Abstract:
Heating, ventilating and air conditioning are provided to a temporary, flexible shelter, especially in a rugged, remote and/or extreme environment, including locations and/or conditions where access to electric power may be limited and/or expensive. A portable system may include a light weight HVAC unit, with variable-speed components that are dynamically managed for efficiency, reliability and safety, and a flexible, self-insulating duct for connecting the HVAC unit to the temporary shelter.

Description:
[0001]    The present disclosure relates to a system and devices for providing heating, ventilating and air conditioning to a temporary, flexible shelter, especially in a rugged, remote and/or extreme environment, including locations and/or conditions where access to electric power may be limited and/or expensive. The system may include a light weight HVAC unit, with variable-speed components that are managed for efficiency, reliability and safety, and a flexible, self-insulating duct for connecting the HVAC unit to the temporary shelter. 
       BACKGROUND 
       [0002]    U.S. Pat. No. 7,735,502 (Hotes) illustrates a heating, ventilating and air conditioning unit for a temporary flexible shelter. 
         [0003]    U.S. Pre-Grant Publication 2009/0228149 (Alston &#39;149) refers to an environment control and power system for a temporary hospital or first-aid shelter operated by military or other emergency response services. The Alston system is said to have a variable speed compressor, and the respective compressor and blower speeds are said to be automatically set. Alston also refers to an electric resistance heater, and a user interface, which is said to control humidity. 
         [0004]    U.S. Pre-Grant Publication 2008/0311842 (Alston &#39;842) refers to a forced-air heating, ventilating and air conditioning system that routes air, through a diverter Y valve, to different locations, based on sensed conditions, including temperature and hazardous conditions, or under the control of a user interface. 
         [0005]    U.S. Pre-Grant Publication 2012/0191253 (Rockenfeller) refers to a heating, ventilation, air conditioning, and refrigeration system for a residential home, commercial building or remote location. Rockenfeller refers to variable frequency drives for controlling the speeds of a blower and a compressor motor. 
         [0006]    The known systems are not sufficiently light weight, efficient, rugged and reliable, and safe, especially for use in remote, extreme environments. There is a need in the art for an HVAC unit that is better suited for operations where the unit has to be moved and installed by hand, where fuel for generating electric power is scarce and expensive, where weather conditions include very high and low temperatures, rapid, large changes in temperature, and high humidity, and/or where rapid set-up, reliability, and safe performance are at a premium. 
       SUMMARY 
       [0007]    The disadvantages and shortcomings of the known systems, devices and processes are overcome to a great extent by the present disclosure, which relates to a portable environment control system for use with one or more portable, semi-permanent or permanent shelters, including fabric shelters and tents. The control system has a refrigerating circuit with an air-cooled refrigerant condenser, a refrigerant-air heat-exchanging evaporator, an expansion valve, and a refrigerant compressor operated by a variable-speed electric motor. Outside air may be circulated over the condenser by a fan that is driven by a variable-speed electric motor, and recycled air may be circulated over the evaporator by a fan that is driven by its own variable-speed electric motor. The refrigerating circuit, which contains a suitable refrigerant gas, may be at least partially contained within a hermetically sealed enclosure, to protect the system from weather, including ultraviolet radiation, wind, rain and snow. The environment control unit may have an electric-resistance heating element in the path of the air that flows over or through the evaporator, for providing heated air when it is cold outside. 
         [0008]    According to a preferred embodiment, the environment control system of the present disclosure has a user interface, a plurality of sensors, and an intelligent (programmed with software and/or firmware, and/or hard-wired) management system. The interface may be used by an operator to input and receive information, including operational information, diagnostic codes, present system settings, and user adjustment controls. The sensors may include temperature sensors, pressure sensors, and an air quality sensor. The temperature sensors may be used to monitor the temperature of the inlet air to the condenser, the temperature of the inlet air to the evaporator, and the temperature of the discharge air from the evaporator. The pressure sensors may be configured to monitor the high-side refrigerant pressure (Pd) and the low-side refrigerant pressure (Ps) (i.e., the pressures of the refrigerant immediately upstream and downstream from the compressor). 
         [0009]    As discussed in more detail below, the intelligent management system may contain information on safe operating limits of various system components, and may be configured to receive additional (input) information from internal and external sensors. In operation, the management system may continuously (or when prompted to do so) adjust one or more of the speed of the condenser fan, the speed of the evaporator fan, and the speed of the compressor, to maintain the best possible operating performance without exceeding the operating limits of the compressor (or other sensitive system components). 
