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re: thermal capacity of floor slabs
28 may 1996
john cain wrote:
>i would like some serious general comments on the relative
>advantages/disadvantages of reducing the thickness of a floor slab
>to reduce its thermal capacity and hence the speed at which it heats-up.
seriously speaking (ahem), it seems like a good idea to reduce the thermal
capacity of a meeting hall floor slab, to _increase_ the speed at which it
heats up, and cools, because it's better to warm up the room quickly before
the meeting, and let the room cool off quickly after the meeting, leaving
less energy on base (hmmm, how do you say that in the uk?) less stored energy
which will be wasted keeping the room warm after the meeting is over...
>the project: a meeting hall with area aprox. 200m2, heated by an air-to-air
>heat pump with heating capacity 19kw (ref) 12kw(aux elec). u-values are
>0.35 w/m2k walls, and 0.25 for ceiling, no windows. warm air is delivered to
>the space via ceiling mounted diffusers, the air distribution is very good,
>not too draughty with adequate temperature gradients althoungh at time of
>occupancy (dense occuppancy in the order of 150)
hmmm. 15 kw of people heat. with some cold feet.
>the existing plant can be started earlier to increase the mean radiant
>surface temperature of the carpeted floor slab (150mm concrete with no
>insulation) which greatly improves the temperature gradient in the space,
the existing slab has no heating therein? no pipes running thru it
with hot water? so you would just heat it with air from the ceiling?
>but for future projects, i would like to consider polystyrene insulation
>in the slab (say 50mm?) with a reduced thickness surface screed to reduce
>its thermal capacity and hence improve the rate at which it heats-up.
sounds good. how about a few pipes in the screed as well?
>is there any real thermal gain in reducing the thermal capacity of the slab?
>say 100mm concrete, 50mm eps, 50mm screed compared with say 100mm concrete,
>50mm eps, 150mm screed?
seems to me you'd be saving the cost of heating up 50mm x 200m of concrete
from, say 50 f to 70 f, per meeting. concrete has a thermal mass of about
24 btu/f-ft^2. "btu"s are "british thermal units," which we use in america.
>what is an approximate u-value for a slab say 100mm concrete, 50mm eps,
>50mm s&c screed? on say clay soil?
concrete has a us r-value of about 0.08 btu/hr-ft^2/in. foam is about 4/inch.
we tend to figure soil has a us r-value of about 10, regardless of thickness,
and a temperature equal to the average yearly temperature above it.
>what calcs can one use to determine the rate at which this slab heat-up?
>newtons laws?
more or less. u = 2 + v/2 for a surface air film moving at v mph, in us units,
if it is rough like stucco (or carpeting?), or 1.5 + v/5 if the surface is
smooth. you can find c for the screed as above. the natural time constant rc
in hours for the top layer would be c/u. if you calculate the final slab temp
tf, and the screed starts out at t(0), t(t) = tf + (tf-(t0)(1-exp(-t/rc)),
where t is in hours.
>this is a serious question therefore general comments from experienced
>engineers would be greatly appreciated.
i am an experienced engineer, and i have tried very hard to be serious
in my comments to you. for more details, consult your local hvac criminal.
nicholson l. pine system design and consulting
pine associates, ltd. (610) 489-0545
821 collegeville road fax: (610) 489-7057
collegeville, pa 19426 email: nick@ece.vill.edu
microprocessor hardware, memory, asic, and computer design. telecommunication
system design. computer simulation and modeling. high performance, low cost,
residential solar heating and cogeneration system design. bsee, msee. senior
member, ieee. registered us patent agent. fluent in french.
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