|• Harder Implementation of change and churn
||• Easier implementation of change and churn
|• Better control at low and ultra–low flow rates
||• May not control well at low flows (<0.012I/s)
|• More likely to give flow repeatability
||• Flow repeatability may not be as good
|• Can cope with up to 12 bar differential pressure
||• Can cope with 4 bar differential pressure
|• Individual branch flows need regulating
||• Flows can simply be set at the valves
|• More measurements points for trouble-shooting
||• Fewer measurement points
As the flow enters the module, a large bodied strainer removes all particles greater than 0.7mm in size. This ensures that valves with narrow clearances (such as 2 port valves and PICVs) will not become blocked during normal operation. The strainer is well placed since all of the pipes downstream of the strainer are made from non-corrosive materials i.e. brass of or plastic. This means that, unlike rigid systems where steel pipes are located downstream of strainers, there is no risk of re-contaminating the water.
The flow manifold incorporates isolating valves so that individual branch circuits can be isolated. Each flow connection also incorporates a modulating 2 port control valve to provide accurate control of flow rates and hence heating or cooling outputs from terminal units.
Flow balancing is achieved by means of double regulating valves installed in the return pipes from the terminal units. Double regulating valves are installed next to each of the ports from a manifold. The regulating valves can be used for flow regulation and isolation.
The main return pipe from the module contains the differential pressure control valve, and the capillary tube governing the DPCV is connected to the flow manifold. This means that in operation, the differential pressure control valve will maintain a constant pressure differential between the flow manifold and the DPCV inlet. Since the DPCV inlet pressure is effectively the same as the pressure in the return manifold, the result is that a constant pressure differential is maintained between the flow and return manifolds. As 2 port valves open and close or the pump varies its speed the DPCV will respond to constantly maintain the pressure differential.
Individual flows through the terminal branches can be measured using the “subtraction method”. The flow through each terminal branch can be measured by closing the branch and registering the drop in flow rate through the flow measurement device.
Pressure independent control valve (PICV) modules incorporate many of the same features as DPCV modules except that the separate DPCVs and 2 port control valves are replaced by PICVs on each terminal branch. Since PICVs incorporate 2 port control and differential pressure control, there is no need for separate valves.
Differential Pressure Control Valves (DPCV)
The function of a differential pressure control valve (DPCV) is to maintain a constant differential pressure between two points in a pipework system that are either side of a variable resistance.
The diagram below shows the basic design for a DPCV. The valve will automatically control a constant pressure between points A and B.
The disk shaped housing at the top of the differential pressure control valve contains a flexible diaphragm. A capillary tube from the flow pipe is connected to the upper side of the diaphragm whilst the lower side is exposed to pressure from the return pipe. Once the DPCV valve is set, any variation in pressure between the flow and return pipes will be sensed automatically causing the diaphragm to flex resulting in movement of the valve stem.
If the pressure available in the particular flow pipe should increase, then the differential pressure valve will close in order to take out the excess pressure. If the pressure available should reduce, then the differential pressure valve will open so that more pressure becomes available.
Similarly, if two port valves within the variable load served should begin to close, then the differential pressure valve will also begin to close in order to maintain the same overall pressure drop between flow and return pipes.
To provide accurate control of pressure, DPCVs must themselves establish some degree of authority over the circuits they control. They therefore typically require a minimum differential pressure across them of 10-30kPa depending on the pressure loss around the circuit in which they are installed.
Pressure independent control valves (PICV)
Pressure independent control valves (PICV) combine the 2 port valve and differential pressure control valve (DPCV) into a single body. Therefore the valve is self-protected against excess pressures. Because the integral DPCV holds the pressure differential constant across the integral 2 port control valve, the result is that whenever the control valve is fully open, the flow rate through the valve always returns, approximately, to its set value (since a constant pressure differential across a fixed resistance results in a constant flow rate) The diagram below shows the design of the PICV.
The opening through the 2 port control valve can be varied manually, and can therefore be used to regulate the flow rate through the valve to the required design value. A flow setting dial on the valve spindle can be used for this purpose. Once set, the valve should perform the function of a constant flow regulator (or “flow limiting valve”) whenever the 2 port control valve is fully open. Only when the control valve begins to close might the flow rate change from its set value.