Chapter 16 A: PUMPS AND SYSTEM EFFECTS T. Agami Reddy (rev- May 2017) Equation of motion- Bernouill’s equation with friction Pressure drop: D’arcy-Weisbach equation, hydraulic diameter of pipe, friction factor, Moody diagram, Pressure drop in pipe fittings (minor losses): equiv length method and pressure loss coefficient for fittings Description of centrifugal pumps Ideal and actual pump power, efficiency, suction head, NPSH Pump characteristics System curves: Water piping characteristics, series and parallel Pump-piping system interaction and control Pump affinity laws HCB 3-Chap 16A: Pumps and System Effects

Review- Total, Static and Velocity Pressures Manometer arrangement HCB 3-Chap 16A: Pumps and System Effects

HCB 3-Chap 16A: Pumps and System Effects Equation of Motion HCB 3-Chap 16A: Pumps and System Effects

HCB 3-Chap 16A: Pumps and System Effects Equation of Motion - HCB 3-Chap 16A: Pumps and System Effects

Equation of Motion- Examples Application to open systems (i.e., open to atmosphere) HCB 3-Chap 16A: Pumps and System Effects

Equation of Motion- Examples Application to closed systems (i.e., a system with no opening to the atmosphere and the fluid is recirculated continuously) Example: Pressure loss due to friction is 24 ft WG. What is the pump head required? There is not net change in elevation, in pressure or in velocity. So H pump = H friction = 24 ‘ WG HCB 3-Chap 16A: Pumps and System Effects

HCB 3-Chap 16A: Pumps and System Effects Pressure Drop For a pipe under pressure, pipe is full and D = Dh For partially filled pipes or for non-circular pipes such as use above equation HCB 3-Chap 16A: Pumps and System Effects

HCB 3-Chap 16A: Pumps and System Effects Pressure Drop (16.8) HCB 3-Chap 16A: Pumps and System Effects

HCB 3-Chap 16A: Pumps and System Effects Fig. 16.2 The Moody diagram with friction factor as a function of Reynolds number and relative roughness as a parameter. The hydraulic diameter is Dh. HCB 3-Chap 16A: Pumps and System Effects

Relative Roughness values for pipes HCB 3-Chap 16A: Pumps and System Effects

Pressure Drop in Straight Pipes Fig. 16.3 (b) Pipe friction chart for seasoned steel pipe in SI units at 20 C. HCB 3-Chap 16A: Pumps and System Effects

HCB 3-Chap 16A: Pumps and System Effects Example: Using the friction chart   Find the flow given the pressure drop and pipe size. Water at 80° C flows through 400 mm cast-iron pipe with a pressure drop of 225 Pa/m. What is the volumetric flow rate? We can solve this problem by rearranging Eq (16.6) as shown in the textbook (Example 16.2). A simpler approach is to use Figure (16.3) directly: flow rate is about 400 L/s However, note that this is approximate since the water temperature and the pipe material does not correspond to the standard conditions assumed in Fig. 16.3 HCB 3-Chap 16A: Pumps and System Effects

Temperature Correction Fig. 16.3(d) Pressure drop correction factor for different water temperatures HCB 3-Chap 16A: Pumps and System Effects

Pressure Drop in Pipe Fittings Straight lengths of pipe are connected by elbows and tees, with valves and ancillary equipment such as filters. These have pressure drops (referred to as minor losses) that are quite significant. Two approaches used for water flow: Kf method and equivalent length method (16.15) (16.16) (16.17) HCB 3-Chap 16A: Pumps and System Effects

HCB 3-Chap 16A: Pumps and System Effects Fig. 16.4 Typical pipe fittings used in HVAC systems. Fig. 16.5 Nomograph for pipe fitting friction factors. HCB 3-Chap 16A: Pumps and System Effects

HCB 3-Chap 16A: Pumps and System Effects (16.19) (16.20) Table 16.4 HCB 3-Chap 16A: Pumps and System Effects

HCB 3-Chap 16A: Pumps and System Effects Piping Material Considerations for pipe material selection: (often dictated by code) Type of fluid, Temperature, Pressure, Oxidation / corrosion, Cost Materials used normally - low carbon steel pipe (black steel pipe), wall thickness specified by a schedule number: example: 20, 30,… - for open systems use galvanized pipe (corrosion prevention) - copper pipe wall thickness specified by a letter: K – thickest, high pressure, refrigerant L – intermediate, hydronic system piping M- low pressure plumbing Copper tubing has lower friction and no corrosion, but is less strong and more expensive Fittings for steel pipe (screwed, welded, flanged) elbows, tees, couplings, unions, bushings Fittings for copper tubing - soldering or brazing ( temp > 10000 F, stronger joint) - flaring - strainers (usually at suction of pumps) HCB 3-Chap 16A: Pumps and System Effects

Centrifugal Pumps- Description HCB 3-Chap 16A: Pumps and System Effects

HCB 3-Chap 16A: Pumps and System Effects Power and Efficiency Hydraulic power (water horsepower, whp) Theoretical power to move fluid Common to express shaft power in HP, flow in gal/min, pressure in ft. H2O. For 68ºF water, HCB 3-Chap 16A: Pumps and System Effects

HCB 3-Chap 16A: Pumps and System Effects Power and Efficiency Brake power (brake horsepower, bhp) Power input to pump shaft required to produce a given hydraulic power Pump efficiency defined by relationship of hydraulic power to brake power HCB 3-Chap 16A: Pumps and System Effects

