Page 73 - Studio International - May June 1975

when the relative humidity is constant,
and, because, for a constant amount of
water vapour in the air, the relative
humidity (RH) depends on
temperature (one degree C temperature
change causing a 4%change of RH), the
temperature must also be constant.
Typical of the published
recommendations is as Figure 1, which
is an adaptation from the paper
`Climatology and conservation in
museums' by H. J. Plenderleith and
P. Philppot, published in Museum
Vol XIII/No. 4196o (which is,
incidentally, an excellent description of
the need and methods then available to
control temperature and humidity).
It is our opinion that, when it is not
possible to have the objects at below,
perhaps —40°C, when no chemical
or bacteriological activity can occur
(which is possible using freeze drying
techniques but clearly an impossible
display temperature) then the actual
temperature does not matter so much
provided it is as constant as possible and
that the RH % is right. Therefore, since
the temperature needs to be near to
human comfort level, it should be at
comfort level (even if not so designed
that is how it will operate.) and we
consider that the ideal is flexible between
19 and 21°C with 45 to 65% RH,
controlled to 5% either way, greater
precision not being reasonably possible.
As already mentioned the cost of
providing ideal conditions can exceed
4o% of the cost of a new building and a
large part of this is in the air
conditioning system. It is therefore
essential to be absolutely convinced that
precise temperature and humidity
control are needed and, if so, the actual
values chosen should appreciate that
relatively small temperature changes
(2 or 3 deg. C) at peak solar condition can
reduce substantially the size of installation
needed and may only occur on a few
days in each year. Also the lower the
temperature selected the greater the
problems of condensation and the more
energy needed to remove the heat gains
in Summer, but, of course, the less heat
needed in Winter. Another energy
factor is the degree of air purity required
and the ideal is probably a combination
air filter in three stages (coarse fabric,
absolute fabric and active carbon — never
electrostatic because ozone is generated)
with the minimum of fresh air so that the
stored bulk is continuously recirculated
to reach an ever increasing degree of
purity. Unfortunately this prevents the
adoption of 'free cooling', that is using
external air to remove internal heat
gains, and in these days of energy
restriction, this is an important aspect
and will also influence the choice of
internal temperature. In Liverpool an
internal temperature of 20°C would
need mechanical cooling for some the conditioned space, but this would
800 hours per annum while at 18°C it not be good enough for precise control
would be needed for some 210o hours and would be very expensive to run. The
per annum. central plant needs to have a means of
To control temperature, humidity and adding moisture and the system should
air purity at all times an air conditioning
system is needed. In its simplest form this
would comprise of a fan drawing in factor is that the control in an area
fresh air from outside through a filter, a
cooler and a heater and delivering it to the average temnerature and RH of that
area and often the heat gain is at
different rates in different areas.
Fortunately the maximum rate of
moisture gain from people is not usually
sufficient to modify the RH beyond the
5% tolerance, indeed is usually much
less, so that, provided the building is
well sealed against an infiltration from
outside, it is only necessary to control
the temperature, and, where 'free
cooling' is adopted, to go progressively
to recirculation as the moisture content
of the external air exceeds that required
inside. To obtain individual temperature
control the possibilities are to fit a heater
and cooler on the duct to each control
area or to arrange for the volume of air
delivered to be at constant temperature
and varied in the amount needed to
maintain a constant space temperature.
Since the former approach involves
water pipes as well as air ducts and also
the heating of air previously cooled, or
vice versa, leads to very poor thermal
economy, it seems that a 'variable
volume' system is essential. The
disadvantage of this system is that, as
the volume introduced reduces, so the
air flow pattern in the space is affected
and a poorly contrived system can result
in stratification, and thus temperature and
relative humidity differentials to be set
up between one part of the controlled
area and another. To reduce these
problems the temperature difference
between the supply and room air should
be as small as reasonable — perhaps
6°C (11°F) — and the variable volume
device should be arranged to maintain
as steady an effective discharge velocity
as can be achieved — either by 'pulsing'
as in the fluidic switch or by inducing a
suitable volume of room air to give
constant mass flow. A diagram of a
typical variable volume system is given
in Figure 2.
A lesser system would be simply
to provide heating with humidity
control and we would suggest that this
will often be quite satisfactory because it
is only low RH that affects many objects,
higher temperatures that occur in the
Summer are not destructive because of
the higher absolute humidity and hence
RH in the Summer. The features
needed for a successful system include
individual thermostatic control of each
heater such as by the cheap method of a
thermostatic radiator valve, where
have means of recirculating expensively
cooled air for running economy. Another
supplied from one plant can only be at
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