Patent GB-524926-A (GB)
Title: Improvements in or relating to the automatic control and indication of temperature in space heating systems
IPC: F24D19/10
Filed: 1938-12-15
Abstract: 524,926. Measuring and controlling temperature. DUNHAM CO., Ltd., C.A. Dec. 15, 1938, No. 36569. Convention date, Oct. 24. [Class 40 (i)] [Also in Group XXXV] Temperature.-An automatic control 'system for modulating the rate of heat output of a heating system comprises a Wheatstone bridge with a plurality of parallel branches, each comprising a pair of series connected arms of which at least one includes a resistance thermostat responsive to temperature variations at a given location, and a further branch containing a resistance, which is mechanically adjustable in accordance with the operation of means adjusting the heat output, for rebalancing the bridge when unbalanced. A galvanometerrelay is connected across the bridge controlling electrical means for increasing and decreasing the rate of heat output from the heating means, e.g., by adjusting a motor-operated steam valve. One branch includes a resistance responsive to temperature variations in an enclosure being heated, another, by resistances responsive to the rate of heat output of the heating system ; and a third, a resistance responsive to variation of outside temperature. Timing means causes each branch in succession to be operative on the galvanometer so that the system restricts the heat output to a range between limits varying with outside temperature and within that range adjusts by such increments the output of heat in response to 'variations from a desired inside temperature. The selection of operative arms is preferably made by changing the points at which direct current is applied to the bridge, the galvanometer always being connected to common terminals of all bridge arms. To avoid errors due to heating of the bridge resistances by the test current certain temperature-controlling resistances are heated by alternating current when disconnected from the direct current used for temperature testing. A heating system of the type described in Specification 291,134, [Class 64 (ii)], and to which the invention may be applied is shown in Fig. 1. The system comprises a steam boiler 13 supplying radiators 24 and a heat balancer C, forming part of the control system, which consists of two temperature responsive resistances C\ C<SP>11</SP> in the air stream through a heat exchanger, one in advance and the other in rear of the heating element. Room thermostats D and an outside temperature thermostat F, preferably secured against a window and consequently subject in part also to internal conditions, each comprise a temperature sensitive resistance. The resistances C<SP>1</SP>, C<SP>11</SP> lie in opposite arms of one branch of the bridge, a single thermostat D or a series-parallel arrangement of such lies in another branch and the thermostat F lies in still another branch. In order to compensate each of the latter branches in respect of the leads to these thermostats, whatever their length, each comprises four terminals between two of which is connected the thermostat and between the two others a short wire loop ; the four terminals are connected to corresponding points in opposite arms of a branch by a fourcore cable. The system comprises a motor operated valve A by which the heat output is adjusted and a pressure switch H adapted to operate this valve irrespective of heating requirements to relieve excess steam pressure. All the electrical connections are brought to a control board B which carries the galvanometer, a manual selector switch, bridge setting and adjusting potentiometer and resistances and indicating lamps. The opening and closing of the valve A is effected by oppositely acting motors or the equivalent which are selectively energized by relays K, K<SP>1</SP> and which operate also a proportioning potentiometer J connected in an additional bridge branch so as to rebalance it after each operation. The relays K, K<SP>1</SP> are energized respectively through switches 160, 137 by switches 130, 131 operated by a depressing coil 132 according to the sense in which the needle of the galvanometer G is deflected by unbalance in the bridge. A timing motor M operates a cam 144 to close in turn. switches 145, 146, 147, which respectively energize relays O, P, Q, and a cam 148 which operates a switch 149 energizing the coil 132. The relays make selected connections to the bridge so that in turn the galvanometer responds respectively to room temperature, to a minimum rate of heat output and to a maximum rate of heat output, the two last varying in accordance with the outside temperature modified to a certain extent by internal conditions also. Thus when at a given outside temperature as measured by the thermostat F the minimum heating rate has been reached the switch 160 is locked open by the relay S by the operation of relay P and the contact 131 of the galvanometer relay and further closing movements of the valve motor when relay O is functioning is prevented. In like manner the relay Q and contact 130 of the galvanometer relay operate switch 13 by relay R to prevent further opening movements when the maximum heat output appropriate to the outside temperature has been reached. The sequential operation of relays O, P, Q continues so long as the selector switch L is set in positions (4), (5), (6) (Fig. 2). The room temperature bridge connections are shown in Fig. 4, wherein D is the room thermostat, and 92 the balancing loop. Potentiometers T, T<SP>1</SP> enable the bridge to be set to balance at independent temperatures for day and night operation, respectively, since the selector switch L causes current supply in position (5) to potentiometer T<SP>1</SP> and in position (6) to potentiometer T. This figure also shows the action of relay O in transferring the bridge to the A.C. supply terminal 133 when de-energized. The lower half of the bridge comprises the proportioning potentiometer J but in order to provide means for initial balance of the bridge and adjustment of the range of the proportioning resistance a branch containing a potentiometer U is paralleled in by means of resistances 157, 158 and an adjustable resistance V through contacts on the relay O. The maximum and minimum heat output control bridge is shown in Fig. 5. The paralleled branches principally comprise the outside thermostat F and its balancing loop 97 and the heat output measuring resistances C\ C" ; these two branches are connected by the aid of potentiometers Y, Z and X, which provide for independent initial adjustments including the maximum and minimum heat output limits for a given outside temperature, and through adjustable resistances Y<SP>1</SP>, Z<SP>1</SP> and connections controlled by the relays P, Q whereby the proportionate effects of the heat balancer and outside thermostat may be adjusted to determine the rate of change of heat output with change in outside temperatures. The selector switch L in position (5) eliminates the connection to the minimum heat output control bridge and substitutes a connection to a fixed resistance branch comprising a potentiometer AA for independent setting of valve opening. This switch in position (3) eliminates the branches containing the elements F, C<SP>1</SP> and C" and makes connection to the aforesaid potentiometer so that the system operates to maintain a constant heat output-or valve opening- as determined by the setting of the potentiometer. In position (4) of the switch G a clockoperated switch CC is caused to operate a relay DD to alternate day and night control conditions automatically. Other positions provide as follows :-(1) " off "-with a load resistance 189 connected to the rectifier 124 to prolong its life. (2) the valve may be manually set to a position determined by adjustment of potentiometer AA (initial balance of the bridge arm is effected by resistances in opposite arms so coupled that one is reduced as the other is increased). (7) and (8) the room temperature existing during day or night operation may be ascertained, zeroizing the galvanometer by operating calibrated potentiometer T or T<SP>1</SP>. (9) and (10) the existing rate of heat output and the valve opening may similarly be read on potentiometers BB and AA respectively. (11) and (12) the valve is fully opened and fully shut respectively, the valve setting indication being simultaneously obtainable as in position (10). Indicating lamps are provided which light as each relay O, P, Q and K, K<SP>1</SP> is energized and a lamp 197 in series with a thermal flasher gives warning when the switch L is not off or in one of the automatic control positions (4), (5) or (6). Specification 523,784, [Group XXXVI], also is referred to.