The controller  25  receives a signal from the moisture sensor  10  via line  30  that contains the level of moisture present in the conditioned space as sampled from the incoming air stream  1 . The controller  25  also monitors a dry bulb temperature sensor  7  in the conditioned space via line  31 . Based on the signals received from dry bulb temperature sensor  7  and moisture sensor  10 , controller  25  can vary the fan speed of fan  3  by sending it signals via line  32 . The preferred moisture sensor is shown in FIG.  3  and measures moisture content in the air using a chilled mirror system. Chilled mirror systems used for the measurement of the dew point of a gas are well known from prior art; however, the inventive moisture sensor  10  is novel owing to the thermal connection of the mirror to the cooling coil  2  and the use of heater  13 , as will be described below. The invention utilizes the temperature of the chilled cooling coil, which is commuted through a thermal conductor  9  to the surface of a mirror  14 . If the temperature of the coil is below the dew point of the incoming air  1   a , condensation will form on the mirror. If the dew point temperature is below that of the coil, no condensation will form on the mirror despite the mirror  14  being thermally connected to cooling coils  2 . The lack of condensation on the mirror  14  indicates that the cooling coil  2  will be unable to reduce the moisture content in the space being conditioned and will only be able to reduce the sensible heat portion of the air in the room. Condensation is detected on the mirror by directing a source of light  12  via a lens  15  to the sensing mirror  14  and measuring the amount of reflected light with a photosensor  11 . The actual dew point can be determined by increasing the temperature of the mirror with a heating element  13  to the point at which the condensation clears from the mirror as detected by the change in reflected light sensed by the photosensor  1   1 . The amount of heat output by the heater  13  and the level of reflected light are both controlled and measured by controller  25 . With this information, controller  25  can determine the temperature at which the mirror unfogs, i.e., the actual dew point of the incoming air  1 . With the dew point known and the dry bulb temperature measured by the temperature sensor  7 , an Effective Temperature index can be determined by the following equation: 
     
       
           ET*=t   0   +wi   m   LR ( p   a −0.5 p   ET*,s   (1) 
       
     
     where 
     ET*=Effective Temperature 
     t 0 =temperature, operative 
     w=skin wetness 
     i m =vapor permeation total 
     LR=Lewis ratio 
     p a =water vapor pressure, ambient 
     p ET*,s =water vapor pressure, saturated at ET*. 
     This equation is the ASHRAE standard for Effective Temperature (ASHRAE Fundamentals 1997, Chapter 8, Section 8.6, Equation (33)) and covers a broad range of different applications; for example, the respective temperature and humidity levels for comfort in a library, health club exercise room, and locker rooms are very different. That is where ‘skin wetness’ and ‘vapor permeation’ come into play. For typical domestic air conditioning applications, average values can be selected for normal clothing in normal activities in the house. If the occupant is not comfortable he/she may adjust the ET set point up or down. 
     Because the operation of moisture sensor  10  is controlled by controller  25 , and because controller  25  receives information from moisture sensor  10 , a feedback loop is present in the system. Line  30  of FIG. 2 should include several components, as will be described herein. As shown in FIG. 4, controller  25  includes analog/digital converter  19  for receiving signals from temperature sensor  7  via line  31  and photosensor  11 . Controller  25  also includes digital/analog converter  20  for sending signals to light source  12  via line  30 B, mirror heater  13  via line  30 C, and fan motor  3   a  via line  32 . Keyboard  16  and display  17  are provided to allow an operator to interface with the system. Power supply  21  is provided to power the entire system. 
     Relative levels of humidity between 40% and 60% have been determined to be the optimum range for both comfort and health. Standard Effective Temperature (SET*) is based on a relative humidity (RH) level of 50%. In addition, relative humidity levels maintained below 60% have been shown to reduce the growth of mold. This is important to people who are sensitive or allergic to mold spores when they come in contact with them. 
     In operation, the invention functions as follows. The occupant of the room where the invention is installed would set an initial dry bulb temperature set point on the controller  25  by means of keyboard  16 , a series of touch switches (not shown), or other similar data entering means. Alternatively, the operator of the system will set a “comfort level”, i.e., Effective Temperature, via the same input means as above, and the CPU  18  will determine from that ET set point what a desirable dry bulb temperature would be. The comfort level may be expressed on a unitless numerical scale of 0-10 for example, on an conographic or pictorial scale, or any other convenient way of expressing different degrees of comfort. In any case, CPU  18  will vary the speed of motor  3   a  of fan  3  in order to achieve the desired dry bulb set point. Dry bulb temperature is monitored by sensor  7 , and CPU  18  determines if the set point has been reached. 
     Once the set point has been achieved, controller  25  sends a signal via line  30 B to activate light source  12  (alternatively, light source  12  can be continuously on). All the while, mirror  14  is being cooled by cooling coils  2  via thermal conductor  9 . If there is no condensation on mirror  14 , photosensor  11  detects light from light source  12  and indicates to CPU  18  that the dew point of the ambient air is below the temperature of the cooling coils  2 , and thus the system cannot reduce the moisture content of the ambient air. If there is condensation on mirror  14 , photosensor  11  will either detect a reduced amount of light or no light from light source  12  reflected off of mirror  14 . Photosensor  11  sends this information to controller  25  via line  30 A. Based on this signal, CPU  18  determines that heater  13  must be activated to clear mirror  14  and does so by sending a signal via line  30 C. Heater  13  generates heat until mirror  14  clears of condensate. When photosensor  11  detects that mirror  14  is clear (because it detects the light reflected therefrom), a signal is sent to controller  25  via line  30 A, and heater  13  is deactivated by controller  25  via line  30 C. CPU  18  is able to calculate the dew point of the ambient air based on the amount of energy delivered to the heater. The amount of energy delivered to the heater can be controlled via a number of different means, such as varying the voltage across the heater, varying the current supplied to the heater, varying the duration of time the heater is on, and the like. Alternatively, a separate temperature sensor (not shown) can be used to measure the mirror temperature directly. 
     CPU  18  will have the value of the dew point of the space by means of the above-described system. An ET* can now be determined, and the fan  3  will be allowed to run at a minimum speed if it is still necessary to reduce the RH to an acceptable value. As the dew point of the space is reduced, the dry bulb temperature of the space will be allowed to rise in order to keep the ET* constant. There is thus a feedback loop that includes the moisture sensor, the temperature sensor, and the controller. 
     Having described the invention with regard to specific embodiments, it is to be understood that the description is not meant as a limitation excluding such further variations or modifications as may be apparent or may suggest themselves to those skilled in the art. It is intended that the present invention cover such variations and modifications as fall within the scope of the appended claims. For example, the above description indicates that the mirror in the moisture sensor is in constant thermal communication with the cooling coils. However, the thermal conductor that connects the two may be selectably disengageable for those periods of time when the moisture sensor is not being used. Similarly, the dry bulb and moisture sensors may be employed to monitor the quality of the room air (or other gas) continuously or intermittently. Also, the specification makes reference to a cooler; this can include any type of cooling device, such as a regular Freon compressor, a hydronic chiller using water, or the like. Additionally, the above description mentions that the CPU can determine if the dew point of the air is below the temperature of the cooling coils and thus cannot be further reduced in humidity. An indicator of this condition can be made as an optional component of display  17  or as a separate warning light. Display  17  can be made to indicate all types of relevant data in text or graphical format, such as temperature, relative humidity, dew point, the change of any of these data over time, or the like.