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Humidity Measurement for Paper Dryer Control

Humidity Measurement for Paper Dryer Control

Industry	: Pulp and Paper
Product	: Zirconia Humidity Analyzer

Introduction

Hood Heat Recovery System

Hood Heat Recovery System - Loop Configuration

Drying is an important part of the papermaking process that evaporates the remaining water in the wet paper web. This is performed with a dryer, and the humidity control in this dryer is essential to maintaining high paper quality (e.g., strength, paper surface) and high heat energy efficiency. The ZR402G/HS Direct In Situ Zirconia High Temperature Humidity Analyzer requires no sampling system and operates stably in high-temperature environments. It has been well received in the marketplace.

Expected Benefits

-	Increases the heat energy efficiency of the dryer in the papermaking process
-	Ensures high paper quality
-	Reduces operating costs

Process Overview

The pulp suspension is dispersed uniformly, dewatered, and pressed to produce wet paper, the moisture content of which is approximately 20 to 50%. The remaining water is removed by evaporation. This is achieved by passing the sheet through a steamheated roll in a closed dryer. The water is evaporated by recirculating hot (50 to 120 °C), low-humidity air through the dryer. To maintain the specified humidity, humidity meters are installed at several locations in the dryer.

Solution Details

Field Data

Measurement system

Process conditions

Measurement point	Closed (semi-closed) hood or exhaust duct of hood with humidistat control
Temperature	50 to 120 °C
Pressure	Negative
Dust	Trace amounts (paper dust)

Measurement system

Detector	ZR22G-040-S-C-E-☐-☐-E-A/☐
Dust protector	ZH21B-040-A*B
Converter	ZR402G-☐-E-E-A/HS/☐
Flow setting unit	ZA8F-A*B
Calibration gas unit	Pressure regulator for gas cylinder: P/N G7013XF (inlet W22, outlet Rc1/4) or P/N G7014XF (inlet W22, outlet 1/4NPT) Case assembly for calibration gas cylinder: P/N E7044KF Note: the calibration gas cylinder must be purchased locally

Utilities

Power supply	rated voltage: 100 to 240 V AC operating voltage range: 85 to 264 V AC rated frequency: 50/60 Hz operating frequency range: 45 to 66 Hz
Power consumption	approx. 100 VA (300 VA max.)
Instrument air (reference gas)	pressure: 300 to 700 kPa

Notes

The humidity measurement system introduced here can also be used in dryers in the flour and starch making processes.

Notes on Installation

The sensor is hot. To remove a possible ignition source, a dust protector should be used to prevent suspended paper dust from coming into contact with the sensor.

Since the process humidity is low, the humidity of the reference air greatly affects the accuracy. Clean, dry air such as instrument air should be used as the reference air. If such air is not available, a customized flow setting unit with a humidifier should be used.