Abstract:
A method for supplying a vaporized gas, including: (a) providing a system having: (i) a storage tank for storing liquefied gas; (ii) a heat exchanger external to the tank, disposed in a heat-exchange relationship with a wall of the tank; (iii) a device for delivering a heat-exchange liquid from a reservoir to the heat exchanger, and (iv) a delivery line for directly transferring the vaporized gas from the heat exchanger to the consumer; (b) supplying the vaporized gas directly to the consumer, according to consumer demand; (c) upon vaporization of a portion of the liquefied gas within the storage tank, and subsequent cooling of the liquefied gas within the storage tank, transferring heat from the heat-exchange liquid to the tank via the heat exchanger, so as to increase the tank pressure.

Description:
[0001]    This is a continuation-in-part (CIP) of U.S. patent application Ser. No. 09/674,700, filed Nov. 6, 2000, now U.S. Pat. No. 6,470,690, and U.S. patent application Ser. No. 10/108,450, filed Mar. 29, 2002. 
     
    
     
       FIELD AND BACKGROUND OF THE INVENTION  
         [0002]    The present invention relates to a method and a system for supplying pressurized gas from a liquefied-gas (LG) storage tank.  
           [0003]    LG systems are widely used in residential, agricultural, and industrial settings and they are expected to be a reliable source of energy, to operate safely, continuously, and to constantly supply guaranteed output. One critical performance criterion of LG systems is the delivery of a constant, stable and reliable flow of vaporized gas to the burners.  
           [0004]    Some commonly used systems and methods of vaporization employ over-capacity storage tanks and vaporizers. The over-capacity storage tanks are expensive and provide inconsistent vapor pressure, especially during extreme ambient conditions. They also waste space, call for surplus gas stock and unnecessarily large and expensive storage area.  
           [0005]    In one known system, the heat of vaporization is supplied by convection with respect to the ambient heat. However, this requires large heat-convection surfaces, according to the demand for the vaporized gas. Moreover, such systems are incapable of delivering gas at pressures exceeding that of the tank pressure.  
           [0006]    In another known system, an external vaporizer is used to heat and vaporize the liquefied gas, with the vaporized gas being recirculated to the storage tank. The vaporized gas supplied to the consumer is delivered via a separate line connected to the storage tank, according to consumer demand. The recirculation of vaporized gas requires a large installation, and correspondingly high investment cost and maintenance expenses. Alternatively, the vaporized gas can be delivered directly to the consumer. However, various mechanical and control-related failures, as well as reduced consumer demand, can cause liquid-phase liquefied gas to be introduced to or to be condensed in the consumer delivery line. Liquid-phase liquefied gas in the vaporized gas is a major problem for many consumers. Consequently, such a process scheme generally requires the addition of vapor/liquid separation equipment for providing the consumer with solely vaporized gas. This equipment increases system complexity, size and cost, and introduces additional reliability and safety problems.  
           [0007]    In the parent application, now U.S. Pat. No. 6,470,690, which is hereby incorporated for all purposes as if fully set forth herein, a system and method are disclosed in which the liquefied gas is circulated through an external heat exchanger and returned to the storage tank as a heated liquid. The sensible heat of the heated liquefied gas provides all of the requisite heat for vaporizing the gas (or at least a substantial portion thereof), which occurs within the storage tank. The vaporized gas is delivered via a separate vapor line connected to the storage tank, according to consumer demand.  
           [0008]    The above-described system has numerous advantages with respect to other known systems. One important advantage is that the system enables major electrical components to be located remotely from the storage tank. This safety advantage is of particular importance due to the flammable nature of the gases disposed within and delivered from the storage tank.  
           [0009]    In some applications, however, it is impractical to install an external heat exchanger through which liquefied gas is pumped.  
