Abstract:
A steam-raising system comprising a boiler having (a) a passageway for water and/or steam, (b) an inlet to the passageway through which water is introduced continuously for given periods when the boiler is in use, (c) a burner to heat the passageway from the outside thereof, and (d) an outlet from the passageway from which steam emerges continuously as water is introduced through the inlet. The system further comprises a pump connected by a flow-path to the said inlet to pump water thereto along the said flow-path. The pump is a variable-flow-rate pump constructed to provide a flow-rate through it which is substantially independent of its downstream pressure.

Description:
TECHNICAL FIELD 
     The present invention relates to a steam-raising system comprising a boiler having (a) a passageway for water and/or steam, (b) an inlet to the passageway through which water is introduced continuously for given periods when the boiler is in use, (c) a burner to heat the passageway from the outside thereof, and (d) an outlet from the passageway from which steam emerges continuously as water is introduced through the inlet, the system further comprising a pump connected by a flow-path to the said inlet to pump water thereto along the said flow-path. 
     BACKGROUND ART 
     EP-A-727,609 describes such a system with flow-control means comprising a plurality of lines which constitute a part of the flow-path, which are connected between the pump and boiler in parallel with one another, and which are independently openable to enable the amount of water delivered to the boiler to be varied, each line having a shut-off valve and a flow regulator which maintains a constant flow through it substantially independently of the pressures upstream of its inlet and downstream of its outlet. 
     A disadvantage of this previously proposed system is the relatively crude control obtained by switching in or out one or more of the lines, and the relatively high expense involved in having a shut-off valve and a further regulator for each line. 
     The present invention seeks to obviate this disadvantage. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to a steam raising system as set out in the opening paragraph of the present specification, in which the pump is a variable-flow-rate pump constructed to provide a flow-rate through it which is substantially independent of its downstream pressure. 
     Advantageously, heated water is fed to the pump, preferably via a further pump upstream of the first-mentioned pump to create an upstream pressure sufficient to avoid cavitation therein. 
     Conveniently, the water fed to the first-mentioned pump is heated by means of some of the steam which emerges from the boiler. 
     The first-mentioned pump may be an electrically operated pump, preferably operated by an alternating electrical current, advantageously constructed so that the flow-rate is dependent upon the frequency of the alternating current. In that case control means may be provided to vary the frequency in dependence upon the required flow-rate of water to the boiler. 
     The first-mentioned pump may comprise a plurality of piston and cylinder arrangements, each connected to draw in water from a water source during a first stroke and then to pump it through an outlet of the pump during a second stroke, the arrangements being in suitably different phases relative to one another to obtain a continuous supply of water from the outlet. 
     The pistons may be moved by means of a rotary surface which slants relative to an axis of the pump to which at least components of the longitudinal extent of the cylinders of the piston and cylinder arrangements are substantially parallel. 
     Temperature monitoring means may be provided downstream of the boiler to measure the temperature of the steam output of the boiler. 
     The temperature monitoring means may be connected to a control unit of the system which adjusts the flow-rate of the first-mentioned pump in dependence upon the measured temperature of the steam output of the boiler. 
     The control unit may thereby maintain a given flow-rate or a given head of steam output. 
     The present invention extends to a method of raising steam by a system in accordance with the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An example of a steam-raising system in accordance with the present invention will now be described with reference to the accompanying drawings, in which 
     FIG. 1 shows a circuit diagram of the system; and 
     FIG. 2 shows an axial-sectional view through a pump of the system shown in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The system shown in FIG. 1 comprises a boiler feed tank  10  having an outlet  12  which is connected to the input of a pump  14  via a passageway  16 . The output  18  from the pump  14  is connected to a feed line  22 . 
     The feed line  22  feeds water under pressure to a frequency-controlled alternating-current electrically operated variable flow-rate pump  32  which is constructed in a manner to be described with reference to FIG. 2 so that it provides a flow-rate of water through it which is substantially independent of its downstream pressure, and also independent of its upstream pressure. 
     The pump  32  feeds water to a water inlet  40  of a boiler  42  via a flowmeter  82 . A steam outlet  44  from the boiler  42  is connected to a header  46  via a steam passageway  48 . The header  46  is hollow and is generally T-shaped with the T on its side so that the part of the header corresponding to what is normally upright in the letter T is horizontal. The passageway  48  is connected at its end further from the steam outlet  44  to the base of the T of the header  46 . A pressure transducer  50  and a pressure switch  52  are connected to the header  46  so as to be exposed to the pressure therewithin. The header  46  is also provided with a pressure safety valve  54  above the main steam outlet  56  of the header  46 , and a header drain valve  58 . A temperature sensor  60  is also provided on the passageway  48  as monitoring means to provide a measurement of the temperature of the steam from the boiler  42 . 
     A further pressure safety cut-out switch  62  is connected to the feed line  22  so as to be exposed to the pressure thereof. 
     Outputs from the pressure transducer  50  and the switches  52  and  62  along with the output from the temperature sensor  60  are all electrically connected to respective inputs of a control unit  84 . Outputs therefrom are respectively connected to the pump  14  and, via an inverter  85 , to the pump  32 . 
     The control unit  84  also has an input connected to the flowmeter  82 , and an output connected to a burner shut-off  83 . If the flowrate of water as measured by the flowmeter  82  falls below a predetermined limit, for example 1 litre/min, the control unit will shut off the burner. 
     The boiler  42  comprises a multi-helical tubular conduit  86  that meanders within the interior of the boiler  42 . A burner  88  of the boiler  42  directs a flame within the helices of the tubular conduit  86  to heat up the water and/or steam therewithin. The boiler  42  is also provided with a flue (not shown) for the escape of the combustion gases from the burner  88 . 
