ASHRAE Journal - September 2009 - 10

90 80 70 60 50 40 30 20 10 0 0 100 200 300 0% Stroke 100% Stroke 90 90 60% Stroke 80 70 60 50 40 30 20 0% Stroke 100% Stroke 60% Stroke 80 70 60 50 40 30 20 0% Stroke 10% 20% 30% 90% 80% 70% 50% 395 gpm 400 455 gpm 500 585 gpm 600 700 10 0 395 gpm 0 100 200 300 400 500 585 gpm 600 700 10 0 365 gpm 0 100 200 300 400 500 535 gpm 600 700 Figure 1: Bypass and circuits unbalanced. Figure 2: Only bypass balanced. Figure 3: Bypass and circuits balanced. The Case for Balancing Valves For several years, Messers. Gil Avery and James B. (Burt) Rishel have contributed articles to ASHRAE Journal on chilled water systems, control valves and balance valves. Appreciably, they stimulate a debate. However, it should not be done at the cost of ASHRAE Journal using “pseudo-engineering.” As a reader of the Journal, it’s hard to lay all of the blame for “Case Against Balancing Valves” by Mr. Avery and Mr. Rishel in the July 2009 issue at the feet of the authors. The Journal’s system of peer review is equally to blame for the presentation of this poor quality and fallacious article. Saving system energy use in chilled water systems has been addressed in ASHRAE Journal for many decades. For an example, see Gilbert F. Carlson’s threepart ASHRAE Journal series from February – April 1972, which was derived from the annual chilled water plant conference at Purdue University. Since the early seventies the industry has recognized, as written in ASHRAE Journal, that application of two-way control valves to water systems, and in particular chilled water systems, allowed for the reduction of pump energy use as a minimum. It has been recommended practice for at least that long. One would hardly expect that in 2009 we would be debating the merits of a “constant”-volume chilled water system with three-way valves. Added to this is the poor economic comparison used to 10 ASHRAE Journal provide a spurious analysis of balancing valves. Designers shouldn’t be designing a system in this manner for occupant comfort control. However, systems designed as “constant volume” with three-way valves were a fact of life, and may still be done in the manner that the authors contend. In that case, the Avery and Rishel article is riddled with mathematical errors that would suggest not using a balancing valve in a three-way valve system. The three figures in this letter illustrate the mathematical system calculations that describe the hydraulic interaction of pump curve, valve, pipe and balance, and use a pump curve similar to that of the article. Using basic knowledge of the system curve and the system flow coefashrae.org ficient (CS) as outlined in the ASHRAE Handbook, a system curve was created for the three-coil system presented, using as much as possible the data provided, to derive equipment head selections. Leaving both the three-way valve bypass and the circuits unbalanced, the result in Figure 1 is seen. The valve authority combined with no balancing yields a maximum flow 63% greater than design, and a closed or open flow to the coil 38% greater. If only the bypass were balanced, the system flow would be adjusted, as seen in Figure 2. This still provides 38% more flow at open or closed states, but moderately less than previous when at the midpoint of stroke. Balanced, the system provides flow, as in Figure 3. Note that design flow occurs only when the valve is fully open to the coil or bypass. This is the age-old problem of three-way valves, and has been welldocumented. Minimally, the balancing valve is reducing the flow at 0% and 100% stroke by 38%. That’s a good thing (in a poor system design); it saves horsepower, a fact that the authors try to ignore. Rethinking their statement, “The energy waste of balance valves is due to their use being based upon constant flow in a system that has a variable load,” the balance valve for the system balance should only reduce head in the two branches closest to the pump. A balance valve in the September 2009

ASHRAE Journal - September 2009

Table of Contents for the Digital Edition of ASHRAE Journal - September 2009