           [0010]    It would, therefore, be highly advantageous to have a method and a system for supplying vaporized gas on consumer demand without a circulation of liquefied gas external to the storage tank, and without the requisite equipment therefor. It would be of further advantage if the system would allow for improved fire safety with respect to prior-art systems, by enabling all electrical components to be located remotely from the storage tank. It would be of yet further advantage if such a system would be simple and energy-efficient.  
         SUMMARY OF THE INVENTION  
         [0011]    The present invention is a method and a system for supplying pressurized gas from a liquefied-gas (LG) storage tank.  
           [0012]    According to one aspect of the invention, the method includes the steps of: (a) providing a system including: (i) a storage tank for storing liquefied gas, the tank having a lower liquid region and a vapor region thereover; (ii) a heat exchanger external to the storage tank, disposed in a heat-exchange relationship with a wall of the storage tank; (iii) a device for delivering a heat-exchange liquid from a reservoir to the heat exchanger, and (iv) a, delivery line for directly transferring the combustible vaporized gas from the heat exchanger to the consumer; (b) supplying the vaporized gas directly to the consumer, according to consumer demand; (c) upon vaporization of a portion of the liquefied gas within the storage tank, and subsequent cooling of the liquefied gas within the storage tank, transferring heat from the heat-exchange liquid to the storage tank by means of the heat exchanger, so as to increase the pressure within the storage tank.  
           [0013]    According to further features in the described preferred embodiments, the method further includes the step of (d) heating the reservoir to supply the heat-exchange liquid with the requisite heat for transferring to the storage tank.  
           [0014]    According to still further features in the described preferred embodiments, the method further includes the step of: (e) circulating the heat-exchange liquid between the reservoir and the heat exchanger.  
           [0015]    According to still further features in the described preferred embodiments, the system further includes a first line for delivering the heat-exchange liquid to the heat exchanger, and a second line for returning the heat-exchange liquid from the heat exchanger to the reservoir.  
           [0016]    According to still further features in the described preferred embodiments, the method further includes the step of: (f) measuring a temperature of the heat-exchange liquid in&#39;the second line, to obtain a measured temperature.  
           [0017]    According to still further features in the described preferred embodiments, the method further includes the step of: (g) heating the reservoir based on the measured temperature of the heat-exchange liquid in the second line.  
           [0018]    According to still further features in the described preferred embodiments, the method further includes the step of: (g) controlling the device for delivering the heat-exchange liquid based on the measured temperature of the heat-exchange liquid in the second line.  
           [0019]    According to still further features in the described preferred embodiments, the method further includes the step of: (f) measuring a pressure within the delivery line to obtain a vapor pressure measurement.  
           [0020]    According to still further features in the described preferred embodiments, the method further includes the step of: (g) controlling the transfer of heat from the heat-exchange liquid to the storage tank based on the vapor pressure measurement.  
           [0021]    According to another aspect of the present invention there is provided a gas supply system for supplying a combustible vaporized gas to a consumer, according to consumer demand, the gas supply system including: (a) a storage tank for storing liquefied gas, having a lower liquid region and a vapor region thereover; (b) a heating system for providing heat to the storage tank, including: (i) a heat exchanger external to the storage tank, the heat exchanger disposed in a heat-exchange relationship with a wall of the storage tank; (ii) a reservoir for storing a heat-exchange liquid for the heat exchanger; (iii) a first line connecting the reservoir and the heat exchanger, and a second line for returning the heat-exchange liquid from the heat exchanger to the reservoir, and (iv) a pumping device for delivering the heat-exchange liquid from the reservoir to the heat exchanger, via the first line; (c) a delivery line for transferring the vaporized gas from the storage tank to the consumer, and (d) a control system associated with the heating system and configured to control a heat supply to the storage tank, by means of the heat-exchange liquid, such that the storage tank produces the vaporized gas at sufficient pressure so as to meet the consumer demand.  
           [0022]    According to further features in the described preferred embodiments, the control system includes a temperature sensor for measuring a temperature of the heat-exchange liquid returning to the reservoir.  