HCB 3-Chap 16A: Pumps and System Effects Fig. 16.6 HCB 3-Chap 16A: Pumps and System Effects

HCB 3-Chap 16A: Pumps and System Effects Solution: From Fig. 16.6 HCB 3-Chap 16A: Pumps and System Effects

HCB 3-Chap 16A: Pumps and System Effects Contd. HCB 3-Chap 16A: Pumps and System Effects

HCB 3-Chap 16A: Pumps and System Effects System Curve Fig. 16.9 Characteristic curve of a basic piping system Head vs. flow characteristic of a piping system Variable head (due to friction, function of flow rate) Fixed head (head at zero flow, due to control, static lift) HCB 3-Chap 16A: Pumps and System Effects

HCB 3-Chap 16A: Pumps and System Effects System and Fan Interaction Fig. 16.12 Typical pump or fan performance curve (solid curve). System curve is approximately parabolic (dashed). Intersection point is the system operating point. HCB 3-Chap 16A: Pumps and System Effects

HCB 3-Chap 16A: Pumps and System Effects Suction Head At inlet to centrifugal impeller Flow accelerates Static pressure decreases If static pressure falls below vapor pressure, cavitation will occur Noise Impeller damage Loss of efficiency Minimum suction head must be provided to prevent flashing Net positive suction head required (NPSHR) Determined by manufacture, a pump characteristic Net positive suction head available (NPSHA) A system characteristic Under all operating conditions, we should have NPSHA > NPSHR HCB 3-Chap 16A: Pumps and System Effects

HCB 3-Chap 16A: Pumps and System Effects Pump Characteristic Graphical representation of pump performance Head vs. flow Pump efficiency Motor power Suction head required Constant speed, multiple impeller sizes Variable speed, fixed impeller size Fig. 16.14 Example pump curve showing constant-efficiency lines (labeled with %), NPSHR, and input power (labeled in hp units) for five impeller sizes (labeled in inches). HCB 3-Chap 16A: Pumps and System Effects

HCB 3-Chap 16A: Pumps and System Effects Example: Calculate the power required by the pump when the flow rate is 6 L/s At 6 L/s, i.e., 0.006 m3/s, we find from the figure HCB 3-Chap 16A: Pumps and System Effects

HCB 3-Chap 16A: Pumps and System Effects Piping networks in series and parallel Fig. 16.18 Equivalent characteristic curves for piping and ducting networks (a) series flow arrangement (b) parallel flow arrangement HCB 3-Chap 16A: Pumps and System Effects

HCB 3-Chap 16A: Pumps and System Effects Pumps in Parallel Fig. 16.19 Two identical pumps in parallel HCB 3-Chap 16A: Pumps and System Effects

Pump and System Interaction Using Valves HCB 3-Chap 16A: Pumps and System Effects

Flow Control Using Valves Not very energy efficient control or balancing valves Generally, head increases as flow decreases Control valve adds resistance- all hydronic systems have several control valves! Efficiency decreases rapidly HCB 3-Chap 16A: Pumps and System Effects

Flow Control with Variable Speed Pump More energy efficient control Lower head than constant speed- so less pumping power Generally, higher efficiency Fixed head influences amount of speed reduction HCB 3-Chap 16A: Pumps and System Effects

HCB 3-Chap 16A: Pumps and System Effects Fig. 16.24 Flow control can be achieved either by partial closure of control valves or by variable speed drives HCB 3-Chap 16A: Pumps and System Effects

HCB 3-Chap 16A: Pumps and System Effects Pump Selection Example 150 gpm at total head = 36 ft Pump output depends on system characteristic Perform system calculations first, then select pump Operating point requires 7” impeller However, this would give 160 gpm which is more than required A valve is specified to reduce the flow to 150 gpm Shaft power required = 2.3 hp So one would specify say a 2.5 hp electric motor HCB 3-Chap 16A: Pumps and System Effects

HCB 3-Chap 16A: Pumps and System Effects Pump Affinity Laws Similarity relationships for homologous (geometrically similar) pumps For diameters or speeds 1 and 2 Flow Head Power HCB 3-Chap 16A: Pumps and System Effects

HCB 3-Chap 16A: Pumps and System Effects Example Consider the pump and system characteristic as shown. The 7” impeller at 1750 rpm pump is originally operated at point 1 (43 ft of total head at 130 gpm). We need to reduce the pump speed until the flow rate is 100 gpm. Find the new pump head, shaft power and efficiency We note that the pump must produce 25 ft of head at a flow rate of 100 gpm. Shaft power Implied assumption while using the affinity laws is that the efficiency is the same (geometrically similar pumps) HCB 3-Chap 16A: Pumps and System Effects

HCB 3-Chap 16A: Pumps and System Effects Outcomes Working knowledge of the equation of motion Understanding the friction factor and the Moody diagram Familiarity with pressure drop determination in open and closed systems Be able to solve problems of pressure drop through straight pipe, both by using the D’arcy-Weisbach relation and by using specialized charts Be able to solve problems of pressure drop through fittings both by the equivalent length method and by the loss coefficient method Familiarity with different parts of a centrifugal pump Understanding notions related to pump characteristics (head, power and effy vs flow) Be able to compute ideal and actual pump power and efficiency Be able to analyze system-pump interactions and select pumps Understanding how pump characteristic curves vary under series/parallel Understanding of water piping characteristics under series/ parallel configuration Understanding flow control using valves and by varying speed Understanding of the pump affinity laws HCB 3-Chap 16A: Pumps and System Effects