Written By : Simon Teo
B. ENG (HONS)
The Ultimate Control Valves Used in Hydronic HVAC System
FlowCon International pressure independent flow control valves (SM) are changing the way control valves function and classified in a hydronic HVAC system. It is no more just an ordinary control valve that performed flow modulation in AHU or FCU only.
FlowCon SM valves can be classified as a combination of a Control Valve, Automatic balancing valve and also a Flow measuring device.
In FlowCon SM valves, the automatic balancing maintains and set a constant DP across the control valve. Looking at the Flow equation, the flow will remain constant of course unless you change the Cv.
Flow Equation Q =Cv dP/sg.
FlowCon SM valve can maintain the desired rated flow independent of any pressure fluctuation in the system.
It also comes with an actuator that can modulate the flow by changing the opening (Cv) of the valve. The microprocessor based actuator come with a self-calibrating feature for ease of installation. The actuator is capable of providing 4-20mA or 2-10 Vdc feedback signal to the control system for flow read out.
The working principle is as shown in Diagram A . P1 and P3 are system pressures. Its function similar to installing a pressure regulator across a control valve. The movement of the internal spring and rolling diaphragm maintain a constant DP (P1-P2) across the control valves. The flow does not change until the computer control system tells the actuators to change the valve position. The actuator controls the rotation movement of the cup and modulates the flow through the valve.
From the Diagram B below, as the minimum differential pressure is achieved the flow will remain constant as pressure differential varies.
Control Valves Used in a Variable Flow Hydronic System
Variable Speed Pump System
There are many advantages in using Variable speed drive in the secondary pumps system. One very obvious advantage is the energy conservation as the speed of the motor that drives the pump lowered due to lower demand for chilled/warm water. Any speed decrease will result in proportional decrease as the cube of the horsepower to drive the electric motor. For example a decrease in 20% of the speed will results in a drop of almost 51% of horsepower.
The decrease in speed will also result in decrease of head/flow of the pump. Under a lower head/flow condition and if no proper control or balancing happen at the coil terminal. The cooling or heating of the coil cannot be achieved and it would results in coils not able to handle the load.
In the past many methods were used for flow modulation and balancing in a variable volume system. The method if used wrongly would ruin the control system.
Running the System Using Conventional Control Valve and Manual Balancing Valve The variable flow system in Figure C 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 control valves have a pressure drop, when wide open, about 10' and balancing valves another 10' losses. Pump selected has total dynamic head (TDH) of 66'is large enough to handle the design GPM when all control valves are wide open. I have assumed no partner valves are installed at the branch and the riser.
Automatic balancing valves are installed at all three chiller 1,2,3 to maintain the rated flow and preventing overflowing. The chilled water from the primary chiller supplies chilled water to the secondary system.
The differential pressure sensor will maintain 34' differential pressure across the ends of the supply and return mains by operating the pump variable speed drive
The system is proportionally balanced by first disable the Differential pressure sensor “DP” and operates the pump at maximum speed. All modulating control valves should be open fully. Balance all balancing valve to get actual flow. To balance a system using manual balancing is a very tedious, time consuming and very often-impossible mission.
The Hydronic system is always dynamic, when adjusting one flow using the static balancing valve, the flow of the already balanced terminal will run out again. Most of the time the final system is left unbalance.
Figure C1 show what can happen after the balancing is completed and the controls are placed in operation. Assuming no load for coil 1 to 4. There will be no loses for supply and return piping. The maximum head available is set at 34'.
Using pump's affinity law where
There would be a flow deficit of 100-72=28%.
The flow deficit will result in AHU/FCU not able to handle the load.