         [0010]    The compressor, with its associated variable-speed motor, may be the heaviest component, or one of the heaviest components, of the environment control system. Thus, an object of the present disclosure is to configure the environment control system so that it can perform as needed even with a compressor that is relatively light weight. Reducing the weight of the compressor makes the HVAC unit easier to transport and install; reducing the weight of the compressor may be especially important where the HVAC unit is transported and/or installed by hand. Reducing the weight of the compressor is also important for units that may have to be transported rapidly, for example by helicopter, over large distances or over rugged terrain or in other difficult conditions. At the same time, the environment control system is proactively and dynamically managed so that the relatively small compressor (which has more constrained operating limits than would be the case for a larger compressor) is reliably maintained within its safe operating limits, while the system continues to meet performance requirements with respect to air temperature, relative humidity, air quality, and safety, to the extent possible. 
         [0011]    According to the present disclosure, the refrigerant gas may have a low critical temperature and/or a low critical pressure. The refrigerant gas may be, for example, an R-410A refrigerant, including but not limited to ones marketed under the brand names Forane 410A, Puron, EcoFluor R410, Genetron R410A, and AZ-20. In operation, the intelligent management system adjusts one or more operational parameters, including condenser fan speed, evaporator fan speed, and compressor speed, to ensure that the system operates below the critical temperature and/or pressure. According to the present disclosure, the critical temperature and/or pressure is incorporated into the algorithm that is run in real time by the management system, and is used by the management system to ensure that the temperature and/or pressure of the refrigerant gas does not exceed safe and desirable limits. Further, the management system may be configured to dynamically adjust the evaporator superheat, by adjusting the extent to which the expansion valve is open, to maintain the compressor inlet and/or discharge gas temperatures within predetermined safe operational limits. If desired, the expansion valve for the refrigerant circuit may be electronically, remotely adjustable. 
         [0012]    According to other aspects of the present disclosure, the evaporator fan motor, the condenser fan motor, and the compressor motor are permanent magnet motors. The intelligent management system may be configured to restrict the maximum acceleration rate of the variable-speed motors to prevent current draw through the components from exceeding predetermined maximum safe levels. In addition, the maximum deceleration rate of the permanent magnet motors may be restricted to prevent the buildup of excessive voltage. 
         [0013]    In addition, the management system may be configured, if desired, to restrict the maximum acceleration rate of the compressor motor to prevent temporary loss of compressor lubrication. According to one aspect of the present disclosure, as part of a compressor startup sequence, the compressor motor may be held at a predetermined speed for a predetermined period of time to ensure that the compressor has proper lubrication. After an initial period of time, during which pressure equalization occurs, the speed of the compressor motor may be linearly increased and then held at a constant predetermined speed for a hold period sufficient to avoid loss of lubrication during compressor start-up, after which the compressor may be operated at target speeds according to performance requirements, as determined by the intelligent management system. 
         [0014]    According to another aspect of the present disclosure, the environment control system may have an electronically controllable/adjustable fresh air vent. The vent may be electronically moved to a fully-closed position, a fully-open position and to positions between the fully-open and fully-closed positions. An air quality sensor may be located within the air stream that is circulated by the evaporator fan. The air quality sensor may be used to monitor the circulating air to determine the presence and/or concentration level of one or more contaminants, including carbon dioxide. 
         [0015]    According to another aspect of the present disclosure, the environment control system may be operated in two modes, that is, a first, adjustable-vent mode and a second, closed-vent mode. The first mode is an active mode in which the position of the fresh air vent (the extent to which the vent is open) is adjusted to prevent the level of contamination within the circulating air from exceeding a first predefined maximum. The second mode is an inactive mode in which the fresh air vent remains fully closed regardless of the level of contamination. Whether the environment control system is operated in the first or second mode may be user selectable. 
         [0016]    According to another aspect of the present disclosure, the environment control system may be operated in a third mode, wherein, when the user-selectable mode of fresh air vent operation is active, the rotational speed of the compressor is monitored by the intelligent management system and, upon the compressor speed exceeding a predefined threshold, the predefined maximum level of contamination within the circulating air is increased from the first level to a second, higher predefined maximum level. This way, a balance is achieved between the amount of contamination that is accepted in the conditioned air and the need to keep the compressor operating without failure or damage. 
         [0017]    According to another aspect of the present disclosure, the environment control system may have a relative humidity sensor located in the return air stream ahead of the evaporator. The intelligent management system may respond to one or more signals from the relative humidity sensor to adjust the compressor speed to increase or decrease the amount of dehumidification of the return air by modifying the target evaporator temperature. 
         [0018]    In a preferred embodiment, the internal components of the environment control system, including the variable-speed compressor, the fans, and the electric resistance heater, operate entirely on direct current (DC). The environment control system may be connected directly to a source of DC electric power or to a source of alternating-current (AC) electric power. The power source may be, for example, a diesel generator, or a wind or solar power device or system. The environment control system may be configured such that any AC electric power that is input to the environment control system is immediately converted to DC electric power. A user-accessible switch may be used to set a maximum total operating power consumption to one of a plurality of levels, for example, by setting one or more of the maximum current draw of the variable-speed compressor, the maximum rotational speed of the variable-speed evaporator fan, and the maximum phase-width modulation (PWM) duty cycle of the electric resistance heater. If desired, the intelligent management system can be configured to automatically limit the maximum current draw and rotational speed of various components so that the devices can be safely and continuously operated at input power voltages that are within at least 20% above and 30% below their nominal operating voltages. 
