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Heating loadsCooling loadsWater flowAir flowAcousticsW2 PIPE SIZING – STRAIGHT LENGTHSOverviewIn order to size pipework both straight lengths and fittings need tobe considered. Initial pipe sizing is done by considering straightruns alone, but for complete system design and pump sizing bothstraight runs and fittings need to be considered.The examples in the pipe sizing, index run and pump sizing sheets(W2-5) are shown as manual calculations. Although calculationsare often done on a spreadsheet or by using a computer sizingpackage, these will still require input and design decisions thatrequire familiarity with the fundamental theory and manual sizingprocedures.Step 5. Size pipe using the CIBSE Guide C pipe sizingspreadsheet. Select an appropriate pipesize using the mass flowrate and the selected value of either pressure drop per metre run orvelocity.Ø Design tip: Pipe sizing and pressure loss data for a system canbe laid out in a tabular format and converted to a spreadsheet.ExampleFind the pipe size for a 10 m straight run of medium steel heatingpipe carrying water at 75 C and serving four fan convectors, eachrated at 4 15 kW output.Design information requiredPipe length in metresThis may already have been provided on schematics.(See also sheet W1.)(See Design Watchpoint 1.)Fluid type and operating temperatureThe specific heat capacity, density and viscosity of fluids willdepend on the type used. (Properties of various fluids are given inCIBSE Guide C Appendix 4.A1.)Temperature drop across system (K) t across the system.Pipe materialCalculation procedure (manual pipe sizing)Step 1. If not already laid out, sketch the system underconsideration, indicating pipe lengths and unit loads (kW).Allocate a reference number to each length of pipework.Step 2. Estimate the heat loss for each section (often taken as apercentage of unit load such as 5-10%, or as a typical value suchas 25 W/m run for insulated heating pipes, 100 W/m run for uninsulated pipes) to give the total load for each section. The heatloss will depend on pipe orientation (vertical/horizontal) and thequality and installation of insulation. Heat losses can be less than5% if the pipes are well insulated.Ø Design tip: Typical values can be obtained by estimating thepipe size and reviewing heat loss data in tables 3.19 - 3.26 ofCIBSE Guide C.Ø Design tip: Part L of the Building Regulations coversConservation of Fuel and Powers. Maximum permissible heatlosses for hot water, heating and cooling pipes can be obtainedfrom the Non-Domestic Heating, Cooling and VentilationCompliance Guide.Step 1. A simple sketch above shows the necessary informationStep 2. Estimate the heat loss with an assumed estimated loss as6% of the load.Hence:Total fan coil load 4 x 4 15 16 6 kW,6% of 16 6 0 996 kWTotal load 17 6 kWStep 3. Temperature drop across system assumed as 10K, fromQ m& Cp DtWhere:Q load (kW)Cp specific heat capacity of water (kJ/kg.K)Dt temperature difference (K)Mass flow rate is therefore:17 6m& 0 42 kg/ s4 2 10Step 4. A nominal pressure drop per metre run of 250 Pa/m isselectedStep 3. Select an appropriate temperature drop across the system,such as 10-20 K for low pressure hot water and, assuming this tobe constant across the system, calculate the mass flow rate in eachsection.Step 5. Using the CIBSE Guide C pipe sizing spreadsheet formedium grade steel pipe with winter at 75 C, size the pipe to givethe required mass flow rate and pressure drop per metre run.Using the individual results section of the spreadsheet it can befound that a 25 mm diameter pipe will give a velocity of 0·73 m/swith a pressure loss of 241 Pa/m. Alternatively the spreadsheetcan be used to produce a pressure-loss data table.Step 4. Select an acceptable design value for either pressure dropper unit length or velocity. (See sheet W1 and seek advice fromsenior engineer as required.)Note that a worked example demonstrating the calculationprocedure used by the CIBSE pipesizing spreadsheet is providedin Section 4.A2.1 of CIBSE Guide C.Cross-check: a velocity of 0 73 m/s is acceptable for 25 mm steelpipe BSRIA BG 30/2007 – LICENSED TO UNIVERSITY OF HONG KONG LIBRARY GUIDE TO HVAC BUILDING SERVICES CALCULATIONS87

