Figure 2. loss of insulated copper pipe is typically a low flow rate. 4. The recirculation return line will be equal to the length of the supply line (usually). For example, as shown in Figure 2, if we had the following residential application: • 100' of ¾" un-insulated copper pipe supplying hot water throughout the house to the furthest fixture. We can see from the sizing chart that 100' of ¾" uninsulated copper pipe loses about 3000 btu/h. The job of the recirculating pump is to provide enough flow in gallons per minute (GPM) to replace this lost energy. Figure 3. Temperature Drop BTU/GPM Relationship 10° 5,000 B/Hr. = 1 GPM 15° 7,500 B/Hr. = 1 GPM 20° 10,000 B/Hr. = 1GPM In sizing residential re-circulating lines, the acceptable temperature drop is 10°F (instead of the standard 20°F we use for hydronic heating systems). We can then calculate the required flow rate by using the following formula: GPM = BTU/H/ 10°F delta T x 500 or GPM = 3000/5000 = .60 gpm The next step is to establish the head-loss of this flow rate through the re-circulating line. In Figure 5, the re-circulating line will be the same length as the supply main, which is 100 feet. Using ½" Type L Copper Piping for Return Line: Head Loss = (Friction Loss from System Syzer) x Length Type L Copper Pipe x 1.5 (to account for fittings) Head Loss = (1 ft friction loss/100ft) x 150 ft equivalent piping = 1.5 ft head loss Head Loss = 1.50 ft head loss So what pump do you use? For domestic hot water systems, the recirculation pump must be suitable for potable water. That means all the working parts exposed to domestic water are brass, bronze, stainless steel or non-ferrous in order to resist the corrosive attack of Figure 5. oxygenated fresh water. Conventional iron body hydronic system pumps should not be used. This pump must be able to pump .6 gpm while overcoming the head loss of 1.5'. As you can see, the smallest traditional 3-piece bearing assembly-style circulator or the modern wet rotor permanent split capacitor (PSC) circulator is more than big enough—of course over the last few years electronically commutated magnetic motors (ECM) circulators are gaining traction in North America—and with this new technology comes nice features. Because of the magnetic motor, all of the ECM pumps have a microprocessor on board and the processor offers intelligence to control the pump and its operating characteristics. So, in a simple residential DHW recirculating application, the pump’s speed could be selected to more closely match the system needs and significantly reduce the KW consumption of the recirculating pump. ICM Questions or comments: gcarey@fiainc.com, call 1-800-423-7187 or follow on Twitter at @Ask_Gcarey. Figure 4. BTUH heat loss per 100 ft for tubing and steel pipe Pipe or Tube Size Insulated Copper Tube or Steel Pipe Non-Insulated Steel Pipe Non-insulated Copper Pipe 1/2" 1,600 4,000 2,300 3/4" 1,800 5,000 3,000 1" 2,000 6,000 4,000 1 1/4" 2,400 7,500 4,500 1 1/2" 2,600 8,500 5,500 2" 3,000 11,000 6,500 2 1/2" 3,400 12,000 8,000 3" 4,000 15,000 9,500 4" 4,800 19,000 12,000 5" 5,700 22,500 6" 6,600 26,000 ICM/January/February 2017 11
ICM January-February 2017
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