         [0019]    If desired, the environment control system may have an exterior enclosure having sufficient rigidity to safely and securely support the weight of at least three such HVAC units stacked vertically on top of each other. Each unit may be provided with pockets for accepting the forks of an industrial lift truck, and one or more concave and/or convex pockets may be located at the top and the bottom to aid in centering the units during stacking. If desired, the system may be safely secured to a vertically-stacked second system for storage and/or transport. 
         [0020]    The present disclosure also relates to a flexible duct for conveying the air discharged from the evaporator into the portable shelter, which may be a tent, or some other enclosure with flexible walls. The shelter may be configured to shield medical personnel and patients from weather, dust, and other adverse environmental conditions. Alternatively, the shelter may be configured to protect stored items, such as food, equipment or other supplies, from weather, dust, and other adverse environmental conditions. 
         [0021]    The flexible duct may be inflated by the pressure of the air discharged by the HVAC unit. The duct may have an outer wall that is formed of a lightweight, gas impermeable membrane. According to one aspect of the present disclosure, the duct may have both an inner wall and an outer wall. The inner wall may have limited porosity to permit air inside the duct to pass to the annulus between the inner and outer walls. Baffles may be located within the annulus to prevent the air that has entered it from actively circulating. As a result, the air within the annulus becomes stagnant and provides thermal insulation between the air flowing through the duct (contained within the inner wall) and the environment outside the outer wall. 
         [0022]    Thus, the environment control system may include a heating, ventilating and air conditioning (HVAC) unit for use with a portable shelter in a rugged environment. The outside environment may be characterized by extreme high-temperature conditions and limited access to fuel for generating electric power. The HVAC unit may be configured to limit one or more of the maximum load of the compressor motor, the maximum temperature of the compressor motor stator windings, the maximum suction gas return temperature, the maximum compressor discharge temperature, and the gas-compression ratio generated by the compressor. 
         [0023]    According to another aspect of the present disclosure, the HVAC unit may have an intelligent management system that selectively operates the unit in a standard mode and a protective mode. The standard mode may be selected where external (environmental) conditions are such that the maximum operating temperature and/or pressure of any one or more system components will likely not be exceeded. The system may be configured to automatically switch to the protective mode of operation where the external conditions are such that one or more system components likely would exceed a design specification (i.e., a safe limit) if the system were operated in the standard mode. 
         [0024]    According to other aspects of the present disclosure, the HVAC unit may be configured to automatically switch from the standard mode to the protective mode based on the temperature of the condenser inlet air, the evaporator inlet air, or both. Moreover, the HVAC unit may be configured to switch from the standard operating mode to the protective operating mode based on one or more of the system low-side pressure, the system high-side pressure, the compressor current draw, the compressor rotational speed, the evaporator fan rotational speed, and the condenser fan rotational speed. 
         [0025]    Thus, the intelligent management system may reduce the speed of the compressor when operating in the protective mode to a level below the speed that would otherwise be set if the HVAC unit were operating in the standard mode. Likewise, the speed of the condenser fan may be increased in the protective mode to a level above the speed that would be set if the HVAC unit were operating in the standard mode, and the speed of the evaporator fan may be set in the protective mode to a level below the speed to which it would be set if the HVAC unit were in the standard mode. 
         [0026]    According to a preferred embodiment, the heating, ventilating and air conditioning system, employing the intelligent management system, may set the compressor rotational speed as the speed necessary to achieve a predefined target evaporation temperature when operating in the standard mode. The management system likewise may be configured to measure the power consumption of the compressor and the power consumption of the condenser fan, and to adjust the speed of the condenser fan to provide the lowest total power consumption, when operating in the standard mode. 
         [0027]    According to another aspect of the present disclosure, the intelligent management system may set the target speed for the evaporator fan via a proportional, integral, derivative (PID) feedback control algorithm based on the difference between the desired temperature of the space being conditioned (i.e., the space within the shelter) and the temperature of the return air (i.e., the air that is returned to the evaporator). 