           [0023]    According to still further features in the described preferred embodiments, the heating system includes a heating element for heating the reservoir.  
           [0024]    According to still further features in the described preferred embodiments, the control system effects the control by operating the heating element based on the temperature measured by the sensor.  
           [0025]    According to still further features in the described preferred embodiments, all electrical components of the gas supply system are disposed remotely to the storage tank. Preferably, the electrical components should be at least 5 meters from the gas supply system, more preferably, 10-25 meters, and most preferably, more than 25 meters from the gas supply system.  
           [0026]    According to still further features in the described preferred embodiments, the delivery line has a pressure sensor for measuring a vapor pressure of the vaporized gas in the delivery line.  
           [0027]    According to still further features in the described preferred embodiments, the control system controls the heating system based on the vapor pressure measured by the pressure sensor.  
           [0028]    According to still further features in the described preferred embodiments, if the vapor pressure measured by the sensor exceeds a pre-determined value, the control system reduces or completely curtails the heat supply to the storage tank.  
           [0029]    According to still further features in the described preferred embodiments, if the vapor pressure measured by the sensor exceeds a pre-determined value, the control system controls the pumping device so as to reduce or completely curtail the heat supply to the storage tank.  
           [0030]    According to still further features in the described preferred embodiments, the heat exchanger includes a flexible heat transfer plate for disposing against an external wall of the storage tank.  
           [0031]    According to still further features in the described preferred embodiments, the heat exchanger further includes a pipe containing the heat-exchange liquid, the pipe being fluidly connected to the first line, and disposed in heat-exchange relation to the flexible heat transfer plate.  
           [0032]    According to still further features in the described preferred embodiments, the heat transfer plate is a corrugated heat transfer plate.  
           [0033]    According to still further features in the described preferred embodiments, the heat-exchange liquid includes water. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0034]    The invention is herein described, by way of example only, with reference to the accompanying drawings. With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice.  
         [0035]    In the drawings:  
         [0036]    [0036]FIG. 1 is a block diagram illustrating a system for supplying vaporized gas according to the parent application, U.S. Pat. No. 6,470,690, and  
         [0037]    [0037]FIG. 2 is a schematic diagram illustrating one embodiment of a system for supplying vaporized gas according to the present invention;  
         [0038]    [0038]FIG. 3 a  is a top, schematic, partially cut-away view of a heating blanket for use in conjunction with the system of the present invention, and  
         [0039]    [0039]FIG. 3 b  is a side view of the heating blanket of FIG. 3 a.   
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0040]    The principles and operation of the system in the invention according to the present invention may be better understood with reference to the drawings and the accompanying description.  
         [0041]    Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawing. The invention is capable of other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting.  
         [0042]    As used herein in the specification and in the claims section that follows, the term “liquid” refers to a non-explosive and, preferably, a non-inflammable liquid.  
         [0043]    Referring now to the drawings, FIG. 1 is a block diagram illustrating a vaporization system  10  for vaporizing gas according to the parent application, U.S. patent application Ser. No. 09/674,700, now U.S. Pat. No. 6,470,690. A storage tank  20  contains gas, such as propane or butane, in liquefied form, in a lower region  22 . Upper region  23 , which is in fluid contact with lower region  22 , contains gas in vapor form. Liquefied gas from lower region  22  is circulated, preferably by a water-driven turbine pump  40 , through a heat exchanger  50 . The liquefied gas is heated in heat exchanger  50 , the heating being controlled such that the liquefied gas is returned in liquid form to storage tank  20 . The sensible heat of the heated liquefied gas provides all of the requisite heat for vaporizing the gas, or at least a substantial portion thereof. The vaporized gas, produced in storage tank  20 , is delivered to a consumer  100 , according to demand, via a separate vapor line  70  connected to storage tank  20 .  
         [0044]    Heat exchanger  50  is a surface heat exchanger typically using water as the heat exchange liquid. The water, which is heated and pressurized externally to vaporization system  10 , is introduced to heat exchanger  50  via line  36 . The water exiting heat exchanger  50  can also serve to drive water-driven turbine pump  40 , before leaving system  10  via line  32 .  