     Whilst many constructions are possible for the pump  32 , FIG. 2 shows one possible construction. It comprises a cylinder block  100  formed with two cylinders  102  and  104  within which respective hollow pistons  106  and  108  are slidable axially. An inlet  110  into the pump  32  communicates with the gallery  112  which in turn communicates with the interiors of the cylinders  102  and  104  via valves  114  and  116  respectively. The pistons  106  and  108  are connected at their ends further from the valves  114  and  116  to respective sliders  118  and  120  via respective ball-and-socket connections  122  and  124 . The latter have planar faces on their sides further from the connections which rest against a slanting planar surface  125  of a cam disc  126  such as to be in sliding contact therewith. The latter is coupled to be rotated about an axis of the pump  32  by an alternating-current electrically driven frequency-controlled hollow motor  128 . An axially extending outlet passageway  130  is in communication at its inside end with the cylinder interiors via the valves  114  and  116 . 
     As the motor  128  rotates the cam disc  126 , the sliders slide over the slanting surface  125 . The pressure of water in the gallery  112  urges the piston  108  away from the gallery end of the cylinder  104  as the slider  120  moves to the part of the slanting surface  125  at the thinnest part of the cam disc  126 . As the cam disc  126  is rotated further, the piston  108  is pushed towards the gallery  112  as the slider  120  climbs the slanting surface  125 . Water is thus urged at a given flow-rate out from the cylinder  104  into the passageway  130  via the valve  116  which has now closed the communication between the gallery  112  and the interior of the cylinder  104  and opened the communication between that interior and the passageway  130 . In the meantime, water is entering the interior of the cylinder  102  via the valve  114 . 
     As the pistons  106  and  108  continue their work in this way, water is urged out from the axially extending passageway  130  at a rate determined by the rotary speed of the motor  128 , substantially independently of the back pressure in the passageway  130 . The flow-rate through the passageway  130  is directly proportional to the rotary speed of the motor  128 . 
     A small amount of the steam from the heater  46  is routed via a passageway  200  with a control valve  202  to the feed tank  10  to heat the water therein to the extent that it will inhibit development of micro-organisms in the tank. The control valve  202  is connected for control by the control unit  84  in dependence upon the temperature of the water in the tank  10  as measured by a temperature sensor  204  therein connected to deliver its output signals to the control unit  84 . The control unit  84  ensures that the valve  202  can only open with a sufficient pressure in the header  46  as indicated by the pressure transducer  50 . 
     When the system is in operation, the pump  14  feeds water from the boiler feed tank  10  to the boiler  42  via the passageways  16  and  22 , the pump  32 , and the passageway  38 . The pressure developed by the pump  14  is sufficient to inhibit cavitation occurring in pump  32 . With the burner  88  switched on, the boiler  42  heats the water which passes through a helical conduit  86  so that the water becomes superheated steam by the time it exits the outlet  44  from the boiler  42 . This superheated steam is then available at the outlet  56  from the header  46  to which the steam is fed from the boiler  42  via the passageway  48 . 
     The pump  32  maintains a constant flow at a rate which, for the given specification of the burner  88 , produces superheated steam at the outlet  44  and consequently in the passageway  48  and at the header  46 . 
     Once the temperature as indicated by the temperature sensor  60  exceeds the boiling point of water at the pressure as indicated by the pressure transducer  50  by more than 5° Centigrade, the control unit  84  increases the frequency of the alternating current supplied to the pump  32  by adjusting the inverter  85  to cause an additional amount of water to be fed to the boiler  42  via the flowmeter  82  and the passageway  38  at a rate determined by the increased speed of the pump  32 . In the event that the temperature of the steam exceeds the boiling point of water for the pressure as indicated by the pressure transducer  50  by more than 10° Centigrade, the control unit  84  further increases the frequency of the alternating current supplied to the pump  32  thus causing a further amount of water to flow into the passageway  38  via the flowmeter  82 , at a flow-rate again determined by the increased speed of the pump  32 . 
     In the event that the temperature of the superheated steam at the temperature sensor  60  falls more than 1° Centigrade below the threshold temperatures for increased flow, the speed of the pump  32  is correspondingly reduced. 
     Such control enables superheated steam to be provided by the boiler at a given temperature, with a substantially constant pressure. 
     In the event that the pressure exceeds the predetermined pressure as indicated by the pressure transducer  50 , by a predetermined amount, the control unit  84  switches off the burner  88  at the switch  83 . It also switches off the pumps  14  and  32 . Once the pressure indicated by the pressure transducer  50  falls below the predetermined amount, the system is switched back on by the turning on of the pumps  14  and  32 , and the switching on of the burner  88  at the burner switch  83 . There may be a hysteresis range between the pressure at which shut-down occurs and the pressure at which the system is switched back on. 
     In the event that either of the pressure switches  52 , or  62  indicate a pressure which exceeds a predetermined value, the control unit  84  will also shut the system down as a safety measure by switching off the pumps  14  and  32  and the burner  88 . 
     Also in the event that the temperature sensor  60  indicates a temperature which exceeds a predetermined value, the control unit  84  will shut the system down as a safety measure. 
     Numerous variations and modifications to the illustrated system may occur to the reader without taking the resulting modification or variation outside the scope of the present invention. To give one example only, the pump  14  may be omitted if the water in the tank  10  is not heated. 
     The pump  32  may comprise a Fenner™ F06 pump manufactured by J. H. Fenner &amp; Co. Limited of Ashton Road, Harold Hill, Romford RM3 8UA, England. 
     The temperature sensor  60  may comprise two temperature sensors, one for regulation and one for safety shut-down.