         [0028]    If desired, a sunlight-readable user interface panel may be permanently affixed to the HVAC unit and used to control the system, and to display operational data. According to another aspect of the present disclosure, a detachable, remote user-interface may allow a user/operator to fully or partially control the system even when the user/operator is located more than five feet away from the environment control system. A dual-function data transfer port may allow a computer to transfer data to and from the remote user interface for operator-system control across a distance of more than five feet. If desired, the environment control system may be operated by a person who is located and protected from the outside elements within the fabric shelter. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0029]      FIG. 1  is a schematic view of an environment control system constructed in accordance with the present disclosure; 
           [0030]      FIG. 2  is a perspective view of a portable shelter connected to the environment control system of  FIG. 1  by flexible conduits; 
           [0031]      FIG. 3  is a flow chart for a process of operating the environment control system of  FIG. 1 , which takes into consideration whether component operating limits are exceeded; 
           [0032]      FIG. 4  is another flow chart for the process of operating the system of  FIG. 1 , showing how load motor currents may be maintained within operating limits; 
           [0033]      FIG. 5  is another flow chart for the process of operating the system of  FIG. 1 , showing how high-side pressure and temperature may be prevented from exceeding limits; 
           [0034]      FIG. 6  is another flow chart for the process of operating the system of  FIG. 1 , showing how an expansion valve can be selectively adjusted to prevent a compressor temperature from exceeding an operational limit; 
           [0035]      FIG. 7  is another flow chart for the process of operating the environment control system of  FIG. 1 , showing how a relative humidity level can be maintained at a desired level; and 
           [0036]      FIG. 8  is a cross sectional view of one of the flexible ducts shown in  FIG. 2 , taken along the line  8 - 8  of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0037]    Referring now to the drawings, where like elements are designated by like reference numerals, there is shown in  FIG. 1  an environment control system  10  constructed in accordance with a preferred embodiment of the present disclosure. The system  10  has a refrigerating circuit  12  which includes an air-cooled refrigerant condenser  14 , a refrigerant-air heat-exchanging evaporator  16 , a relatively small, variable-speed refrigerant compressor  18 , and an expansion valve  20 . Incoming air  22  is circulated over the condenser  14  in the direction of arrow  24  by a fan  26  that is driven by a variable-speed electric motor  28 . 
         [0038]    Recycling air  30  is circulated over the evaporator  16 , in the direction of arrow  32 , by a fan  34  that is driven by a variable-speed electric motor  36 . The refrigerant compressor  18  causes a suitable refrigerant gas to flow through the circuit  12  in the direction of arrow  38 , and is driven by a variable-speed electric motor  40 . The expansion valve  20  is variably controlled/adjusted by a suitable controller  42 . The enclosed parts  18 ,  20 ,  16  of the refrigerant circuit  12  are protected from the weather by an enclosure  44 . The condenser  14  is located functionally outside of the enclosure  44 , to transfer heat to the ambient outside air  22 . 
         [0039]    An electric-resistance heating element  46  is located in the path of the recycling air  30  that flows over the evaporator  16 . The heating element  46  is used to provide heat for the flexible shelter  80  ( FIG. 2 ) when the shelter  80  is located in a cold environment. 
         [0040]    The system  10  has a plurality of sensors for providing operational information to a user interface  60  ( FIG. 1 ), and to an intelligent management system  62  contained within the enclosure  44 . The management system  62  may have a computer processor  64 , a main data-storage memory device  65 , and a working data-storage memory device  66 , each of which may be encoded with software and/or provided with hardware to implement the functions of the intelligent management system  62  described herein. The management system  62  has suitable signal wires, receivers or transmitters  145  for communicating with other components of the system  10  via signal wires, receivers or transmitters  145  of those components. Although  FIG. 1  shows the management system  62  within the enclosure  44 , the management system  62  may be located outside of the enclosure  44  and remote from the other parts of the environment control system  10 . In addition, the processor  62  may be in the form of, or supplemented by, one or more processors, distributed processors, and/or a cloud computing environment. 
         [0041]    A first temperature sensor  70  is used to monitor the temperature of the inlet air  22  upstream from the condenser  14  (the inlet air  22  is the outside air  22  that is located outside of the HVAC enclosure  44 ). A second temperature sensor  72  is used to measure the temperature of the inlet air  30  upstream from the evaporator  16 . The heating element  46  may be located between the second temperature sensor  72  and the evaporator  16 . A third temperature sensor  74  is located at the downstream, discharge end of the evaporator  16 . The third temperature sensor  74  is used to measure the temperature of the conditioned (or heated) air  76  that is forced into a flexible duct  78  to flow into the shelter  80 . 
         [0042]    A first pressure sensor  82  is used to monitor the high-side refrigerant pressure (Pd), downstream from the compressor  18 , and a second pressure sensor  84  is provided to monitor the low-side refrigerant pressure (Ps), upstream from the compressor  18 . 
         [0043]    The intelligent management system  62  may contain (or be loaded with) information on safe operating limits of the various system components  28 ,  20 ,  36 ,  18 , and the system  62  may be configured to receive additional (input) information from internal and external sensors  72 ,  70 ,  74 ,  84 ,  82 . In operation, the management system  62  continuously adjusts one or more of the speed of the condenser fan motor  28 , the speed of the evaporator fan motor  36 , and the speed of the compressor  18 , to maintain optimum HVAC performance without exceeding the operating limits of the sensitive system components, especially the limits of the small compressor  18 . 
