DriSteem Vapor-Logic Instalační příručka stáhnout pdf (Strana 25)

Off-On Control

The first and simplest control scheme, this operation

does what its name implies: the output device turns

fully on then fully off. The off-on humidistat differential

is designed into the control action between off-on

switching points. The differential is established at a

range sufficient to prevent output short cycling. See

figure 25-1. VAPOR-LOGIC offers single output control

or multiple output control on larger humidifiers by using

integral pneumatic-electric switches or external stage

or step control switches by others. Off-on control may

also be used in multiple-zone humidifying applications

by controlling the opening and closing of area zone

valves.

Time Proportioning Modulation

For more precise control, the time proportioning system

operates in the same way as off-on control when the

controlled relative humidity is far enough away from set

point. But when controlled relative humidity approaches

set point, the system enters a control range which

provides proportional control called the throttling range,

which is positioned symmetrically around set point. In

this range, the output device is switched on and off

inversely proportional to the actual RH relative to the

set-point at an established cycle rate. This cycle rate is

adjustable from 15 to 99 seconds; for TP modulation

the VAPOR-LOGIC default is 20 seconds.

Because of the limitations of mechanical contactor

cycling rate design, to increase the effective control

potential of time proportioning, a solid state SCR power

control is also offered. The SCR cycle rate is reduced

to one second; this provides a shorter interval of output

modulation.

The TP modulating control operates as follows, at the

lower limit or below set point, within throttling range, the

on time is greater than the off time. As the process

relative humidity approaches set point and just above,

the amount of on time decreases as the off time

increases. This action, in effect, is time proportioning

modulation. The power delivered to the humidifier over

time is reduced. This on and off cyclical action

continues until a stable relationship is achieved around

set point. See figure 26-1.

VAPOR-LOGIC

CONTROL SCHEMES

Desired Set-

point

Switching

Differential

Humidity % RH

OM-587

Time

Figure 25-1: On-Off Differential

1 2 ... 20 21 22 23 24 25 26 27 28 29 30 ... 51 52

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