Book - School of Science and Technology

236 Piping design for indirect heating systemsTable 9.1 Pressure loss factors for single resistancesItem Item Item Sharp elbow (90 ) 20 mm 1.7 Reducer d i /d 2 0.2 0.5 Values50 mm 1.0 0.5 0.4 Standard globe 20 mm 8.0100 mm 0.7 0.8 0.2 40 mm 4.9Long elbow/bend 20 mm 0.9 Expansion d i /d 2 0.2 0.9 100 mm 4.1(90 ) 50 mm 0.4 0.5 0.6 Angle globe 20 mm 2.7100 mm 0.2 0.8 0.1 40 mm 2.4Pipe joints Entry to vessel 1.0 100 mm 2.2Metal negligible Exit from vessel 1.0 Gate spherical seal 0.03Welded plastic 0.4 Column radiator 5.0 Gate plain parallel 0.3Flanged plastic 0.13 Panel radiator 2.5 Non-return flap 20 1.7Through tees =0.5 plus reducer/expander 40 6.6Branch tees =0.2 plus equivalent elbow/bend 60 30.0180 bend =1:6 90 elbow/bend45 bend =0:7 90 elbow/bendTable 9.2 Water velocities in pipeworkPipesnominal bore(mm)Application groupVelocitiesin normal use(m/s)10, 15, 20 Small domestic 0.25±0.7525, 32, 40 Domestic 0.5±1.050, 65, 80 Small commercial 0.75±1.75100, 125, 150 Commercial 1.25±2.5than 2±3 m/s in straight pipes are not likely to give rise to a noise level which isunacceptable. As to minimum velocities, these are of importance only as far as airentrainment is concerned and a value of 0.5 m/s has been proposed although this wouldseem to be rather on the high side for small pipes. The ranges of values listed in Table 9.2represent normal practice for pumped circulations.Water systems ± applicationsIn applying theory to practice, it is as well to remember that a system made up fromcommercial pipework bears little resemblance to a laboratory test rig. Manufacturingtolerances for internal diameters vary with the material, being about 10% for steel, andother variables such as internal roughness with ageing, thickness of scale deposition, etc.,add to the uncertainty. Furthermore, for one reason or another, site work may notproceed exactly to the design: additional bends may be introduced, tee branches may bearranged in a different way and welded joints in awkward positions may not be quite asfree of protrusions into the bore as they should be. These comments are not intended tosuggest that pipework sizes should be selected solely by a plumber's rheumy eye nor thatdata included in the Guide Section C4 are too exact for practical use. They are offeredrather to emphasise that calculations made with a slide rule are more pertinent in thiscontext than results displayed on a pocket calculator, or from computer output, tuned toprovide answers to many decimal places.