DESIGN MASTERCLASS 7 OPERATIVE TEMPERATURE

velocity, it is useful to have a temperature- only index for thermal comfort. CIBSE now uses ‘operative temperature’ as the index for comfort to align with the ASHRAE and ISO standards, which is directly equivalent to dry resultant temperature, as follows:

tc = tr + 10v tai 1 + 10v

where tc is the internal air temperature, tr

is the operative temperature, tai is the

mean radiant temperature, and v is the

air velocity. In this expression, 10v represents the ratio of the convective to radiative heat transfer coefficients at the surface of the body. Thus, at higher air velocities heat transfer by convection dominates, but at very low velocities the primary means of heat transfer would be radiation. In a room with no forced air movement – either mechanical or natural – it is

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assumed that the air velocity due to natural convection is 0.1m/s and so the expression for operative temperature simplifies to the following:

tc = tai

+ tr 2

This tells us that, for normal indoor

design conditions, the air and mean radiant temperatures are equally important. For buildings with lightweight

finishes, which are heated or cooled with conventional systems that act on the room air, the air temperature, mean radiant temperature and therefore operative temperature are all likely to be similar due to the heating of surfaces by the air. This is probably why so many of us only consider air temperature for the majority of heating and cooling installations. If we introduce thermal mass, or any form of surface heating or cooling system,

June 2011 CIBSE Journal 53

Embedded heating and cooling systems work by changing the surface temperature of building elements. The energy is transferred to the room environment by convection, conduction and radiation. Since occupants sense the radiant temperature of the surfaces, as well as the air temperature, this component of heat transfer can play a significant role in comfort, reducing the need to control tightly the air temperature

Doug King

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