DriSteem Vaporstream Especificaciones descargar pdf (Pagina 59)

P a g e 5 4 • D R I - S T E E M D e s i g n G u i d e

D R I - S T E E M D e s i g n G u i d e • P a g e 5 5

where it begins time-proportioning modulation control – that is,

the burner system turns on for a period of time and then turns

off for a period of time. With a high quality valve and responsive

control at low demand, a gas humidifier should be able to provide

steam output rangeability at a ratio of 40 to 1 and, especially in a

large capacity system, can yield RH control to ±1%.

SSRs deliver energy most consistently

Electric resistive heating elements are controlled using on-off

time proportioning modulation control. Heaters cycle on and

off for durations ranging from one second, using electronic

controllers such as solid state relays (SSRs) or silicon controlled

rectifiers (SCRs), to 100 seconds using mechanical contactors.

Every time the heaters are off, there is an interruption to steam

production. However, when cycle times are at or near one second,

steam interruption is unnoticeable. So, while at first it may seem

counterintuitive that an on-off modulating system provides tighter

control than a modulating valve system, an electronic system

can cycle heaters at such a rapid rate that the steam interruption

does not impact controllability. In addition, electronic controllers

operate as accurately at low demand as at high demand.

Float-fill valve with SSRs provides tightest RH control

In conclusion, there are many variables that affect isothermal

humidity control, and the tighter the control required, the more

variables that must be carefully managed. There are two main

humidifier mechanical issues that affect RH control: replenishing

water and modulating energy. The humidification system type

you choose will be defined by the requirements of your particular

application. But if your goal is to achieve the tightest RH control

possible, then for most applications, the preferred humidification

system will replenish water levels using a float-fill valve and will

have electronically controlled electric resistive heating elements.

Output control basics

Time-proportioning modulation control

of electric heaters

Humidifiers with electric heaters responding

to a modulating demand signal use time-

proportioning (TP) modulation control.

With TP modulation, outputs cycle on and

off at a rate corresponding to the demand

signal. Cycle times range from 1 second to

100 seconds. Mechanical contactors are

typically used when cycle times are above

60 seconds; electronic controllers such as

solid-state relays (SSRs) or silicon-controlled

rectifiers (SCRs) are used for rapid cycling,

tight control and quiet operation. The faster

the heaters cycle on and off, the closer the

humidifier output tracks humidity set point.

When a humidifier has more than one

heater, heater duty time is shared. For

example, if a humidifier has four output

stages controlled by four contactors, to

achieve a 55% system demand using a

60-second cycle time, two contactors are

on (each providing 25% of the output)

and a third contactor is on for 5/25 of 60

seconds, or 12 seconds on and 48 seconds

off. On-off cycling duty is typically rotated

to reduce contactor wear. To increase

the cycling rate (up to 1 second), a single

SSR or SCR can be added and do all the

cycling (this is called SSR modulation with

contactors). Or, all heat stages can be

controlled by SSRs or SCRs (called 100%

SSR or SCR modulation), allowing the

tightest possible control because all heat

stages can cycle rapidly.

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