Design temperatures and heat gains

The choice of inside and outside summer dry-bulb design temperatures affects the heat gains and hence influences the capital cost of the installation and its running cost, the latter implying the energy consumption of the system. Any change in the room design condition, particularly dry-bulb temperature, will have an effect on the comfort of the occupants. Similarly, any change in the chosen outside dry-bulb temperature will influence the system performance and the satisfaction given. Thus a relaxation of the outside design dry-bulb temperature to a lower value may give a small reduction in the sensible heat gains and the

Capital cost but this must be balanced against the fact that the system will only be able to maintain the inside design conditions for a shorter period of the summer. The relative merits of any such decisions must be carefully considered and the client advised. Example 7.19 considers this.

Exercises

1. The human body adjusts itself, within limits, to maintain a relatively constant internal temperature of 37.2°C.

(a) How does the body attempt to compensate for a cool environment which tends to lower the internal temperature?

(b) How does the body attempt to compensate for a warm environment approaching body temperature or exceeding it?

(c) State, giving your reasons, whether an increased air motion as provided by a large rate of air change or the action of a ‘punkah’ fan is likely to be beneficial to comfort in a room at 29.5°C, 75 per cent relative humidity.

2. It is required to visit a site to obtain measurements of comfort conditions at a point

1.5 m above floor level in the centre of a particular room. The measurements required are dry-bulb and wet-bulb temperatures, globe temperature and mean air speed. State what simple instruments and ancillary aquipment should be taken to site in order to obtain the required measurements. Briefly describe the instruments, with the aid of sketches and explain how to use each one, mentioning any precautions which should be taken to ensure accuracy of the values obtained.

3. (a) Write down an equation expressing the thermal balance between the human body and its environment.

(,b) Under what conditions is the temperature of the deep tissues of the human body going to change? Discuss the physiological mechanisms which the body employs to adjust such an imbalance. How can the air conditioning engineer, through an appropriate manipulation of the environment, assist the body in feeling comfortable?

4. List the factors in the environment which affect the body’s feeling of comfort and describe how they influence the rate of heat loss from the body.

5. Briefly discuss the influence of clothing on human comfort. How is the effect of clothing expressed? Why would you expect a mixed group of people in an air conditioned room to have different attitudes to thermal comfort?

6. Define plane radiant temperature and explain how it is used to describe the effect of asymmetrical radiation on comfort. State limiting values for vertical and horizontal radiant asymmetry.

7. Explain briefly how Fanger’s equation defines thermal comfort, stating the physical variables used for this purpose. Explain also the meaning of the terms: predicted mean vote, predicted percentage dissatisfied and lowest possible percentage of dissatisfied persons.

8. Discuss the difference between the original concept of effective temperature and that of standard effective temperature.

9. State the conditions of the indoor environment that should be satisfied for a person to feel comfortable. Which of these conditions are under the control of the air conditioning system? How can the designer arrange for the other conditions to have values likely to achieve comfort?

10. Quote suitable inside design conditions for the provision of comfort conditions in summer and winter for sedentary workers in the UK. Explain how different design conditions would be chosen for summer in a tropical environment and suggest indoor design conditions for an outdoor state of 40°C dry-bulb, 30°C wet-bulb.

Notation

Symbol

Description

Unit

Du Bois bodily surface area

M2

C

Bodily rate of heat loss by convection

W or W rrr;

E

Bodily rate of heat loss by evaporation

W or W itT’

Ed

Heat loss by vapour diffusion through the skin

W

Ere

Latent heat loss by respiration

W

P

*-•sw

Heat loss by sweating from the skin

W

Ev

Ventilation effectiveness

H

Internal rate of bodily heat production

W

H

Height of a person

M

Hc

Convection heat transfer coefficient

W nr2 K-1

K

Evaporative heat transfer coefficient at the clothing surface

W itT2 kPa~

Hr

Radiation heat transfer coefficient

W m“2 K“1

H i

Insulating value of clothing

M2 K W“1

^clo

Thermal insulation of the total clothing ensemble

M2 K W"1

^clui

Effective insulation of garment i

M2 K W“1

Moisture permeability index for clothing

L

Sensible heat loss by respiration

W

M

Metabolic rate for a particular activity

W

M

Body mass

Kg

PD

Percentage of people dissatisfied

%

PMV

Predicted mean vote

PPD

Predicted percentage of people dissatisfied

%

LPPD

Lowest possible percentage of people dissatisfied

%

Pt s

Saturated vapour pressure at temperature tef

KPa

Ps

Vapour pressure of humid air

Pa

Pw

Vapour pressure of water

Pa

R

Bodily rate of heat loss by radiation

W or W rrr"

S

Bodily rate of heat storage

W or W nT

T

Ja

Air dry-bulb temperature

K

T

Globe temperature

K

T

1 rm

Mean radiant temperature

K

T

Ju

Intensity of air turbulence

K

Ambient air dry-bulb temperature

°c

Fed

Effective draught temperature

°c

^ef

Standard effective temperature

°C

^0

Operative temperature

°c

H

Mean room dry-bulb temperature

°c

Hes

Dry resultant temperature

°c

Local air dry-bulb temperature

°c

V

Mean air velocity

M s"1

Vsd

Standard deviation of the local air velocity

M s“1

V

Relative air velocity

M s’1

Vx

Local air velocity

M s-1

W

Rate of working

W or W m

W

Fraction of the skin surface that is wetted

<t>a

Ambient air relative humidity

%

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