         [0045]    The above-described system has numerous advantages with respect to other known systems. One important advantage is that the system enables major electrical components to be located remotely from the storage tank. This safety advantage is of particular importance due to the flammable nature of the gases disposed within and delivered from the storage tank.  
         [0046]    In some applications, however, it is impractical to install an external heat exchanger through which liquefied gas is pumped. Hence, it would be advantageous to have a system for supplying vaporized gas on consumer demand without a circulation of liquefied gas external to the storage tank, and without the requisite equipment therefor. It would be of further advantage if the system would allow for improved fire safety with respect to prior-art systems, by enabling all electrical components, including electrical actuators and indicators, to be located remotely from the storage tank.  
         [0047]    [0047]FIG. 2 illustrates a storage tank  2  that is heated by a heating system  100  of the present invention, which includes a remote water heater  51 , a heating blanket  50 , a circulation pump  52 , temperature indicator  55 , and optionally, CPU  60 . Heating blanket  50 , which is directly applied to the external surface of the storage tank, is heated by hot water circulated to heating blanket  50  via a circulation pump  52 . A remote water heater  51 , typically gas-fired or electric, heats the circulated water to heating blanket  50 .  
         [0048]    Heating blanket  50  may be disposed substantially anywhere on the body of storage tank  2 . However, it is generally preferable to position heating blanket  50  towards the bottom of storage tank  2 , well below the interface between liquid region  2   a  and vapor region  2   b  within storage tank  2 .  
         [0049]    Heating system  100  is designed to provide heat to storage tank  2 , so as to vaporize the requisite amount of fuel upon consumer demand. Remote water heater  51  supplies heat to the liquefied gas in storage tank  2 , along with the influx of ambient heat to boost the internal heat of the liquefied gas within, the storage tank, so as to provide the heat of vaporization for vaporizing, within the storage tank, the liquefied gas, according to the consumer demand therefor.  
         [0050]    In simplest form, heating system  100  is controlled as follows: the temperature within remote water heater  51  is controlled by thermostat  58 , which is electrically connected to remote water heater  51 . Temperature indicator  55  is preferably disposed in the flow of water returning from heating blanket  50  to remote water heater S 1 . As long as the temperature measured by temperature indicator  55  is below a predetermined value assigned to thermostat  58 , remote water heater  51  is heated by heating element  54 , such that the water pumped from water heater  51  to heating blanket  50  is relatively hot, enabling substantial heat transfer, via heating blanket  50 , to storage tank  2 . When the temperature measured by temperature indicator  55  reaches a predetermined value, the heating of remote water heater  51  by heating element  54  is curtailed, such that the transfer of heat to storage tank  2  is gradually reduced, and the temperature of the water within heating blanket  50  approaches the temperature of the liquefied gas within storage tank  2 .  
         [0051]    When consumer demand resumes or increases, vaporized gas is discharged from storage tank  2  via consumer gas line  4 . Liquefied gas within liquid region  2   a  vaporizes, with the heat of vaporization being drawn from the liquefied gas. Consequently, the temperature within storage tank  2  is decreased. The increased AT between heating blanket  50  and storage tank  2  results in an increased transfer of heat from heating blanket  50  to storage tank  2 . The temperature in the return flow decreases, such that the temperature measured by temperature indicator  55  falls below the pre-determined value assigned to thermostat  58 , and heating element  54  is activated to heat the water delivered to heating blanket  50 .  
         [0052]    While the activation of thermostat  58  has been described as a simple on-off control, it will be apparent to one skilled in the art that various known programs for activating thermostat  58  could be used. In addition, in some applications, particularly those in which remote water heater  51  is maintained at a relatively constant temperature, the rate of heat delivered by heating blanket  50  to storage tank  2  can be controlled by controlling the flowrate provided by circulation pump  52 .  
