DKCD
VD.51.Q1.
12 © Danfoss 10/2003
5
Sensor operating principle
The RA2000 thermostatic sensor consisting of a bel-
lows system and saturated vapour charge provides
proportional control, as opposed to On/Off. The sen-
sor is actuated by ambient temperature changes.
The calibrated pressure in the bellows corresponds
to the temperature of the charge. This pressure is
balanced by the force of a regulating spring.
On a rise in ambient temperature, the vapour pres-
sure increases in the bellows, moving the valve
cone towards the “closed” position, until equilibrium
exists between the bellows and spring. On a drop in
ambient temperature, the vapour pressure falls, al-
lowing the bellows to contract and the valve cone to
move towards the “open” position until a state of
equilibrium is re-established.
The vapour charge will always condense at the cold-
est part of the sensor, usually that furthest away
from the valve body. The radiator thermostat will
therefore always respond to changes in room tem-
perature and will not be affected by the water tem-
perature itself. However, when the air around the
sensor is heated by the hot valve body and
pipework, then the sensor can register a higher tem-
perature than that of the rest of the room. For this
reason, Danfoss recommend that the sensor is
mounted horizontally whenever possible.
To ensure that the valve capacity of a fully opened
valve is not significantly greater than that at 2
degrees C P-deviation, all RA-FN valves have a
modified valve characteristic. This improved valve
characteristic maintains hydraulic balance during
start-up.
Proportional band
Proportional temperature controls try to maintain
equilibrium between heat loss and heat input. They
react proportionately to any deviation from a set
temperature level, until heat input is either com-
pletely shut off or is at its maximum. The tempera-
ture levels at which this occurs are presettable, and
the band between the two levels, measured in K
(degrees C) is the proportional band.
Danfoss radiator thermostats can be sized to oper-
ate across a defined proportional band. A range of
0,5 – 2K is recommended. In providing proportional
control, they ensure smaller swings in room tem-
perature than, say, a conventional room thermostat
which gives on/off control and needs to cycle
throughout its switching differential each time it op-
erates.
1.
Thermostatic element
2.
Bellows
3.
Setting dial
4.
Setting spring
5.
Limit pin
6.
Pressure pin
7.
O-ring seal
8.
Valve cone
9.
Valve body
10. Nozzle
Data Sheet
Radiator thermostats for one-pipe and two-pipe systems
Hydraulic balance
Definitions
kv-Coefficiant of flow
The kv-value of a valve shows the quantity of liquid,
in m3/h, that the valve will pass with a pressure
drop of 1 bar across the valve. This value is used in
the following formulae to calculate flow rates for
valve sizing purposes.
