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Data Sheet

Sensor operating principle

Thermostatic element

Bellows

Setting dial

Setting spring

Limit pin

Pressure pin

O-ring seal

Valve cone

Valve body

10. Nozzle

Hydraulic balance

Definitions

DKCD

Radiator thermostats for one-pipe and two-pipe systems

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.

VD.51.Q1.12 © Danfoss 10/2003

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.