Determine Total Dynamic Head
In order to properly select a type of pump, the engineer must know which type is most applicable to the situation. Sizing a pump depends on two criteria, (1) the flow rate and the (2) total dynamic head. The flow rate is determined by the needs of the HVAC and Refrigeration system. The pump may be a chilled water pump serving several air handlers, so the flow rate (GPM) can be found by adding up the design flow rates to the air handlers and any diversity required. The (2) second criteria is the total dynamic head. Determining total head is a must-have skill for the aspiring professional engineer.
Pump Selection:(1)GPM and (2)TDH [total dynamic head]
Total head or total dynamic head is the total equivalent height of water that a fluid must be pumped against.
Head is a unit of pressure and has the units of feet of head, which is the total pressure exerted by a certain amount of feet of a water column.
Total head can be broken up into the following components, (1) Static head or Elevation Difference between the inlet and the outlet of a piping system (2) Friction loss. In a closed system, both static (elevation) head and friction loss are present. However, in a closed system there is no elevation difference, the beginning and the end of the piping system are the same, therefore there is no elevation difference. Refer to the following figures, which describe the different pressure losses in a open and closed system.
The typical example of an open system in the HVAC and Refrigeration field is the condenser water system serving a cooling tower. The pump moves the condenser water from the cooling tower basin through piping, then the chiller and back to the top of the cooling tower. The pump must provide a total dynamic head to account for the (1) Static [Elevation] head and (2) the Friction Head through the piping, chiller, fittings, other equipment and appurtenances.
(1) The static head is the difference between the inlet and the outlet. The elevation difference between the inlet and the pump, on the suction side of the pump is called the suction static head and the elevation difference between the outlet and pump, on the discharge side is called the discharge static head. The difference between discharge and suction static head is the static/elevation head that the pump must pump against.
(2) Friction head. Friction head consists of pressure losses due to equipment like chillers, cooling towers, filters, strainers, heat exchangers, air handlers, etc. The amount of friction head from these pieces of equipment are provided by the manufacturer and are typically provided in a table format with total friction head or pressure loss for the equipment versus the flow rate. Friction head also consists of pressure losses due to the piping and the various fittings like elbows, tees, valves, etc. Calculating friction had due to piping will be discussed later in this section.
The typical example of a closed system in the HVAC and Refrigeration field is the chilled water system serving the air handlers and chillers. The pump moves chilled water to and from the chiller and through the air handlers. The pump must provide a total dynamic head to account for only the Friction Head through the piping, chiller, fittings, other equipment and appurtenances. There is no static/elevation head because the system is closed.
Friction Loss: Friction loss is found through the use of either the Darcy Weisbach equation or the Hazen-Williams equation. The Darcy Weisbach equation is slightly more involved and will be explained below, starting with the equation.h
During the exam, in order to quickly complete a friction loss question using the Darcy Weisbach, the aspiring professional engineer must have the necessary tools readily available to find the values necessary to complete the equation. These include the following, 1) Inner Diameter tables of common pipe materials and sizes, 2) Flow unit conversions, 3) Inner Area table of common pipe materials and sizes, 4) Kinematic viscosity tables of common fluids at various temperatures and 5) the Moody Diagram. 1) Inner Diameter Table of Common Pipe Materials
Collect inner diameter [ft] tables of schedule 40/80 steel [Pipe sizes to 30"], type K, L, and M copper tubing [Pipe sizes to 6"] and schedule 40/80 PVC [Pipe sizes to 30"]. Provide inner diameters in feet for ease in using the Darcy Weisbach Equation.
2) GPM to FT^3/sec Conversion Factor
Multiply GPM by 1/448.83 to get (FT^3)/sec.
3) Inner Area Table of Common Pipe Materials
Collect inner area [ft^2] tables of schedule 40/80 steel [Pipe sizes to 30"], type K, L, and M copper tubing [Pipe sizes to 6"] and schedule 40/80 PVC [Pipe sizes to 30"]. Provide inner areas in feet^2 for ease in finding the velocities through the pipes.
4) Kinematic Viscosity Tables [used to get Reynolds number which leads to the friction factor]
5) Pipe Roughness
Collect pipe roughness factors for common pipe materials, steel, PVC, copper, etc.
6) Moody Diagram: The Moody diagram uses the Reynold's number and the relative roughness factor to determine the friction factor. The relative roughness factor is found by first finding the roughness value corresponding to the pipe material. Then dividing the roughness factor by the inner diameter of the pipe. Ensure that the roughness factor and the diameter are in the same units. The Reynold's number is found by multiplying the velocity of the fluid through the pipe by the diameter of the pipe and dividing by the kinematic viscosity of the fluid. Once these two values are found (a) Relative Roughness and (b) Reynold's Number, then the friction factor can be found by finding the intersection of the vertical Reynold's number line shown in black and the Relative Roughness factor curves shown in red.
Step 1: Find relative roughness factor, step 2: find intersection of reynold's number and relative roughness factor. step 3: read corresponding friction factor.