The oversized control valve will magnify the valve authority problem, which results in hunting of the conventional control valve. The consequences of hunting are shortening actuator and seat life expectancy. More hunting also means decrease occupant comfort. An oversized control valve cost more, may require a larger and more expensive actuator, and have a larger leakage and low close off pressure.
Variable Flow System that control by Using Flowcon SM Control Valve
The FlowCon SM control valves works effectively in both the fixed speed and the variable speed pump system.
Figure E show a typical primary-secondary pump system, where FlowCon SM pressure independent automatic balance modulating control valve were installed for ultimate energy saving and control. Here the SM valve function as a modulating control valve for precise temperature controls and also automatic balancing valve. The SM valve selected has a pressure drop of about 35kPa, this pressure drop will be less than a conventional control valves in combination with balancing valves.
Above all no partner balancing is required at the branch and risers. This eliminates unnecessary valves with big losses at the branch and risers if conventional control valves and manual balancing are used.
This result in a system with lower totals losses (10% - 40% lower heads).
A smaller secondary variable speed pump and smaller piping can be selected. Pump selected has a head of 60' compare to 66' for the other design. A smaller motor is needed to drive the secondary pump, end up huge saving in utility bill. This type of system can quickly pay for themselves with savings in energy and power.
The selection of valve does not require any Cv calculation. This type of valve only depends on flow and pipe size for valve selection.
FlowCon SM valves used in a Dynamic Variable Speed Pump System results in energy saving. Also it lower the initial system cost due to smaller pumps, smaller piping and lesser total valve involved. The saving could be as much as 20% to 30% of the total cost for pumps, piping and valves.
Running the System
The variable flow system in Figure E has five coils of the same size of 10' losses. The mains are sized for a 4' drop between adjacent coil branch supplies and a 4' drop between adjacent coil branch returns.
The SM control has a pressure drop, when wide open, about 14'. Pump selected about 60'is large enough to handle the design GPM when all control valves are wide open. Automatic balancing valves are installed at all three chiller 1,2,3 to maintain and preventing overflowing. The chilled water from the primary chiller supplies chilled water to the secondary system.
The differential pressure sensor will maintain 28' differential pressure across the ends of the supply and return mains by operating the pump variable speed drive. Also as the zone require the SM valve to decrease the flow of the valve. The differentiate pressure of the SM valve will increase. The increase of the differentiate pressure for the index terminal will facilitate VSD to change speed of pumps through signal from the DP sensor transmitter.
During start up when the differential pressure sensor is by-pass and the pump is at maximum speed, all the flow at the fully opened coil 1-5 are automatically balance and the system is ready for full operation. SM valve is a 100% authority valve, it will not hunt as the internal pressure regulator across the control valve ensure same differentiate pressure regardless of any opening of the control valve. The SM valve controls the flow accurately from fully open to full close position to provide precise temperature control. There will not be any flow deficit at any operating condition when use with variable speed pump. Looking at Figure E1 when coils 1 to 4 are at no load condition. The maximum head now dropped to 28'. The losses of the SM valve at coil 5 dropped from 46' to 14'. There would be no flow deficit.
The system will be very stable, as pressure fluctuation cannot change the flow.
This type of system with SM control valve optimizes system control and performance and maximizes energy consumption .
From the finding, I concluded the type of balancing and control valves selected will affect the performance of a variable speed Hydronic HVAC system. Using manual balancing valve has balancing and flow problem that lead to coil under performing. Automatic balancing valve solved most of the flow deficit problem. The system using this method will work only if the control valves are sized properly and the actuators are large enough to position the valve plugs properly.
There is a saying that one does not need any balancing valve for flow control. This statement is grossly wrong, as the system will not be balance. All the problems that were discussed regarding valve authority due to valve over-sizing of a conventional control valve would apply. The system using only control valve will not be steady as when one control valve start to modulate the rest of control valves will also start to move too, as the pressure in the system changes. Eventually the system will start to hunt until the right temperature is achieved.
The ultimate solution for all the above would still be using the state of art Flowcon SM Pressure Independent Modulating Control Valves.