Ashrae Journal - October 2008 - 25

Head Loss ft/100 ft Volumetric Flow Rate, gpm Figure 1: Typical friction rate/velocity limits for Schedule 40 steel pipe.1 This article discusses how the spreadsheet works and presents example system calculations. Optimum Pipe Size Calculations The optimum pipe size for a given design flow rate is a function of: • Location of pipe in the system (whether or not it is in the critical circuit*); • First costs of installed system including piping, fittings, valves, pumps, pump motors, and variable speed drives; • Pump energy costs, which depend on the pressure drop through the system at full load, pump and motor efficiency, hours of operation, energy rates, distribution system type (constant or variable flow), annual flow profile, type of pump control (variable speed or riding pump curve), etc.; • Erosion considerations (high velocities can contribute to hastening of pipe wall deterioration); • Noise considerations (high velocities and turbulence can cause noise problems in occupied areas); • Physical constraints; and • Budget constraints. The spreadsheet addresses all but the last two bullets, which are project specific. The spreadsheet models a single circuit in a system. Typically only the critical circuit is analyzed, selected by the user by inspection or by testing multiple circuits in the spreadsheet to see which requires the highest pump head. For each segment of the circuit, the user inputs pipe lengths and properties, valves and fittings, pressure drop of coils and other equipment, and optionally applies noise and erosion constraints. If the modeled circuit is indicated to be the critical circuit, the spreadsheet (within noise and erosion constraints) sizes the pipe for each segment to minimize first costs plus life-cycle energy costs. The spreadsheet includes a cost database of the following hydronic system components: • Piping;† • Fittings such as 90° elbows, 45° elbows, and tees; • Valves and accessories including calibrated balancing valves, check valves, ball valves, butterfly valves, wyestrainers, suction diffusers, and flow limiting valves; • Piping insulation (for hot and chilled water); and • Pumps, pump motors, and variable speed drives. Costs are based on RS Means Mechanical Cost Data,3 or local suppliers when items were not covered in Means. Adjustments for inflation can be entered to keep the cost data up-to-date. Adjustments for labor costs are included for various California cities or can be manually entered. Component costs can also be entered manually by the user. Energy Calculations Pressure drop through piping is calculated from the DarcyWeisbach Equation with the friction factor determined from the Moody chart4 as a function of Reynolds Number (including fluid temperature effects) and pipe roughness. Pressure drops through valves, fittings, and accessories are determined from manufacturer’s data (K-value or Cv) for representative products. Pressure drops for control valves, heat exchangers, etc. are entered by the user. Pump efficiency is assumed to vary with flow from a minimum of 55% to a maximum of 80% (user adjustable). The values were determined from typical centrifugal pump selections. Motor size for cost calculations is assumed to be the next size above the calculated brake horsepower. Motor efficiency corresponds to the minimum required for NEMA Premium Efficiency designation. All values may be overridden by the user. *The critical circuit is the circuit within the system with the highest pressure loss. This is often the longest run in the system, but not always. The pressure loss in this circuit, and only this circuit, determines the required pump head and drives annual energy consumption. † At this time only Type L copper and standard weight black steel piping are included since they are the most commonly used for HVAC applications. The spreadsheet allows the user to enter data for any piping materials. October 2008 ASHRAE Journal 25

Ashrae Journal - October 2008

Table of Contents for the Digital Edition of Ashrae Journal - October 2008

Ashrae Journal - October 2008

Contents

Commentary
Industry News
Letters
ASHRAE HQ Dedication Booklet
ASHRAE Rebuilds HQ to Reflect Mission
Feature Articles
ASHRAE's Living Lab
Meetings and Shows
Sizing Pipe Using Life-Cycle Costs
Feature Articles
Energy-Efficient Data Centers: A Close-Coupled Row Solution
The Future of 90.1
Ventilation Standard for Health Care Facilities
Grosvenor Humidity Chart
ASHRAE Research Report
ASHRAE Honor Roll
Building Sciences
Special Products
Emerging Technologies
Product Showplace
Classified Advertisting
Advertising Index
Ashrae Journal - October 2008 - Ashrae Journal - October 2008
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Contents