National Oilheat Research Alliance 29 The 1” valve has a Cv of 3.8 and creates 1 psi of pressure drop at a flow rate of 3.8 gallons per minute. One 1 psi of pressure drop equals 2.31 feet of head loss, and any additional head loss through the valve needs to be added to the total head loss of the zone. However, the actual flow rate through the valve is six gallons per minute. To get the correct calculation for total head loss, use the following formula: Head loss = ( Flow/Cv)2 x 2.31 In this example, divide 6 by 3.8 to get 1.58. Then square 1.58 to get a pressure drop of 2.5 psi. To turn that into head loss, multiply 2.5 by 2.31 for a total pressure drop through the valve of 5.8 feet. Add that to the combined head loss of the hard piping to obtain the total head loss the circulator will need to overcome, which in this case is 9.4 feet of head. To determine the appropriate circulator for a project, you would need to know the head loss and the flow rate. Assuming a 6 gallon-perminute flow rate and 3.6 feet of heat, it would appear that Circulator #2 or Circulator #5 would do the job. However, if a zone valve is present that increases the head loss (9.4 feet of head) then the proper choice is Circulator #6 for a single speed or the high speed setting on a three-Speed circulator Expansion tanks Sizing expansion tanks is easy—use the tank manufacturer’s guidelines if they are available. For example, some manufacturers’ recommendations can be found at this link: http://www.elmcoduddy.com/htx-sxht_sizing. pdf If manufacturers’ guidelines are not available, there are rules of thumb for sizing tanks: Diaphragm tanks—1 gallon for each 7,000 Btuh of heat load. For example, if the heat load is 100,000 Btuh, the tank should be a minimum of 15 gallons (100,000/7.000 = 14.3). Steel compression (air cushion) tanks—1 gallon for each 5,000 Btuh of heat load. For example, if the heat load is 100,000 Btuh, the tank should be a minimum of 20 gallons (1,000/5 = 20) For detailed information on properly sizing expansion tanks see page 3-28 in IBR Manual.
NORA Advanced Hydronics
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