HVAC

Two of the best references I can give for discussion of re-heat in HVAC
are:

    1. Reading, A. "A control philosophy for the economical air
        conditioning of museums and galleries."  BUILDING SERVICES
        ENGINEERING RESEARCH AND TECHNOLOGY.  V.4 (1983) 97-105.

    2. Thompson, Garry.  THE MUSEUM ENVIRONMENT. 2nd ed. ISBN
        0-408-01536-5. LCCN 85-25446.  Especially pages: 219ff.

Like most texts on this topic, these, however, tend not to discuss HVAC
from the point of the (near) tropics.

HVAC units recommended for use in archives, libraries and museums are
designed for fine tuning of climates.  They generally contain three
sections: pre-heat, chilling and re-heat.

Pre-heat allows air to absorb moisture.  In winter, it functions as a
humidifying aid (often in conjunction with humidification systems).  In
summer, it may pull some moisture out of artifacts, books, etc. into the
air, and so effecting (slowing) rates of (acid) hydrolysis.

Chilling allows air to be cooled to tolerable levels in the summer. It
also removes some humidity from the air.  Pre-heat and chilling can work
together to condition air without dehumidifying it.  In (near) tropical
climates chilling often is considered all that is needed for climate
control.  Who would want to re-heat air just cooled?  Well, in winter,
chilling must still be used in this climate to reduce the still high
humidity.  Re-heat makes this cold air in the seasonal chill bearable.
The same is often true of summer conditions.

If re-heat is not built into the system, the only way to bring humidity
down is by additional chilling.  Because humidity may be so high that
chilling to tolerable human levels is incapable of reducing RH levels
to those recommended for archives, libraries and museums, climate either
must be chilled to below tolerable levels OR to tolerable levels and
let the RH go. the latter often happens.  The workings of the 3 parts
are far more complex than all this.

Re-heat would (if we had it) allow us to chill air to below tolerable
levels, removing RH, then heat it to tolerable levels for both humans
and materials. Of course re-heat, if air has been dehumidified, would
force some moisture out of materials into the air.  So, the process
requires fine tuning of the temperature/RH balance.

Erich J. Kesse
Preservation Office
University of Florida Libraries
904-392-6962
Fax: 904-392-7251

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                  Conservation DistList Instance 5:37
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