         [0044]    As illustrated in  FIG. 3 , following start of operations (S 10 ), the processor  64  loads component operating limits into the working memory device  66  (S 12 ). The operating limits include a compressor discharge temperature limit  100 , corresponding to the first pressure monitor  82 , a compressor return gas temperature limit  102 , corresponding to the second pressure monitor  84 , a current limit  104  for the compressor motor  40 , a compression ratio limit  106 , corresponding to a relationship between pressures measured by the first and second pressure monitors  82 ,  84 , a system safe operating pressure limit  108 , and an electronic switch temperature limit  110 . If the system  10  is determined to exceed either the system safe operating pressure limit  108  or the electronic switch temperature limit  110 , the system  10  is automatically shut down and stops operating. 
         [0045]    The processor  64  then reads one or more internal and external sensor readings from the sensors  72 ,  70 ,  74 ,  84 ,  82  (S 14 ), and then the processor  64  determines whether the sensor readings indicate that component operating limits  100 ,  102 ,  104 ,  106 ,  108 ,  110  are exceeded (S 16 ). If NO, then the processor  64  causes the system  10  to continue to run (S 18 ), and the process returns to Step S 14 . If YES (i.e., if the sensor readings indicate that one or more component operating limits  100 ,  102 ,  104 ,  106 ,  108 ,  110  are exceeded), then the processor  64  begins to process readings according to a decision tree (S 20 ), as discussed in more detail below. 
         [0046]    In operation, the management system  62  reads the speeds of the fans  26 ,  34  and the compressor  18  (S 22 ), and the processor  64  determines whether one or more of the speeds can be changed in a way that would prevent the component operating limits  100 ,  102 ,  104 ,  106 ,  108 ,  110  from being exceeded (S 24 ). If NO, the processor  64  causes the system  10  to stop on an emergency basis, and causes the display unit  60  to display an error message (S 26 ). If YES (i.e., if a determination is made that a desired change can be implemented), then the change is implemented (as discussed in more detail below) (S 28 ), and the management system  62  returns the process to Step S 14 , and the environment control system  10  continues to operate with the implemented change. 
         [0047]    The interface  60  ( FIG. 1 ) has a user accessible switch  140  that sets the maximum total operating power consumption ( 110 ,  FIG. 3 ) to one of a plurality of levels by setting one or more of the maximum current draw of the variable-speed compressor  18 , the maximum rotational speed of the variable-speed evaporator fan  28 , and the maximum pulse-width modulation (PWM) duty cycle of the resistance heater  46 . 
         [0048]    Turning now to  FIG. 4 , the intelligent management system  62  automatically limits the maximum current draw and rotational speed of various components so that the devices can be safely and continuously operated at an input power voltage that varies within a range of from not more than 20% above to not less than 30% below their nominal operating voltages. Following start of operations (S 10 ), the processor  64  loads motor current limits  142 ,  144 ,  146  into the memory device  66  (S 50 ). Then the processor  64  measures the input power voltage level (S 52 ), and then a determination is made as to whether the measured input voltage is greater than the nominal voltage (S 54 ). The measured voltage is the voltage that is applied to the respective motor  28 ,  36 ,  40 . If NO, then a determination is made as to whether the measured input voltage is less than the nominal voltage (S 56 ). If YES from Step S 54  (i.e., if a measured input voltage is greater than the nominal voltage), then the processor  64  makes a determination as to whether the maximum motor PWM duty cycle has been referenced to the measured voltage (S 58 ). 
         [0049]    If NO from Step S 56  (i.e., if the processor  64  determines that a measured input voltage is not less than the nominal voltage), then the process return to Step S 52 . If YES from Step S 56  (i.e., if the processor  64  determines that a measured input voltage is less than the nominal voltage), then the process moves to the determination at Step S 58 . If NO from Step S 58  (i.e., if the maximum motor PWM duty cycle has not been referenced to the measured voltage), then the maximum motor PWM duty cycles are reduced to maintain the present peak current limit (S 68 ), if the determination is YES at Step S 54 . On the other hand, the maximum motor PWM duty cycles are increased to maintain the present peak current limit (S 70 ), if the determinations are NO at Step S 54 , YES at Step S 56 , and NO at Step S 58 . After Steps S 68 , S 70 , the process returns to Step S 52  and the environment control system  10  continues to operate. By providing the YES path from Step S 58 , the environment control system  10  can continue to operate (S 52 ) and does not have to make any change with respect to the maximum motor PWM duty cycle (i.e., Steps S 68 , S 70  can be bypassed) in the event the maximum motor PWM duty cycle is already within a desired range relative to the measured input voltage. 
