Data Sheet for Product

CA1N4028E / 11.1998 Siemens Building Technologies
8/12 Landis & Staefa Division
High resolution
The fast, sensitive response of the magnetic actuator and the short stroke of
the valve disc combine to give a valve with a high resolution. Even the
smallest control deviations result in a change in the stroke and hence a
change in the flow rate.
The valve controls the flow accurately across the full stroke, even at the
point where the valve characteristic (K
v
as a function of the stroke) is
relatively steep. The average resolution (∆H : H
100
, where H = stroke) of the
standard Landis & Staefa disc valves is 1 : 200.
The ideal valve characteristic for control of partial loads
Figures 15, 16 and 17 show the most common types of valve characteristic in
the field of HVAC controls. Figures 18 and 19 are examples of characteristic
curves measured with Landis & Staefa valves across the entire stroke, and
Fig. 20 depicts the curve in the low opening range (0 … 10 % stroke).
This shows the following:
– A typical feature of the Landis & Staefa valve is the flatness of the curve
in the low opening range near the zero point. This is the area where the
curve differs significantly from the normal linear response. The curve is
concave, and there is no jump on start-up. The valve is optimised in the
low opening range resulting in “drop-by-drop” control.
– In the curve in Fig. 20, the slope tolerance, as defined in the VDI/VDE
guidelines, is exceeded at the point where kv = 4 %. However, these
tolerances are not relevant for the Landis & Staefa valves, because as
stated above, the flow is still fully controllable even where the curve is
this steep. In the case of the Landis & Staefa valves, the leakage rate k
vo
corresponds to the relevant (critical) limit value k
vr
as defined in the VDI/
VDE guidelines.
The k
vr
value is normally used to calculate the rangeability, S of a valve:
S = k
vs
: k
vr
Conventional valves have a rangeability of between 50 and 100. Since Landis
& Staefa valves are not subject to a maximum slope tolerance in the normal
sense, there is no corresponding kvr. Hence, S would need to be calculated,
using k
vo
, giving the theoretical rangeability of the valve. With a leakage
rate k
vo
= 0.05 % k
vs
, the rangeability is 1 : 0.0005 = 2000. The ability of the
Landis & Staefa valve to control the flow rate in the low opening range is
thus so good, that the concept of rangeability as a criterion of quality is irre-
levant. The VDI/DE guidelines were written with conventional valves in
mind, and do not take account of the Landis & Staefa magnetic valve.
Theoretical valve characteristics
The linear curve (see Fig. 15) is based on the following equation:
k
v
= k
vo
+ n
lin
· H
where H = Stroke [mm]
k
v
= Specific flow rate [m
3
/h]
k
vo
= Flow rate where H = 0
(Jump on start-up)
n
lin
= Steepness of curve
( = 1 - ––– )
k
vo
k
vs
94.0144
H
k
v
k
vs
k
vo
H
100
Leakage rate =
Jump on start-up
Fig. 15: Linear curve