Four buildings, shown in Figure as A, B, C, and D, are to be cooled using a central chilled-water facility, shown as Supply. The figure is close enough to a scale drawing to be used for preliminary layout purposes. Ethylene glycol, a commonly employed heat transfer liquid medium, is to be used. Average properties of the ethylene glycol in the temperature range of interest are density = 68.6 lbm/ft3, specific heat = 0.619 Btu/lbm-R, and viscosity = 0.0133 lbm/ft-s. Cooling requirements are as follows:
The head loss through the air-handling unit in each building is a function of the flow rate (cfs) through the unit. An approximate expression is 15.5Q2, where Q is the flow rate in cubic feet per second (cfs) through an AHU and the head loss is in ft-lbf/lbm. The head loss in ft-lbf/lbm across the chiller is taken to be 1.25Q2, where Q is the total flow rate in cfs. The temperature exits the chiller 25°F cooler than it enters. The lines are well insulated, so that heat loss is minimized. In addition, the following specifications must be met:
- Provide valves to isolate each building.
- Provide valves to isolate supply and return lines.
- Provide 35 ft of pipe in each mechanical room.
- Minimize tunnel length by placing lines in the same tunnel.
- Use a “Z” network (like Fig. P1-7).
- Pipe velocities more than 10 ft/s should be avoided.
- Limit the number of sizes (ANSI schedule 40) to 4 (buildings A and B, building C, building D, and the supply/return loop to the chiller and main pump).
- Provide valves to isolate each pump. (Note that items 1 and 2 may do the job.)
Accomplish a preliminary system design. At least the following should be specified for the system:
- Layout of the network
- Actual physical layout
- Loops arrangement for Hardy-Cross input
- Lengths and sizes of all pipes required
- Number and type of valves and fittings
- Increases in head and power delivered to the fluid for all required pumps
- List of materials (diameters, lengths, valves) required