         [0050]    In the illustrated embodiment, the refrigerant gas has a low critical temperature and/or a low critical pressure, and the intelligent management system  62  adjusts one or more operational parameters, including the condenser fan speed, the evaporator fan speed, and the compressor speed, to ensure that the HVAC unit  10  operates below the critical temperature and/or pressure. In addition, the intelligent management system  62  may be configured to dynamically adjust the evaporator superheat to maintain the compressor inlet and/or discharge gas temperatures within predetermined safe operational limits. If desired, the extent to which the expansion valve  20  is open may be electronically adjustable. 
         [0051]    As illustrated in  FIG. 5 , following start of operations S 10 , the processor  64  loads refrigerant critical pressure and temperature data into the memory device  66  (S 100 ). Then the management system  62  retrieves a measured value of the high-side pressure from the first pressure monitor  82  ( FIG. 1 ) (S 102 ,  FIG. 5 ), converts the measured high-side pressure ( 82 ) to a temperature (S 104 ), and then makes a determination as to whether the system high-side pressure and/or temperature exceed the critical limits of the refrigerant (S 106 ). If NO, the process continues to Step S 102  and the environment control system  10  continues to operate (S 108 ). 
         [0052]    However, if YES from Step S 106  (i.e., if the high-side temperature does exceed the critical limit of the refrigerant), then the management system  62  reads the current speeds of the condenser fan  26 , the evaporator fan  34 , and the compressor  18  (S 110 ). Subsequently, the processor  64  makes one or more determinations, in turn, as to whether the condenser fan  26 , the evaporator fan  34 , and the compressor  18  are at their maximum, minimum and minimum speeds, respectively (S 112 , S 114 , S 116 ). If the condenser fan  26  is determined to be not at its maximum speed (NO from S 112 ), then the speed of the condenser fan  26  is increased (S 118 ), to reduce the heat of the refrigerant in the condenser  14 , and thereby reduce the high-side pressure (Pd) and/or temperature immediately downstream from the compressor  18 . 
         [0053]    If the condenser fan  26  is determined to be at its maximum speed (YES from S 112 ), and if the evaporator fan  34  is determined to be operating at greater than its minimum speed (NO from S 114 ), then the speed of the compressor  18  is decreased (S 120 ), to thereby reduce the high-side pressure (Pd) and/or temperature immediately downstream from the compressor  18 . On the other hand, if the evaporator fan  34  is determined to be operating at its minimum speed (YES from S 114 ), then the speed of the compressor  18  is decreased if possible (S 122 , via NO from S 116 ) or, if the speed of the compressor  18  cannot be further decreased (YES from S 116 ), then the HVAC unit  10  is stopped on an emergency basis and an error message is displayed on the user interface  60  (S 124 ). 
         [0054]    Another way in which the system  10  can maintain the compressor inlet ( 84 ) and the discharge ( 82 ) gas temperatures within predetermined safe operating limits is illustrated in  FIG. 6 . Following start of operations (S 10 ), the processor  64  loads compressor temperature limits into the memory device  66  (S 160 ). Then the processor  64  reads measured values of the low-side and high-side pressures from the second and first pressure monitors  84 ,  82 , respectively (S 162 ), and the processor  64  reads a measured value of the evaporator outlet temperature from the corresponding temperature sensor  74  (S 164 ). 
         [0055]    Then the processor  64  calculates the compressor discharge and inlet gas temperatures (S 166 ), and makes a determination as to whether the compressor discharge and/or inlet temperature is too high (S 168 ). If NO, the process returns to Step S 162 , and the environment control system  10  continues to operate. If YES (i.e., if the compressor discharge and/or inlet temperature is too high), then the extent to which the expansion valve  20  is open is increased (by controller  42 ) to decrease the superheat (S 170 ), the process returns to Step S 162 , and the environment control system  10  continues to operate. 
         [0056]    According to a preferred embodiment, the evaporator fan motor  36 , the condenser fan motor  28 , and the compressor motor  40  are permanent magnet-type motors. The intelligent management system  62  may be configured to restrict the maximum acceleration rate of the variable-speed motors  36 ,  28 ,  40  to prevent respective current draws above predetermined maximum safe levels. In addition, the maximum deceleration rate of the permanent magnet motors  36 ,  28 ,  40  may be restricted to prevent the buildup of excessive voltage in the environment control system  10 . 
         [0057]    Moreover, the intelligent management system  62  may be configured to restrict the maximum acceleration rate of the compressor motor  40  to prevent temporary loss of lubrication within the compressor  18 . For example, as part of a compressor startup sequence, the compressor motor  40  may be held at a predetermined speed for a predetermined period of time to ensure that the compressor  18  has proper lubrication. After an initial period of time, during which pressure equalization occurs, the speed of the compressor motor  40  may be linearly increased and then held at a constant predetermined speed for a period of time sufficient to avoid loss of lubrication in the compressor, after which the compressor  18  may be operated at target speeds according to performance requirements, as determined by the intelligent management system  62 . 
