Book - School of Science and Technology

602 Piping design for central hot water supply systemsTable 21.3 Discharge rates and pipe sizes for draw-off pointsFitting Rate of flow (litre/s) Size of connection (mm)Bathprivate 0.3 20institutional 0.6 20±25Basinbib tap 0.15 15spray tap 0.05 10Showerspray 0.15 15100 mm rose 0.4 15150 mm rose 0.6 15Sink 0.3 20Outflow from draw-off pointsFundamental to the design of secondary outflow pipework is the quantity of hot waterrequired at the various points of draw-off. Table 21.3 gives details of the capacity of avariety of standard fittings and the rates of water flow which are necessary to produce thiscapacity in a reasonable time. These are the data used to produce the values quoted in theGuide Section B4.Where there are only a few draw-off points to be served, as in a private house, it isreasonable to assume that at some time all the taps, etc., will be in use simultaneously,although this is unlikely. With any sizeable system it would be extravagent to make suchan assumption since, as the number of taps served increases, it becomes less probable thatthey will all be open at once.For instance, a hot water tap at a wash basin may be open for 30 seconds but at least aminute will elapse before the tap is opened again. In a row of ten basins, this would meanthat a maximum of about three taps is likely to be open simultaneously even if people arequeuing to use them. The type of building, the type of fitting and the pattern of use are allimportant matters for consideration. Thus, a group of showers in a sports club house, atthe end of a match, is likely to be in full use at the same time but, in a hotel, it is probablethat only a small proportion of the baths provided will be filling at the same time. Whilespecial cases must always be considered on their merits, a generalised approach forapplication to a wide spectrum of buildings is necessary.Various attempts* have been made to establish a basis which may be used withconfidence and a method based upon the theory of probability was developed for theGuide Section B4, adopting the concept of a scale of demand units, a wash basin tap beingtaken to represent unity. No values can, unfortunately, be allocated to showers and basinspray taps since they require water flow for the whole period of fitting use. Otherwise, thetype of application is taken into account in this scale by weighting the units according toassumed intervals of use varying from 5 to 20 minutes for a basin and from 20 to 80minutes for a bath. From this approach, a much simplified list of `demand units' may beproduced as Table 21.4, with three categories of use:`Congested ' (where times of draw-off are regulated).`Public' (normal random usage).`Private' (infrequent or spasmodic).* Bull, L. C., `Simultaneous demands from a number of draw-off points', JIVHE, 1956, 23, 445.