Calculating Pump Head How to calculate the Pump Head for Open and Closed Systems. 6/27/2012 Vemco, Inc.

Transcription

1 Hw t calculate the Pump fr Open and Clsed Systems 6/27/2012 Vemc, Inc.

2 Befre we can discuss pumpp head, we must understand the difference between an pen hydrnic system and a clsed hydrnic system. It is imprtant t knw whether the pump serves an pen r a clsed system, because the pump head calculatin depends n the type f system that the pump serves. In a clsed system, the fluid is nt expsed t a break in the piping system that interrupts frced flw at any pint. In an pen system, it i is. In a clsed system, the fluid travels thrugh a cntinuus clsed piping system that starts and ends in the same place there is n break in the piping lp. The vast majrity f hydrnic piping systems are clsed. The mst cmmn pen system is the cling twer prtin f a chilled waterr system, as depicted belw. A break in the piping system ccurs where the water exits the spray nzzles, and is expsed t air in the fill sectin f the twer. The water cllects in the cling twer sump befree being pumped arund thee lp again. Nte that the chilled water side f this diagram (the right side) is clsed. Becausee it is clsed, an expansin tank absrbs any thermal expansin f the fluid. Open systems dn t requiree expansin tanks, as the fluid is naturallyy free t underg thermal expansin. Figure 1: Clsed and Open Hydrnic Systems What is Pump? Units f Measure: In the U.S. system, head is measured either in PSI r in "feet f head" (usually abbreviated t "feet"). Pump is the ttal resistance that a pumpp must vercme. It cnsists f the fllwing cmpnents: Static : Static head represents the net change in height, in feet, that the pump must vercme. It applies nly in pen systems. Nte that in a clsed lp system, the static head is zer because the fluid n ne side f the system pushes the fluid up the ther side f the system, s the pump des nt need t vercme any elevatin. Frictin : This is als called pressure drp. When fluid flws thrugh any system cmpnent, frictin results. This causes a lss in pressure. Cmpnents causing frictin include bilers, chillers, piping, heat exchangers, cils, valves, and fittings. The pump must vercme this frictin. Frictin head is usually expressed in units called "feet f head." A ft f frictin head is equal t lifting the fluid ne ft f static height. Pressure : When liquid is pumped frm a vessel at ne pressure t a vessel at anther pressure, pressure head exists. Cmmn applicatins includee cndensate pumps and biler feed pumps. Cndensate pumps ften deliver water frm an atmspheric receiver t a deaeratr perating at 5

3 PSIG, meaning that in additin t the ther heads, the pump must vercme a pressure head f 5 PSIG. One PSIG equals 2.31 feet, s the differential head in this applicatin is 5 X 2.31 = Pressure head is a cnsideratin nly in sme pen systems. Velcity : Accelerating water frm a standstill r lw velcity at the starting pint t a higher velcity at an ending pint requires energy. In clsed systems the starting pint is the same as the ending pint. Therefre the beginningg velcity equals the final velcity, s velcity head is nt a cnsideratin. In an pen system, the velcity headd is theretically a cnsideratin, but the pipeline velcities used in hydrnics are s lw that this head is negligible, and is ignred. (Nte thatt the velcity head is defined by the frmula V 2 /2g wheree V is the fluid velcity in feet per secnd and g is the gravitatinal cnstant 32 feet/secnd 2. Therefre at typical velcities f 2 6 fps, the velcity head is a fractin f a ft. Since head lss calculatins are really estimates, this small figure becmes insignificant). S, fr hydrnic applicatins, we can say that: 1. Fr clsed systems: Pump head = the sum f all frictin pressure drps Where: Frictin pressure drp = piping pressure drp + terminal unit pressure drp + surce unit pressure drp* + valve pressure drp + accessries pressure drp. 2. Fr pen systems: Pump head = the sum f all frictin lsses plus the static lift f the fluid plus the pressure head. * The "surce unit" is defined as the biler, chiller, r heat exchanger, which creates the ht r chilled water. Steps in Calculating the Pump Basically, we need t plug values int the prper frmula abve. Step 1: Lay ut the piping system using lgical ruting as determined by the building requirements. Nte each terminal unit and its GPM. Step 2: Select pipe sizes fr each segment, based n prper velcityy and pressure drp. The graphs belw are frm the ASHRAE Fundamentals Bk. Recmmended velcities are: Pipe Sizes f 2" and Under: 2 fps minimum t 4 fpss maximum Pipe Sizes f ver 2":.75 ft. f P/100 equivalent feet minimum t 4 ft. f P/100 equivalent feet maximum Where P is the head lss (als called frictin lss r pressure drp).

