An automatic flow control valve is a device which controls a flow (fluid/steam) to within +/- 5% of a specified flow rate regardless of head pressure provided they are selected within there control range of which there are a number of ranges from 2-32 PSI up to 8-128 PSI. This means that as system diversity sets in, the valve automatically either exposes more flow area (increase in system header pressure) or reduces the amount of exposed area (increasing system resistance pressure) to deliver the correct flow to the applicable piece of equipment. It achieves this feat using a machined stainless steel piston engineered to proportionally reduce/increase the surface area exposed to the media flow path by compressing/elongating an engineered, preloaded and individually tested spring assembly; the piston/spring assembly are enclosed in a tamper proof piston cartridge.
They typically replace manual throttled circuit balancing valve or simple "non flow meter" devices; think of them as a manual valve with a dedicated person to open and close the valve as system pressures change. Yes, they are more expensive than the valves they typically replace (except in the case of a steam application). However, as we have all come to learn at one time or another, what you see is not always what you get!
Consider a typical HVAC system with HW or CHW coils, manually balanced; this system is going to require the following valves not required when automatic flow control valves are used:
* triple duty or circuit balance valves each pump
* circuit balance valves at the mains
* circuit balance valves at branches/risers
* throttling devices on by-pass associated with 3-way ATC valve terminal units
The reason these devices are not required is that when using automatic flow control valves the system is balanced from the terminal units in that system flow is determined by the simple addition of the terminal unit loads (as well as any future capacity loads). Automatics flow control valve designed systems are self-balancing when properly designed and installed. Add up the costs of these components, the installation of these items, the additional T & B (test and balance) fees associated with manual devices and its should be easy to perceive from a strictly material/installation stand point, automatics do not add as much money to a job as some may perceive.
Now, lets consider operating costs. In a two way or three way ATC valve application, automatics prevent overflow. A manually balanced system is set up to deliver 90-100 % of the design flow by the T & B contractor to handle loads associated with "peak" loads.
With two way ATC valve terminal units, as diversity sets in, the ATC valves begin to modulate or close and header pressures go up. A manually throttled device will simply allow more water to flow through the fixed orifice and pump horsepower is wasted.
When examining a 3-wayt ATC valve application, how does an automatic prevent overflow? First, it is imperative to accept and believe that "two equal resistors in parallel will flow 1.41% of the original (if you don't believe this, ask someone in your electrical department). Secondly, it is important to understand how a terminal unit using a 3-way ATC valve is balanced by the T & B Contractor. The terminal unit is balanced by flowing all water through the coil and setting the throttling device to achieve design flow; then, all the water through the by-pass leg, throttling another device to create artificial head equal to the resistance of the terminal unit pressure drop. Here's where it eliminates overflow - as the 3-way ATC valve modulates to the by-pass mode, you begin to have two equal resistors in parallel. The result is overflow and water pump brake horsepower.
Automatic flow control valves create artificial head in both examples above to prevent any unnecessary overflow and the resultant additional wasted brake horsepower required by the pumps. This in mind, automatic flow control valves also prevent underflow problems associated with variable speed systems as header pressures decrease.
ABSOLUTE MYTH: "Automatic flow control valves add too much head to the system"
*automatics require 2 PSI to be added to the pump head "index circuit" (i.e., that which determines pump head ; worse case scenario pressure loop ); typically, with a manual balanced system, an additional 10 to 15' is added to overcome the balancing valves as there are typically three of these valves required "in series". Not so with automatics!
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