         [0058]    In the illustrated embodiment, air  98  ( FIG. 1 ) returning from the portable shelter  80  ( FIG. 2 ) is recycled through the evaporator  16  ( FIG. 1 ) by way of a suction conduit  100 . The environment control system  10  may have an electronically controllable fresh air vent  102  for mixing fresh air  22  into the air  98  that is recycled from the shelter  80  through the suction conduit  100 . A motorized controller  104  is used to move the vent  102  to a fully closed position, a fully open position, and intermediate positions between the fully open and fully closed positions. When the vent  102  is in the fully closed position, essentially no fresh air  22  is mixed with the recycled air  98 . When the vent  102  is in the fully open position, a large amount of fresh air  22  is mixed into the recycled air  98  to flow through or over the evaporator  16 . When the vent  102  is in one of the intermediate positions, some fresh air  22  flows through the evaporator  16 , though not as much as when the vent  102  is in the fully open position. 
         [0059]    An air quality sensor  110  may be located within the air stream  76  circulated by the evaporator fan  34 . The sensor  110  can monitor the circulating air  76  to determine the presence and/or concentration level of one or more contaminants, including, among other things, the concentration of carbon dioxide. 
         [0060]    In the illustrated embodiment, the environment control system  10  operates in two modes, that is, a first, adjustable vent mode and a second, closed vent mode. The first mode is an active mode in which the position of the fresh air vent  102  (i.e., the extent to which the vent  102  is open) is adjusted by the management system  62  to ensure that the level of contaminants within the circulating air  76 , as measured by the air quality sensor  110 , does not exceed a first predefined maximum. The second mode is an inactive mode in which the vent  102  remains fully closed regardless of the level of contamination measured by the air quality sensor  110 . 
         [0061]    Whether the environment control system  110  is operated in the first or second mode may be user selectable (via the user interface  60 ). The first mode may be used, for example, when the shelter  80  is occupied by humans, especially patients, who may be especially sensitive to the level of contamination. The second mode may be used when the shelter  80  is first set up and not yet occupied, or when the shelter  80  is used to store inanimate objects, and not persons or animals that are sensitive to the level of contamination in the interior air  76 . 
         [0062]    Further, the illustrated system  10  may be operated in a third mode, wherein, when the user selectable mode of fresh air vent operation is active, the rotational speed of the compressor  18  is monitored by the intelligent management system  62  and, upon the compressor speed exceeding a predefined threshold, the predefined maximum level of contamination within the circulating air  76  is increased from the first level to a second, higher predefined maximum level. The third mode may be especially useful where the first maximum level of concentration is preferred for comfort or other factors but the second maximum level of contamination may be acceptable and/or tolerated by the occupants or items stored in the shelter  80  under certain conditions, including conditions that would otherwise require operation of the compressor  18  outside the safe operating limits of the compressor  18 . 
         [0063]    The illustrated environment control system  10  has a relative humidity sensor  112  located in the air stream  30  ahead of (upstream from) the evaporator  16 . The processor  64  may respond to signals ( 145 ) from the relative humidity sensor  112  to adjust the speed of the compressor  18  to thereby increase or decrease the amount of dehumidification of the circulating air  76  by modifying the target evaporator temperature. 
         [0064]    As illustrated in  FIG. 7 , following start of operations (S 10 ), the processor  64  loads a predetermined relative humidity level into the memory device  66 , and then the processor  64  reads the relative humidity level of the return air  98  from the corresponding sensor  112  (S 202 ). The processor  64  then determines whether the measured humidity level is greater than the desired level (S 204 ). If NO, the processor  64  determines whether the measured humidity level is less than the desired level (S 206 ). If Yes from Step S 204  (i.e., if it is determined that the measured humidity level is greater than the desired level), then the speed of the compressor  18  is increased (by applying a suitable signal to the compressor motor  40 ) to maintain a lower target evaporator temperature (S 208 ), then the system  10  waits a predetermined amount of time (S 210 ), and then the system  10  measures the relative humidity level of the return air  98  again, using the sensor  112  (S 212 ). 
         [0065]    The processor  62  then determines whether the measured humidity level has fallen since the last reading (S 214 ). If NO, the humidity-related effect of increasing the compressor speed may be understood to have been stabilized, and the process return to Step S 202 , and the environment control system  10  continues to operate. If Yes from Step S 214 , the system  10  waits again (S 210 ) and then takes another measurement of relative humidity (S 212 ) to determine whether the humidity level within the returning air  98  has stabilized (S 214 ). 
