Product Overview | Valve
CORRECT VALVE SIZING Made Easy
!
Curtain ½ open Core full open
Curtain ¾ open Core rotated ¼
Curtain ¾ open Core rotated ½
Figure A. Turning curtain adjusting knob changes port opening so valve can constitute optimum resistance in the system for good control.
For a furnace operated by a modulating type air control valve, either a linear or an equal percentage characteristic is preferred for most applications. These characteristics are shown in Figure B, together with two other curves indicating very common but usually undesirable flow patterns: In their lower parts, change of flow is too fast, causing an "over correction;" while at the upper end there is practically no flow change in response to valve movements. Shape of a flow curve depends (1) on valve area characteristic , and (2) on size of valve port relative to downstream resistances (piping, orifices, and burners), the sum of which is difficult to predict in some cases. An Adjustable Port Valve solves the problem. Its linear area characteristic permits proper "sizing" of valve after installation without involved calculations.
Figure B. Flow characteristics of valves installed in systems with downstream resistance. The three solid linesrepresent characterizable adjustable port valves.
Recommended procedure is to fully open valve control handle and valve curtain, then close curtain until a change in pressure is noted downstream of the valve. Flow characteristics shown as solid lines in Figure B can be realized with full valve stroke, enabling desired flow response to movements of the control motor.
Linkage between motor and valve can be "characterized" to produce the desired flow curve, e.g., linear or equal percentage.
PACKAGING
The adjustable port valve can be sold alone or with a bracket and linkage to be mounted to an actuator in the field. They are also available pre-packaged with an actuator mounted and linked from North American. See Bulletin 1020 for standard llinked motors. The valve can also be direct coupled to an actuator. See Bulletin 1199.
CURTAIN ADJUSTING KNOB
CONTROL HANDLE
raises or lowers
rotates
CORE
CURTAIN
to change width of previously adjusted
to adjust height of
PORT OPENING
PORT OPENING
in response to control instrument.
for best flow characteristics.
Figure C . Adjustable Port Valve (APV) for cold or hot air, 3 psi or less. All APVs have sliding gate action and rotary plug action.
Bulletin 1004/1014 Page 2
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