         [0053]    In many of the prior-art systems for heating gas storage tanks, an external evaporator vaporizes a stream of liquefied gas as long as consumer demand is maintained. This type of system is extremely inefficient from an energy standpoint, because the energy-intensive evaporation is linked to consumer demand, and not to the intrinsic thermodynamic properties of the gas in the system. In a preferred embodiment of the present invention, a pressure indicator or switch  62 , disposed on consumer gas line  4 , is operatively connected to heating system  100 . When the pressure measured by pressure switch  62  is above a pre-determined value, pressure switch  62  deactivates heating system  100 , because the pressure of the vaporized gas supplied to the consumers via consumer gas line  4  is sufficiently high. As demand continues, the pressure in consumer gas line  4  is reduced, until the pressure measured by pressure switch  62  drops below a pre-determined value, at which point pressure switch  62  activates heating system  100 , which then provides heat to storage tank  2  as described hereinabove.  
         [0054]    Pressure switch  62  may directly or indirectly activate and deactivate circulation pump  52 . Preferably, heating system  100  is configured such that the deactivation of circulation pump  52  deactivates, in turn, heating element  54 . Similarly, when circulation pump  52  is activated, the heating of remote water heater  51  by heating element  54  is enabled.  
         [0055]    In another embodiment of the present invention, CPU  60  receives a data signal from temperature indicator  55 , and is pre-programmed to control heating element  54  (and/or circulation pump  52 ) based upon the value received. In addition, CPU  60  may be configured to receive a data signal from pressure indicator or switch  62 , and to activate or deactivate heating element  54  (and/or circulation pump  52 ) based upon the data signal received.  
         [0056]    It will be evident to one skilled in the art that many other control relationships between pressure switch  62  and heating system  100  are possible. It must be emphasized, however, that the control of heating system  100  is by no means trivial. The design criteria include:  
         [0057]    Minimization of safety risks associated with electrically-powered components in the vicinity of a system containing inflammable or explosive gases  
         [0058]    Quick response to consumer demand  
         [0059]    Ability to cope with periods in which there is no consumer demand in a safe and energy-efficient manner  
         [0060]    Substantially 100% reliability  
         [0061]    The system illustrated in FIG. 2 is particularly useful in underground storage tanks, and involves a minimal modification to the existing equipment, both in terms of equipment and in terms of ease of installation.  
         [0062]    It must be emphasized that various designs and configurations for achieving the requisite heat-exchange relation will be apparent to those skilled in the art.  
         [0063]    [0063]FIG. 3 a  is a top, schematic, partially cut-away view of heating blanket  50 , which includes an inlet tube  72  for supplying hot water for heating the storage tank, an outlet tube  74  for returning water to the reservoir, a serpentine tube  76  connecting between inlet tube  72  and outlet tube  74 , and a flexible, conductive mat or plate  78 . The difference between the temperature of the water flowing through serpentine tube  76  and the temperature of the liquefied gas in the storage tank provides the driving force for the transfer of heat from the water, through the wall of serpentine tube  76 , via conductive plate  78 , to the wall of the storage tank, and from there, to the liquefied gas contained therein. Conductive plate  78  is typically made of aluminum, and preferably has a corrugated surface  79  to provide for an increased surface area for heat transfer. The flexibility of conductive plate  78  allows for a good fit with the external wall of the storage tank, such that efficient heat transfer is achieved.  
         [0064]    A side view of heating blanket  50  of FIG. 3 a  is provided in FIG. 3 b . It should be appreciated that the thin, compact heating blanket  50  described herein is advantageously installed in new gas storage systems, and is also advantageously retrofitted in existing gas storage systems.  
         [0065]    Although the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims. All publications, patents and patent applications mentioned in this specification are herein incorporated in their entirety by reference into the specification, to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated herein by reference. In addition, no citation or identification of any reference in this application shall be construed as an admission that such reference is available as prior art to the present invention.