Heating loadsCooling loadsWater flowAir flowAcousticsW2 PIPE SIZING – STRAIGHT LENGTHSNote that as an alternative approach to step 2, an experiencedestimate of pipe size of 25 mm gives a heat loss for uninsulatedsteel pipe of 92 W/m run ( CIBSE Guide C, table 3.19), in otherwords 920 W. This is 5 55 % of the total load, therefore theoriginal assumed loss of 6% is reasonable.Ø Design tip: Review the impact of a change of temperature dropacross the system. Larger temperature drops (20K is common inEurope) can reduce pipe sizes considerably, but this will requiremore careful balancing of the system when commissioning.Ø Design tip: For occupied spaces consider omitting heat lossesfrom radiator sizes. Any heat loss from the pipes to the occupiedspace is useful heating. As long as the total design capacity isavailable, it does not matter that some comes from the pipes andsome from the emitters. This will also reduce pipe size, andreduce unnecessary system over sizingReferencesCIBSE Guide B1, Heating, Section 5.1.3, and appendix A1.3, 2002,ISBN 19032 8720 0/CIBSE Guide B, Section 1.5.1.3, appendix1.A1.3, 2005, ISBN 1 9032 8720CIBSE Guide C, Reference Data, Section 1, 2007,ISBN 978 1903287 80 4BSRIA, Rules of Thumb, BG 14/2003, BSRIA 2003, ISBN 0 86022626 3Lawrence Race G, Pennycook K, Design Checks for HVAC – AQuality Control Framework for Building Services Engineers –sheets 26 and 46, BG 4/2007, BSRIA 2007, ISBN 978 086022 6697Department for Communities and Local Government, NonDomestic Heating, Cooling and Ventilation Compliance GuideSee also:Sheet W1 Pipe sizing - GeneralSheet W3 Pipe sizing - Pressure drop across fittingsSheet W4 System resistance for pipework - Index runDESIGN WATCHPOINTS1.2.88LIBRARYIf taking dimensions from drawings use dimensions shownrather than scaling off, as drawings can become distorted ifphotocopied.If pressure drop was selected, check that the velocity falls withina reasonable range and vice versa.GUIDE TO HVAC BUILDING SERVICES CALCULATIONS BSRIA BG 30/2007 – LICENSED TO UNIVERSITY OF HONG KONG

Heating loadsCooling loadsWater flowAir flowAcousticsW3 PIPE SIZING – PRESSURE DROP ACROSS FITTINGSOverviewThere are two methods of calculating the total pressure lossthrough a fitting:DP 5 Pv z 0 5 r v 2 CIBSE Guide C, equation 4 11)Where:DP z r v Pv total pressure loss (Pa),pressure loss factordensity (kg/m 3 )velocity (m/s)0·5x p v2 velocity pressureCalculation procedureTo calculate the pressure loss through a fitting the following stepsare required:Step 1. Find the appropriate velocity pressure loss factor ( z)value for the fitting in tables 4.20-4.39 of CIBSE Guide C.Step 2. Determine the velocity pressure in the pipe using theGuide C pipe sizing spreadsheet.Step 3. Calculate DP using CIBSE Guide C equation 4.11.Note that velocity pressures (P v ) are also available in CIBSEGuide C Table 4.10.Values of DP/1 can be determined using the CIBSE Guide C pipesizing spreadsheet. The spreadsheet also calculates the velocitypressure (Pv).ExampleCalculate the pressure loss through a 90 sharp elbow with asmooth radiused inner of 25 mm diameter connected to a mediumgrade steel pipe and passing a water flow rate of 0 5 kg/s at 75 C.The z values represent the fraction of one velocity pressure thathas the same pressure loss as the fitting.Step 1. From CIBSE Guide C table C4.21 (pressure loss factorsfor elbows) the z value for a 90 sharp elbow with a smoothradiused inner with a 25 mm diameter is 0 8.Design information required(See also sheet W1 pipe sizing – general)Step 2. From the CIBSE Guide Cspreadsheet a velocity pressure of369 Pa is obtained.Pipe system layoutStep 3. The pressure lossassociated with the elbow can becalculated fromPipe lengths, location, number and type of fittings,Pipe sizesWhen the straight length diameters have been calculated, thecorresponding l e and DP/l for each section may be required. Whencalculating the pressure drop across a branch fitting, the z for thestraight section and branch section will be determined separatelyThese will then be used to calculate the pressure drop across bothparts of the branch.Mass flow rateThe mass flow rate through each section and therefore through thefittings is required. Again, with a fitting such as a swept divergingtee, (such as a branch), the mass flow-rate at each part of the teewill be required. For example, one flow into the tee (1) and twoflows out of the tee (2 and 3) as shown below:DP z Pv 0·8 369 295PaReferencesCIBSE Guide B1, Heating, Section 5.1.3, and appendix A1.3,2002, ISBN 19032 8720 0/ Guide B, Section 1.5.1.3 and appendixA1.3, 2005, ISBN 1 903287 58 8CIBSE Guide C, Reference Data, Section 1, 2007,ISBN 978 1903287 80 4BSRIA, Rules of Thumb, BG 14/2003, BSRIA 2003,ISBN 0 86022 626 3Lawrence Race G, Pennycook K, Design Checks for HVAC – AQuality Control Framework for Building Services Engineers –sheets 28 and 46, BG 4/2007, BSRIA 2007, ISBN 978 086022 6697See also:Sheet W1 Pipe sizing - GeneralSheet W2 Pipe sizing - Straight lengthsSheet W4 System resistance for pipework - Index runDESIGN WATCHPOINTS1.A common mistake is to apply the z for a tee piece to the wrongpart of the branch. The values of z in CIBSE Guide C should beused with the velocity pressure (P v ) of the combined flow. BSRIA BG 30/2007 – LICENSED TO UNIVERSITY OF HONG KONG LIBRARY GUIDE TO HVAC BUILDING SERVICES CALCULATIONS89

Heating loadsCooling loadsWater flowAir flowAcousticsW4 SYSTEM RESISTANCE FOR PIPEWORK – INDEX RUNOverviewThe index run within a system is the circuit that has the highestresistance to the flow of water and supplies the index heat emitter.This is the worst case possible when considering pressure losseswithin a system. It is usually, but not always, the longest circuit inthe system. Sometimes a shorter run with a greater number offittings or items of equipment can be the index run.The index pd (pressure drop) is required in order to successfullysize the pump for the system. If the pump can work to the pressuredemands of the index run then all other circuits will work.To identify the index run, the pressure drop for several circuitsmay need to be found. The pressure drop across each length ofpipe, fitting and terminal within these circuits will need to becalculated to determine the total pressure drop for each circuit. Theone with the largest pressure drop will be the index.Ø Design tip: Balancing a system is necessary to achieve thecorrect pressure losses and flow rates through the differentcomponents of a system. If the layout of the system issymmetrical, then the amount of commissioning required tobalance the system is reduced. This is true for both water andair systems.Design information requiredSee also sheet W1 Pipe sizing – general.Step 2. Add up the total direct pressure losses from each sectionwithin a circuit to give a circuit pressure drop.Step 3. Identify the index run by examining each circuit pressuredrop to identify the highest pressure drop.ExampleIdentify the index run and calculate the index run pressure drop forthe following two-pipe system serving two panel radiators, eachrated at 4 15 kW. The water temperature is 75 C.Design dataHeat EmittersFor two heat emitters at 4 15 kW each, a 6% emission loss frompipework has been added when calculating the mass flow rate.Assume a Dt of 10K across the system.Cp 4 2kJ/ kgKQ (2 4 15) 1 06 8 8kwFrom:Q mcp& Dtm& 8 8 0 21kg/ s4 2 10Number of circuitsFrom the CIBSE Guide C pipe sizing spreadsheet, a 20 mm pipewith a flow of 0 2 kg/s gives a DP/l of 212 Pa/m and a velocitypressure (Pv ) of 165 Pa.Each circuit within a multi-circuit system needs to be clearlyidentified.Step 1. Identify the different sections and their design details.Pipe sizing detailsAll pipework and fittings should have been sized with pressuredrop per unit length ( DP/l) velocity pressure (P v) and pressure lossfactor (z). Data is available from tables such as those found insection 4 of CIBSE Guide C and the pie sizing spreadsheet.Ø Design tip: Often the longest circuit is the index run but there isalways the possibility of a shorter circuit with many fittings beingthe index run.Calculation procedureStep 1. Identify each section and where it begins and ends. Forexample, section 1 starts at the combined flow side of the teefitting in the return, goes through the boiler and ends at thecombined flow side of the tee fitting in the flow. Assign areference to each section. Identify each fitting within each sectionand determine the design details: pressure drop per unit length(D/Pl) velocity pressure (P v) and pressure loss factor (z).Section 1: GreenSection 2: RedSection 3: BlueStep 4. Identify each circuit by the sections it consists of, forexample: circuit A 1,2,3, circuit B 2,3,4 and so on.Section detailsSection 1Step 5. Calculate all direct pressure losses across fittings andpipe work in each sectionFor fittings use:DP z PvFor straight pipe use:DP (DP/l) l90LIBRARYGUIDE TO HVAC BUILDING SERVICES CALCULATIONSStraight Pipe work: length 25 m, m& 0 21 kg/s, diameter 20mm.Elbow: Smooth radiused inner diameter: 20 mm, z value: 0 75(Table 4.21 CIBSE Guide C.)Boiler: Sectional boiler z value: 1 5. BSRIA BG 30/2007 – LICENSED TO UNIVERSITY OF HONG KONG

Heating loadsCooling loadsWater flowAir flowAcousticsW4 SYSTEM RESISTANCE FOR PIPEWORK – INDEX RUNSection 2Straight pipe work: length 8 m, m& 0 105 kg/s, diameter 15mm, DP / l 251Pa/ m,Pv 133Pa.Radiator: panel radiator z value: 2 5.Diverging tee with reduction: z value 1 30 (Table 4.33 CIBSEGuide C).Converging tee with enlargement: z value 2 98, (Table 4.34CIBSE Guide C).Section 3Straight pipework: length 13 m, m& 0 105 kg/s, diameter 15 mm, DP / l 251Pa/ m,Pv 133Pa.Elbow (two off): Smooth radiused inner diameter 15mm, z value 0 93 (Table 4.21 CIBSE Guide C).Radiator: panel radiator z value 2 5.Diverging tee with reduction: z value 0 57, (Table 4.33 CIBSEGuide C).Converging tee with enlargement: z value 1 63, (Table 4.34CIBSE Guide C).Values of z for radiators and boilers are found in table 9.1 ofHeating & Air Conditioning of Buildings Faber & Kell, NinthEdition. (See Design Watchpoint 1.)Step 2. Now identify the circuits in the system;Circuit A consists of section 1 and 2,Circuit B consists of section 1 and 3.Step 3. Calculate the pressure loss across each fitting and thepipework for each section. (See Design Watchpoint 2.)There are two branches in this example. For both, the combinedflow forms part of section 1; therefore as the velocity pressure (P v)to be used is that of the combined flow, the value for this exampleis 165 Pa.Section 1: GreenSection 2: RedSection 3: BlueSection 2Straight pipe: 8 m 251 Pa/m 2008 PaRadiator: 2 5 133 Pa 333 PaDiverging tee: 1 3 165 Pa 215 PaConverging tee: 2 98 165 Pa 492 PaTotal 3048 PaSection 3Straight pipe: 13 m 251 Pa/m 3263 PaElbow (2 off): 2 0 93 133 Pa 247 PaRadiator: 2 5 133 Pa 333 PaDiverging tee: 0 57 165 Pa 94 PaConverging tee: 1 63 165 Pa 269 PaTotal 4206 PaStep 4. Add up total section losses for each circuit.Circuit A Total pressure loss 5672 Pa 3048 Pa 8720 Pa, (8 7 kPa)Circuit B Total pressure loss 5672 Pa 4206 Pa 9879 Pa, (9 9 kPa)Step 5. The index circuit in this example is Circuit B.Ø Design tip: Often, as in this case, the index circuit is obvious byinspection. But, if in doubt, check all circuits. Otherwise there isa risk of undersizing the pump and poor system performance.ReferencesCIBSE Guide A, Environmental Design, 2006,ISBN 1 903287 669CIBSE Guide B1, Heating, Section 5.1.3, and appendix A1.3,2002, ISBN 19032 8720 0/Guide B, Section 1.5.1.3 and appendixA1.3, 2005, ISBN 1 903287 58 8CIBSE Guide C, Reference Data, Section 1, 2007,ISBN 978 1903287 80 4BSRIA, Rules of Thumb, BG 14/2003, BSRIA 2003,ISBN 0 86022 626 3Lawrence Race G, Pennycook K, Design Checks for HVAC – AQuality Control Framework for Building Services Engineers –sheets 28 and 46, BG 4/2007, BSRIA 2007, ISBN 978 086022 6697Heating & Air Conditioning of Buildings , Ninth Edition, Faber &Kell. ISBN 075 064 642 XSee also:Sheet W1 Pipe sizing - GeneralSheet W2 Pipe sizing - Straight lengthsSheet W3 Pipe sizing - Pressure drop across fittingsSheet W5 Pump sizing.Pressure loss for each sectionSection 1Straight pipe: 25 m 212 Pa/m 5300 PaElbow: 0 75 165 Pa 124 PaBoiler: 1 5 165 Pa 248 PaTotal 5672 PaDESIGN WATCHPOINTS1.2.This example has the same heat emitters in both circuits. Thismay not always be the case.When calculating the pressure loss through a fitting such as thebranch fittings in this example, the velocity pressure (P v) of thecombined flow is used. BSRIA BG 30/2007 – LICENSED TO UNIVERSITY OF HONG KONG LIBRARY GUIDE TO HVAC BUILDING SERVICES CALCULATIONS91

Heating loadsCooling loadsWater flowAir flowAcousticsW5 PUMP SIZINGOverviewPumps are required to transport the required fluid at a given massflow rate around a system against the resistance to flow.Centrifugal pumps are normally used for most building servicesapplications. There are other types of pumps, such as positivedisplacement pumps, that are normally used in applications wherehigh viscosity fluid is the system medium, such as heavy fuel oil.The two main designs of pumps that are used in building servicesare the in-line pump and the end-suction pump.System characteristicsThe system performance can be expressed in the form of theequation ΔP RQ 2 for turbulent flow. The constant R is required asthe equation is derived from P µ Q2 . Most building servicessystems will use the turbulent flow equation.Graph 1 shows the system characteristic on a pressure and volumeflow rate graph.Graph 1The basic information required to size a pump is the total massflow rate required for the system and the total pressure drop (theindex pressure drop).Once these details have been confirmed, the next step is todetermine what configuration of pumps are to be used (such assingle, series or parallel), and then to compare the pumpperformance curve or characteristic to the system performancecurve or characteristic. These issues are explained below and canbe determined graphically or by calculation.Pump lawsVarious pump laws show the relationships between pressure, flowrate, efficiency and power. These can be used to calculate eachfactor:Q µNP µ N2W µ N3P µρW µρQ µ D3P µ D2W µ D5Where:Q N P W D ρ volume flow ratespeedpressure developedpowerdiameter of the impellerdensity.Any system will have a resistance to flow due to the fittings,components, (such as heat exchangers) and the materials used. Theflow of fluid through a system will vary according to the pressuredeveloped by the pump.Pump characteristicsChanges can be made easily to the points of oper

Ø Design tip: If the system fluid is chilled water containing glycol, then the specific heat capacity will need to be adjusted. Fluid temperature For example whether hot or chilled water. Typical flow temperatures for low temperature hot water systems are 70-95 oC, and for primary chilled water are 6-12oC. Design flow and return temperatures