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5 The recmmended ranges ensure that the piping system will be quiet, cnsume reasnable pump hrsepwer, and be reasnably ecnmical t install. Nte that the minimum velcities are recmmended based n the fact thatt lwer velcities will allw air t cllect at high pints, with the pssible result f air binding. Once the layut and pipe size fr each sectin has been determined fllw these steps: Step 3: Determine Frictin Due t Surce, Terminal and Accessry Equipment Including: Surce and terminal Equipment: Cnsult manufacturer s catalgss r cmputer selectins. Accessry items i include filters, strainers, check valves r multi purpse valves that culd have a significant pressure drp that wuld nt be cveredd under the equivalent feet f piping rule f thumb. T determine valve D P refer t curves r Cv ratings. A Cv is defined as the flw at which the valve will have a resistance f 1 PSIG (2.31 feet) ). Since the pressure drp iss prprtinal t the square f the flw rate, use the fllwing frmula t calculate the pressure drp thrugh the valve fr any flw rate: PD In Feet = (Flw Rate/ Rated Cv) 2 X 2.31 Example: A valve has a Cv f 10. Flw thrugh the valve is 21 GPM. What is the valve D P in feet f head? PD in Feet = (21/10) 2 X 2.31= 10.2 Special Cnsideratin: Pressure Drps In PSI and Cnverting PSI t Smetimes pressure drps will be given in PSI units instead f feet f head. T cnvert PSI units t feet f head: PD in feet = PD in PSI X 2.31 Example: A plate and frame heat exchanger printut shws a pressure drp f 8.5 PSI. What P in feet must be added t the pump fr this item? Answer: Feet in = 8.5 PSI X 2.31ft./PSI = ft. Step 4: Determine the Static (Open Systems Only) The static head is simply the ttal height that the pumpp must lift thee fluid. It applies nly in pen systems. Remember that the static head is the difference in height that the pump will be required t prvide. In the drawing belw, shwing a cling twer, the static height might appear t be 40. Hwever, the water level in the twer sump is 28 abve the pump, s the pump must nly prvide a net lift f 12. Therefre, the static head is 12.

6 Figure 2, Static Height Example Step 5: Determine the Pressure (Sme Open Systems Only) If the system is pen, determine the pressure differential required, if any. Dn t frget t multiply pressure differentials in PSI X 2.31 /PSI. Step 6: Determine the "Wrst Pressure Drp Lp" and Estimate the Frictin Lss fr that Lp by Using Equivalent Feet" Because fittings result in mre pressure drp than plain pipe, we accunt fr them by using "equivalent length." The equivalent length f a piping circuit is the actual measured length plus an allwance fr all the fittings (elbws, tees, valves, etc.). Lng methd fr determining equivalent length: The table belw lists the number f equivalent feet f piping fr varius fittings and accessries:

7 T use this methd, add the equivalent length f each item in the fluid s path t the actual length f piping t get the ttal equivalent feet f piping. Shrtcut methd fr determining equivalent length: Designers ften skip the abve methd and simply multiply the actual piping length times 1.5 t 1.75 t get the equivalent length. This prvides speed and a reasnably accurate estimatin fr "typical" hydrnic piping systems. As with any rule f thumb, hwever, watch ut fr ddball situatins (the biler rm is 2 blcks away frm the building, a piping system with an extreme number f fittings, etc). In such situatins, the lng methd prvides better accuracy. Nw multiply the frictin lss per 100 f piping frm the ASHRAE charts times the equivalent length in the "wrst" lp t get the ttal piping frictin lss. Select the wrst lp by inspectin, if pssible. Calculate several branches if there is a dubt. The frictin in the wrst lp is used as the frictinn head.

8 Figure 3, Wrst Pressure Drp Circuit Nte that the wrst lp is simply that the lp that results in the largest ttal pressure drp. D nt add pressure drps frm ther parallel lps. In the drawing abve, assume that the pressure drp thrugh Cil 3 and its valve are higher than the pressure drp thrugh Cils 1 and 2. Assuming that the branch piping fr all three circuits is similar, the "wrst case" ttal frictin lss lp is shwn in light blue. It wuld be errneus t add the pressure drps f the piping shwn in black. Ntes 1. Thse circuits with less pressure drp than the "wrst" circuit will be balanced in the field by partially clsing balancing valves (nt shwn abve). 2. If there are different pipe sizes n the circuit, the circuit may have t be analyzed in sectins, because the pressure drp/ft may vary by sectin. This is ne gd reasn fr selecting all piping at the same pressure drp per 100. It simplifies the calculatins cnsiderably. Safety factrs Yu may wish t add a safety factr t the calculated head fr tw reasns: Jbsite cnditins may nt allw direct ruting f piping as shwn n the plan. Extra length and extra elbws result in added frictin. The interir pipe walls becme rugh ver time due t crrsin, especially in pen systems, wheree fresh water makeup brings in a steady supply f crrsin causing f 15 25% fr frictin calculatins. Nte that the frictin tables assume cld xygen. This increases frictin. Varius surces recmmend ttal safety allwances water, which results in mre frictin than ht water. Therefre, if yu are designing a ht water system, yu already have a safety margin f arund 12%. Be careful f excessive safety factrs. They result in versized pumpp impellers that cause wasted energy! Pressure Drp Crrectins fr Glycl. Sme systems utilize either ethylene r prpylene glycl mixtures inn lieu f water. These fluids result in higher pump heads than des water. Fr discussins f the effects f glycl, cntact yur lcal Vemc, Inc. ffice

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