         [0066]    If YES from Step S 206  (i.e., if the measured humidity level is determined to be less than the desired level), then the speed of the compressor  18  is decreased to maintain a higher target evaporator temperature (S 216 ), and then the system  10  iteratively (YES from S 222 ) waits (S 218 ) and then measures the relative humidity (S 220 ) again, until the humidity-related effect of decreasing the compressor speed stabilizes (NO from S 222 ), at which point the process returns to Step S 202 , and the environment control system  10  continues to operate. 
         [0067]    As illustrated in  FIG. 2 , the environmental control system  10  may be in the form of a HVAC unit that has a first opening  200  through the enclosure  44  for receiving the discharge duct  78  and a second opening  204  for receiving the suction conduit  100 . The other ends of the discharge duct  78  and the suction conduit  100  are attached to respective openings in the fabric shelter  80 . The exterior enclosure  44  for the HVAC unit  10  may have sufficient rigidity to safely and securely support the weight of at least three like HVAC units  10  stacked vertically on top of each other. Each unit  10  may be provided with pockets  202  for accepting the forks of an industrial lift truck, and one or more concave and/or convex pockets may be located at the top and the bottom to aid in centering the stacked units  10  for storage or transport. If desired, a weatherproof cover  206  may be used to cover the user interface  60  ( FIG. 1 ). 
         [0068]    The duct  78  ( FIG. 8 ) for discharging the conditioned air  76  into the shelter  80  may be inflated by the pressure of the discharged air  76  (and by no other source of pressure or inflation). The duct  78  has an outer wall  210  formed of a flexible, lightweight and gas impermeable material, and a flexible inner wall  212 . The inner wall  212  has limited porosity to permit air  76  inside the duct  78  to pass into the annulus  214  between the inner and outer walls  212 ,  210 . Baffles  216  are located within the annulus  214  to prevent the air that has entered the annulus  214  (by passing through the inner layer  212 ) from actively circulating within the annulus  214 . As a result, the air within the annulus  214  becomes stagnant and thereby provides thermal insulation between the air  76  flowing inside the duct  78  and the external environment  22 . 
         [0069]    According to another aspect of the present disclosure, the heating, ventilating and air conditioning system  10  may be configured to operate in a standard mode and a protective mode. The standard mode may be selected (via the interface  60 ) where external (environmental) conditions are expected to be such that the maximum operating current, temperature and/or pressure limits of any one or more of the system components  28 ,  36 ,  110 ,  84 ,  40 ,  82  will likely not be exceeded. The processor  64  may be configured to automatically switch the system  10  to the protective mode of operation where the external conditions are such that one or more of the system components likely would exceed its design specification if the system  10  remained in the standard mode. 
         [0070]    Moreover, the system  10  may be configured to switch from standard operating mode to protective operating mode based on the temperature of the condenser inlet air  22 , the evaporator inlet air  30 , or both. Moreover, the system  10  may be configured to switch from the standard operating mode to the protective operating mode based on the one or more of the system low-side pressure (Ps,  84 ), the system high-side pressure (Pd,  82 ), the compressor current draw, the compressor rotational speed, the evaporator fan rotational speed, and the condenser fan rotational speed, as communicated through the corresponding signal lines, receivers or transmitters  145 . 
         [0071]    Thus, the intelligent management system  62  may reduce the speed of the compressor  18  when operating in the protective mode to a level below the speed that would otherwise be set if operating in standard mode. Likewise, the speed of the condenser fan  26  may be increased in the protective mode to a level above the speed that would otherwise be set for it if the system  10  were operating in standard mode, and the speed of the evaporator fan  36  may be set in the protective mode to a level below the speed to which it would be set in the standard mode. 
         [0072]    According to a preferred embodiment, the heating, ventilating and air conditioning system  10 , employing the intelligent management system  62 , sets the compressor rotational speed at the speed necessary to achieve a predefined target evaporation temperature when operating in the standard mode. The illustrated system  62  is likewise configured to measure the power consumption of the compressor  18  and the power consumption of the condenser fan motor  28 , and to adjust the speed of the condenser fan  26  to provide the lowest total power consumption, when operating in the standard mode. 
         [0073]    If desired, the interface  60  may include a permanently affixed sunlight-readable user interface panel, for controlling the system  10  and for displaying operational data. According to another embodiment, the interface  60  may include a detachable device that allows a user/operator to fully or partially control the system  10  even while the user/operator is located more than five feet away from the system  10 . A dual-function data transfer port (not shown) may be employed to allow a computer to transfer data to and from the remote user interface for operator-system control across a distance of more than five feet. This, way, the environment control system  10  may be operated by a person located within the fabric shelter  80 . 
         [0074]    The invention is not limited to the structures, methods and instrumentalities described above and shown in the drawings. The invention is defined by the claims set forth below.