Book - School of Science and Technology

Water systems ± applications 239Pipe section Total load Pipe sizeA 22kW 32mmB 44kW 40mmC 66kW 50mmD 88kW 50mmE 110 kW 65 mmIt will be noted that the loads on pipe sections B and D, 44 and 88 kW, are slightlyabove those listed in the last column of the first schedule for 40 and 50 mm pipesrespectively. By reference to Figure 9.1, however, it will be apparent that the unitpressure loss for these sections will be increased only slightly if these sizes are selected:the next pipe size up in each case would be much too large. The short connections tothe individual fan convectors would, continuing with the same approach, be 32 mm ineach case but, alternatively, might each be considered separately and used to balancethe system a little better. For example, since the total length related to the first itemserved is not much more than half that of the index circuit, 25 mm pipe might be usedfor the connections.Parallel single-pipe circuitsBy inspection of Figure 9.3(b), it will be apparent that each single-pipe circuit here may betreated as if the points of connection to pipework common to all circuits were flow andreturn connections to a boiler. The size of the single pipe may then be selected on thatbasis, as discussed previously. Thereafter, each single-pipe circuit may be considered as ifit were a compact heat emitting unit connected to a two-pipe circuit and sizes for thecommon pipework then selected accordingly.The pressure loss which is used to select the pump will be that of the longest or indexcircuit and the volume handled will be the sum of the individual requirements. If a singlevalue of unit pressure loss were to be used then those circuits which are shorter or carryless load than the index run will lose less pressure and it will be necessary to take accountof this when sizing the common pipework and, in all probability, also necessary to provideregulating valves to overcome out-of-balance conditions.As in the case of the example for a simple two-pipe arrangement, the process outlinedabove is approximate only but would serve also as a basis for making a preliminary costestimate.Calculations for final designMoving now from approximations to the calculation routine required to complete adesign, the system shown in Figure 9.5 will serve as a basis to illustrate the necessarysteps which are as follows:. The iterative process of allocating guessed preliminary pipe sizes to a system in order toestablish the amount of heat emitted by each section (guessed or estimated as apercentage in the previous examples), is tedious and use may be made of Figure 9.6as a short cut. From the curves included, a near approximation to heat emission from apipe may be read from one or other of the vertical scales for a chosen temperature dropt, flow to return, against the energy flow rate shown on the bottom scale. Thus, forthe system in Figure 9.5, a list may be made as follows for a 12 K temperature drop: