From Constant To Variable FlowWith the TAB portion of the work beginning to grow significantly as the commissioning role grows, it got me thinking. Why do we seem to specify balancing valves and pressure-independent regulating valves to compensate for what I would consider to be marginally efficient water control valves?
In the past, when design engineers were providing chilled water and hot water constant flow systems, these balancing valves were needed at terminal unit coils to assist in maintaining a constant design flow to a coil or around that coil if the unit had a three-way control valve.
The energy crisis changed the way we designed, and we began to specify two-way control valves that provided variable chilled water and hot water flow. To make this energy measure work efficiently, we added pump head pressure control followed by variable-frequency pump motor drive to these variable flow systems.
While we made this transition from constant water flow to variable water flow, a similar situation was occurring on the airside of the HVAC business. Back in those constant airflow days, we were specifying constant volume supply air systems. In these air systems, you could find volume dampers and splitter dampers to help direct and control flow of air to the individual air devices.
Once the energy crisis took hold in the 1970s, we began designing VAV systems to reduce the energy waste associated with constant volume systems. In the process of changing the way we designed duct distribution, we abandoned the use of volume dampers, volume extractors, and splitter dampers. The conversation to variable volume air systems raised our awareness to how we could make these systems more energy efficient with a little more engineering expertise.
Bringing Waterside Up To SpeedNow think about this. Some 25 years ago, we were able to begin the elimination all those volume control devices, so why haven't we progressed on the waterside design of HVAC systems? This new generation of variable volume air systems seems to provide reliability when it comes to delivering the supply air we need, so why haven't we re-engineered how variable volume water systems function? If we can eliminate what we considered to be control devices in duct distribution design, why can't we eliminate all those pre-energy crisis water-balancing devices and specify efficient chilled water and hot water control valves that function without ancillary devices?
Today, I see more and more installations specifying a water pressure-regulating device upstream of a terminal unit automatic temperature control valve. These devices added resistance to the pumping system, which, in turn, consumes more pump horsepower energy. From a TAB point of view, it seems to me we could reduce energy by eliminating these devices if we understood pumping a little better. Why has it been that we took a giant leap of faith and eliminated all those volume devices in air systems without question but, when it comes to water systems, we have added more devices to control variable flow?
Have we added these water-regulating devices because we want to be conservative and safe with our design? Will this make our design work better because we don't have confidence in the TAB contractor doing the job and so these devices will save the day? Shouldn't we look at how a water control valve works and clearly understand how it will respond to pumping system pressure as the terminal devices receive the flows it need? Wouldn't better automatic temperature control water valves improve system performance at less first cost and operating cost then the cost associated with furnishing, installing, and maintaining these pressure-regulating devices? Isn't this what engineering is all about, designing the optimum system for the optimum cost? I don't think engineering is all about putting in enough devices to be safe with the design. It appears we have a double standard for how we design air systems vs. how we design water systems, and it's time to reassess our water system design guidelines.
A good start to understanding water system design is to understand control valve performance and how a TAB contractor will set up the water system. Don't add pressure-regulating valves until you understand the impact of these devices on the efficiency of the system you are designing. Let's rethink water balancing. ES