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Plumbing Engineer - Features: February 2010: Hot Water Recirculation in High-Rise Buildings

Hot Water Recirculation in High-Rise Buildings

 

Pumping systems are contributing significantly to the success of this trend through the use of alternative applications including variable frequency drives, water heaters with minimal or no storage, and innovations such as incorporating expansion tanks and jockey pumps in pressure booster systems to maximize off-peak performance.

 

Hot water recirculation pumps are being applied to systems in order to decrease the unnecessary “dumping” of cold water, and the resulting convenience coupled with water savings, both offer significant benefits to the user. Such recirculation pumps typically have fractional horsepower motors and run constantly. In a typical 2- 3-story hospital or school building, recirculation works well because the circulators only have to overcome friction losses in the piping and function simply as closed loop pumps.

 

However, in a high-rise building, the issue is more complicated. A typical 26-story high rise building may have three recirculation pumps and hot water heaters at the top of the building in a down-feed style supply. Water is boosted from the basement to the top of the building, and in the illustration shown in Figure 2, pressure of 38 psi must be maintained at the top.

This forces the water through PRVs in each lower zone in order to avoid excessive pressures at the fixtures. The pump must act not only as a recirculation pump but also as a pressure booster. Pump P3 in Figure 1 would be sized around 15 GPM @ 210 FT to overcome friction losses and the static height of the building, and it must be capable of pumping potable water but the standard 175 psi seals become marginal and pump efficiency at this low flow is poor at best.

 

One way to separate the negative effects of the PRVs is to ‘decouple’ the recirculation loop, and this can be done with brazed plate heat exchangers, which provide a cost effective solution to the issue. We can save operating horsepower by using a low-head (pressure) pump instead of a high-head pump. The initial cost of a small single-wall brazed plate heat exchanger and the low-head pump is comparable to the cost of a high-head pump. Each isolated group does need its own tank, isolation and balancing valves, but the balancing act is simplified since we don’t have to try to balance multiple high-pressure pumps on various floors with each other. We can even run the secondary zones with aquastats. The net effect is a more efficient and quieter running plumbing system.

 

To illustrate a more efficient and quieter running plumbing system, see the drawings, left, above. One shows the traditional method of piping a domestic hot water recirculation system, and the other shows an alternate system using a “decoupled” approach. De-coupled recirculation offers significant benefits and should be considered for greener high-rise building applications.

Information for this article was provided by ITT/Bell & Gossett.

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