Danfoss RA 2000 Datenblatt - Seite 4

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Data Sheet

Setting the temperature

Radiator thermostats for one-pipe and two-pipe systems

Radiator thermostats are sophisticated proportional

control.

1) The operating principles of thermostats

The function of the actuator unit is to sense the am-

bient temperature, compare it with the required tem-

perature, and correct the valve setting in accord-

ance with the difference, as illustrated in fig. 1.

European Standard EN215 requires that radiator

thermostats meet various operating criteria. Danfoss

radiator thermostats meet or exceed these require-

ments in every respect. A brief description of the key

considerations is given below.

2) Differential pressure dependence.

The closing characteristic of a radiator thermostat

depends on the pressure conditions in the valve. In

fig. 2, curve 1 applies to the actuator unit without

counterpressure from system differential pressure,

while curve 2 shows the offset when the

counterpressure increases proportionately to the

valve closure. The effect is a result of the elasticity

of the actuator unit which should, therefore, not be

too large.

3) Time constant

To give reasonably fast and stable control, a radia-

tor thermostat must not be too slow in its reaction to

temperature variations. The time constant (fig. 3)

depends on the specific heat and surface of the sen-

sor.

At a sudden change of the ambient temperature at

time 0, the thermostat will react as shown, relative

to time. The time constant is defined as the time

used by the spindle for 63% of its full movement.

4) Flow temperature dependence

Flow temperature dependence occurs largely in the

case of actuator units with built-in sensors since

heat transmission takes place from the valve body

to the sensor of the unit (Fig. 4), with the possible

effect, under full load conditions, of controlling the

room temperature at a lower temperature than re-

quired. Insulating plastic materials for thermostat

base, internal heat shields and good product design

in positioning the sensing element away from the

heat source, help reduce this dependence. Danfoss

recommend that radiator thermostat sensors are

mounted horizontally, away from the effects of heat

rising from hot valve and pipework.

5) Hysteresis

If the spindle movement of an actuator unit is

measured at, first, increasing and then decreasing

temperatures, there will be a difference between the

point where the valve just closes and the point

where it opens again – see fig. 5. This difference is

termed hysteresis and is caused by friction in the

actuator unit and valve body. It is desirable that

hysteresis should be as small as possible.

DKCD