Operative temperature is defined as a uniform
temperature of a radiantly black enclosure in which an occupant
would exchange the same amount of heat by radiation plus
convection as in the actual non-uniform environment.
Some references also use the terms 'equivalent
temperature" or 'effective temperature' to describe combined
effects of convective and radiant heat transfer.
In design, operative temperature can be defined
as the average of the mean radiant and ambient air temperatures,
weighted by their respective heat transfer coefficients.
The instrument used for assessing environmental
thermal comfort in terms of operative temperature is called a
eupatheoscope and was invented by A. F. Dufton in 1929.
I say,
if building codes dropped the reference to controlling air
temperatures and switched the requirements to controlling
mean
radiant temperature, building performance specifications would
have to change overnight. Bad buildings have low MRT’s in winter
and high MRT’s in summer; this contributes to large differences
in the vertical air temperature; creates excessive drafts and
increased radiant asymmetry. Bad buildings also have
uncomfortable floor temperatures and are challenged to maintain
reasonable levels of humidity.
Operative Temperature (To): What happens
when MRT and
Tdb mate?
Copyright (c) 2012, Robert Bean, R.E.T., P.L.(Eng.) and
www.healthyheating.com
When it comes to thermal
comfort, operative temperature
(To) is what humans experience
thermally in a space. It is the combined effects of
the mean radiant temperature and air temperature (Tdb).
Mathematically,
operative temperature (To
or
Top)
can be expressed as;
The chart is familiar to
virtually all seasoned Architects and HVAC designers except most designers fail to note
the x-axis in thermal comfort analysis is not dry bulb (Tdb
or Ta, air or ambient)
temperature but operative temperature (To).
Figure 1. Modified psychrometric "thermal
comfort" chart as
published in ANSI/ASHRAE Standard 55 - Thermal Environmental
Conditions for Human Occupancy considers
humidity (y-axis), operative temperature (x-axis), metabolic rate,
clothing and air velocity (reprinted with permission
from ANSI/ASHRAE Standard 55, see
www.ashrae.org)
Mean Radiant Temperature: An intimate part
of operative temperature
Most people understand the concept of dry
bulb or air temperature but few really grasp the concept of
mean radiant temperature. Unfortunately the focus on air
temperature has led to bad indoor climate solutions comprising of blowing
hot and cold air instead of focusing on the mean radiant
temperature which is first and foremost a
building enclosure
performance element. This element can if necessary be
augmented with radiant cooling and heating systems designed
using the graph below.
Figure 2. ASHRAE Design Graph for
Sensible Heating and Cooling with Floor and Ceiling Panels1
reprinted with permission from Section/page 6.8, Panel
Heating and Cooling, 2012 ASHRAE Systems and Equipment
Handbook (see
www.ashrae.org).
Figure 3. During the hot summer months, watching an
outdoor sports match or concert can be tantamount to baking
uncomfortably in the sun -- but it doesn’t have to be. At
the TEDxSummit in Doha, physicist Wolfgang Kessling reveals
sustainable design innovations that cool us from above and
below, and even collects solar energy for later use.
