The sensible heat is transferred by increasing the
temperature of incoming air. The latent heat is
removed as water vapor from evaporation and
transpiration. Sensible heat transfer can be estimated
by:
hsa = 0.02 M (ti – to) (4–2)
where hsa = air exchange heat, Btu/hr.
M = air exchange, ft.3/hr.
The rate of air exchange between inside and outside
is affected by wind and the type and quality of
greenhouse construction. Reasonable estimates of air
exchange are given in Table 4–5 on the next page.
hf = hc + hsa = AU (ti – to) + 0.02 M (ti – to) (4–3)
where hf = furnace heat (Btu/hr.) needed to
keep the greenhouse air
temperature at the desired level.
The solar gain on sunny days will replace some or
all furnace heat needed to maintain temperature at
the proper level. If solar gain exceeds heat loss, the
greenhouse air temperature will rise and cooling
may be required. The solar gain can be estimated by:
hs = T Is AF