Data Sheet for Product

ManualsBrandsSiemens ManualsMagnetic (BC2)Stetige Regelventile, PN16, MXG461.., MXF461..

Siemens Building Technologies CA1N4028 / 11.1998

Landis & Staefa Division 1/12

Construction and principle of operation

Introduction: The valve in the HVAC control loop

Both the static and the dynamic behaviour of the valve play a major part in

ensuring fast and accurate control of HVAC systems (Fig.1). The precise

conversion of the control signal into a stroke movement has a significant

influence on the stability and accuracy of the control loop.

This data sheet describes the technology applied (Fig. 2) and considers

aspects of the valve’s construction (Fig. 3) with regard to the specific

demands of HVAC.

Fig. 2: The valve: An outline of the basic technology

Fig. 1: The valve in the HVAC control loop

94.0204

Static

Dynamic

MMI

Controller

Sensor

Valve

Heat

exchanger

electrical

mechanical

hydraulic thermic

Phase cut

Pressure

balancing

Valve

seat

Hydraulic

circuit

Control signal

Power supply

94.0205

Electrical

interface

Actuator Valve body

amplified /

converted

mechanical hydraulic

Hydraulic / thermal capacity

Valve authority

Flow rate (k

)

Input characteristic

Linearity

Resolution

Positioning speed

–

Valve

Magnet

The modulating

magnetic valves

(original product range of

Staefa Control System)

028

Summary of content (12 pages)

PAGE 1
4 028 Construction and principle of operation The modulating magnetic valves Introduction: The valve in the HVAC control loop Both the static and the dynamic behaviour of the valve play a major part in ensuring fast and accurate control of HVAC systems (Fig.1). The precise conversion of the control signal into a stroke movement has a significant influence on the stability and accuracy of the control loop.
PAGE 2
The technology in practice / Features of the valve 94.0226 1 2 3 a b c d e f g h i k Inlet port Bypass Outlet port Magnetic coil Core Spring Stem Disc Upper / lower valve seats Bellows Aperture for pressure balancing Handwheel Electrical interface i k I a b 0 ... 20 V– c d g 1 h 1 3 f 3 e 2 2 Fig. 3: Diagram and cross-section of a magnetic valve Conversion from positioning signal to magnet voltage The electrical interface (Fig.
PAGE 3
Conversion from electrical to mechanical energy a) Magnet voltage → magnetic force In the ferromagnetic circuit, which consists of the casing (yoke) and the moving core, the voltage applied to the coil (Fig. 3) produces a magnetic field. Where the greatest magnetic resistance occurs – in the air gap – this field causes a force of attraction which becomes stronger as the voltage increases. 94.0207 Coil (a) (armature) Core (b) Casing (yoke) Air gap Pole Fig.
PAGE 4
c) Valve control range The full control range consists of three partial ranges: – Upper valve seat opening / closing range: The closing force of the flexible disc also works here against the force of attraction. The behaviour of the valve is characterised in this range by its extremely high resolution, making very fine modulating control of fluids possible. – Stroke operating range: Here, the force of attraction works only against the force of the spring.
PAGE 5
• Overcoming the pressure forces Differing pressures across the control path affect the magnetic force, but this potential problem can be virtually eliminated through pressure balancing. This ensures that the magnetic force is primarily used for flow control. Pressure balancing is an extremely effective solution and is achieved in practice with the aid of a bellows (Fig.10).
PAGE 6
Relationship between flow rate and heat quantity When the Landis & Staefa magnetic valve is used as a simple straightthrough valve (port 2 closed) the disc provides throttle control in response to the positioning signal from the controller. When used as a three-port control valve, water from two temperature sources is mixed in a proportion determined by the positioning signal from the controller. The correct hydraulic circuit and the correct installation of the valve are vital factors for control quality.
PAGE 7
The control benefits The quality of a control loop depends on a number of separate factors: the characteristics of the controlled system and its various elements (expressed quantitively as the ‘degree of difficulty’), the controller settings and, to a large extent, the control properties of the hydraulic valve.
PAGE 8
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 (Kv as a function of the stroke) is relatively steep.
PAGE 9
The equation for the equal percentage (exponential) curve (Fig. 16) is: kv = kvo • e ngl • H H100 The slope is not constant here, but changes with H, in accordance with the same exponential law. The value ngl determines how steeply the curve rises. For any given valve, it is a constant. kv 94.0145 kvs kvo Fig.
PAGE 10
kv [%] kvs kv [%] kvs 94.0149 94.0148 10 110 ± 10 % 100 9 8 kv100 = 92.63 90 7 80 Measuring point 6 70 Theoretical characteristic Measuring points 60 5 Theoretical characteristic 50 kvr = 4 % 4 40 3 30 2 20 Slope tolerance 10 Fig. Fig. 20 5 1 Slope tolerance kvr = 0.3 % Stroke 0 0 10 20 30 40 50 60 70 80 90 100 [%] Fig. 19: M3P25F valve characteristic (across full stroke) Stroke 0 0 1 2 3 4 5 6 7 8 9 10 [%] Fig.
PAGE 11
Qmin [%] QN 94.0150 70 50 40 a 30 25 0 0.1 20 30 0. 20 0. 15 0. 10 60 0. 8 5.8 50 0. 40 70 0. 90 0. 80 0. .0 1 B 5 4 3 2 B' 1.4 1 1.0 Pv 0.80 0.60 0.50 A A' 10 0.40 0.30 Sv 20 25 0 50 10 0 20 0 30 0 50 0.20 0.15 0.10 0.4 0.6 0.8 1 2 3 4 5 6 8 10 15 20 V min [%] Fig.