Herrmidifier Load Calculator User Manual

Page 8

L o a d C a l c u l a t i o n G u i d e

w w w. h e r r m i d i f i e r- h v a c . c o m

WHY DOES WOOD CRACK OR WARP?

As wood loses moisture it shrinks. However, the tangential

shrinkage is much greater than the radial shrinkage. This

causes dimensional changes and instability in the wood and

the wood pulls apart, along the grain causing cracks. If the

wood is strong enough not to crack it will warp as the un-

even shrinkage occurs. This is why it is important to condi-

tion wood to the proper Regain for best workability and then

stabilize it at the corresponding EMC by maintaining proper

humidity control.

WHAT IS THE BEST R.H. FOR WOOD?

The Regain should be between 5%-9% depending on the

species of wood and its use. This regain corresponds to the

EMC with 35%--45% R.H. air at 75°F. If you are able to

maintain the proper EMC Regain in the wood by maintain-

ing the proper R.H. in the air surrounding it, rejects will be

sharply reduced, production efficiency increased and profits

increased!

WHAT ABOUT GLUING AND FINISHING?

When dry thirsty air pulls moisture from it, the wood also be-

comes thirsty, so that when a varnish or other coating is ap-

plied the dry wood will act like a sponge and quickly absorb

the liquid. This leads to a dull finish or excessive use of coat-

ing to produce a bright finish. Glue will not cure properly or

quickly if the air and wood that it is in contact with are too dry.

WHAT R.H. DO I NEED FOR GLUING AND FINISHING?

Generally, 40%-50% R.H. is recommended for gluing and a

minimum of 60% R.H. recommended for finishing areas. You

must remember that if wood has not been conditioned previ-

ously and enters the humidified gluing and finishing areas

dry and thirsty, it will still produce the problems experienced

before. It is important to maintain the stability of your wood,

from start to finish, by proper humidity control. The econom-

ic gains from proper humidity control, can often pay for the

equipment in less than one year.

Paper Products And Humidification

The common problems experienced in every paper industry,

whether it involves manufacture of cardboard boxes, sheet

paper, etc. are: curling of stock, cracking on folds of paper

or boxes, failure to feed properly into machines and glue

that won’t hold. Generally these problems are experienced

in the winter heating season when the R.H. indoors is low.

Once again, the cause of these problems is moisture loss

from materials. Paper and paper products are made up of

fibrils. When these fibrils lose moisture they shrink and be-

come brittle.

Woodworking And Humidification

In the woodworking industry controlled humidification can of-

ten mean the difference between a profitable operation and

bankruptcy. A company, manufacturing wood products, that

must reject one (1) out of ten units due to cracking, warp-

ing, splitting or checking suffers losses due to wasted raw

materials, wasted labor and wasted energy in the production

of nonsaleable items. These losses can be profound- par-

ticularly the energy loss, as this goes all the way back to the

cutting of the tree and transporting of the lumber. If the dam-

aged unit can be reworked there is still the added cost of do-

ing so. These losses and added costs must be added to the

costs of producing the other nine out of ten saleable units. It

is not hard to see why controlled humidification can result in

substantial savings for the woodworking industry.

HOW IS WOOD AFFECTED BY HUMIDITY?

Wood is a hygroscopic material, able to take on or give up

moisture to the surrounding air. As wood takes on moisture

it swells. As it gives up moisture it shrinks. It does so in re-

sponse to the changing atmospheric conditions of tempera-

ture and humidity, humidity having the greater effect. The

amount of moisture in the wood, expressed as a percentage

of its dry weight, is referred to as its REGAIN. This Regain

varies with temperature, relative humidity and type of mate-

rial. When the Regain of a piece of wood reaches a balance,

where it is no longer taking on or giving up moisture, it is said

to have reached its EQUILIBRIUM MOISTURE CONTENT

(EMC) for that particular temperature and R.H. At this state

the wood is stable and will not change dimensionally. The

EMC changes with temperature and R.H. For example, a

piece of wood with an EMC regain of 9.3% at 75F 50% R.H.

will have an EMC regain of only 4.4% at 75°F 20% R.H. (See

Table 1-C.)