Data Sheet for Product
CA1N4028E / 11.1998 Siemens Building Technologies
10/12 Landis & Staefa Division
94.0149
Measuring point
Theoretical
characteristic
0
k
vr
= 4 %
[%]
k
v
k
vs
Stroke
012345678910[%]
Slope tolerance
1
2
3
4
5
6
7
8
9
10
k
vr
= 0.3 %
The practical benefits: an even smaller minimum controllable
heat quantity
In HVAC control engineering, hydraulic valves are normally used in
conjunction with a heat exchanger. The valves control the flow of cold or
hot water. The heat exchanger generally has a steep characteristic in the
partial load range (see Fig. 21). Thus, even with low rates of flow, a
relatively large amount of thermal energy is transferred to the air. If the
valve responds with a “jump” on start-up, the minimum controllable heat
quantity is significantly greater. This is not at all desirable in control terms,
because it can cause the system to hunt. What is essentially a modulating
form of control will become on/off control at this point.
A diagram (Fig. 21) is used to determine the minimum controllable heat
quantity. For this purpose, two key values are required: the a-value (design
characteristic) of the heat exchanger (i.e. the relationship between the
difference in water temperature and the differential between the water
inlet and air outlet temperature) and the valve authority p
v
(= relationship
between the pressure differential at the valve and the overall pressure diffe-
rential in the variable-volume section of the hydraulic circuit).
Typical values are: a = 0.5 and P
v
= 0.5. We can now compare a Landis &
Staefa valve with a conventional valve on the basis of these values:
– Conventional valve: S
v
= 50
– Landis & Staefa valve: S
v
= 200
For the Landis & Staefa valve, the resolution should be inserted here, as it is
lower than the rangeability of the valve body. (In the case of the
conventional valve, the influence of the actuator has already been taken
into account).
The following values can now be extracted from the diagram:
– Conventional valve: Q
min
= 5.8 % Q
N
– Landis & Staefa valve: Q
min
= 1.4 % Q
N
Fig. 19: M3P25F valve characteristic (across full stroke)
94.0148
Measuring points
Theoretical
characteristic
0
10
20
30
40
50
60
70
80
90
100
110
k
v100
= 92.63
[%]
k
v
k
vs
Stroke
0 102030405060708090100[%]
± 10 %
Slope tolerance
Fig. 5
Fig. 20
Fig. 20: M3P25F valve characteristic (low opening range)