1. Refrigeration & Air-Conditioning
UNIT IV
Psychrometry and Air Conditioning Load Estimation

2. Example 1

3. Example 1

4. Comfort Air Conditioning (May 13, May 14, Dec. 16)
This class of air conditioning is meant to ensure human comfort in the space.
A comfortable environment would increase the efficiency of a worker in the office.
It is essential for human life in operation theatres, ICUs, burn sections, etc.
It is a common thing now a days which people expect in multiplexes, malls, shops, restaurants, households, etc.

5. Factors Affecting Human Comfort
(May 09, May 11, May 13, Dec. 14, Dec. 16)
Factors Affecting Human Comfort:
Temperature
Humidity
Air Movement
Air Purity

6. Factors Affecting Human Comfort
(May 09, May 11, May 13, Dec. 14, Dec. 16)
Factors Affecting Human Comfort:
Temperature:
Q = h A (dT).
This dT ensures convection heat transfer.
Preferred temperatures are in range of 20 to 25°C.

7. Factors Affecting Human Comfort
(May 09, May 11, May 13, Dec. 14, Dec. 16)
Factors Affecting Human Comfort:
2. Humidity:
Higher humidity would reduce evaporation of sweat from our body.
In dry climates, excessive loss of moisture leads to drying of skin.
So, humidity is maintained within 30 to 70 %.

8. Factors Affecting Human Comfort
(May 09, May 11, May 13, Dec. 14, Dec. 16)
Factors Affecting Human Comfort:
3. Air Movement:
Air movement is helpful in heat dissipation and gives greater comfort.
But excessive air movement would cause noise and discomfort.
So, air movement in air conditioned space is limited to air velocity of 8 to 15 m/min.

9. Factors Affecting Human Comfort
(May 09, May 11, May 13, Dec. 14, Dec. 16)
Factors Affecting Human Comfort:
4. Air Purity:
Atmospheric air has many impurities in it ranging from large impurities like dust, pollen, particles to micro-organisms like bacteria and virus.
In some cases, odorous gases cause equal discomfort.
So, air conditioner removes impurities as per requirements of user.

10. Factors Affecting Human Comfort
(May 09, May 11, May 13, Dec. 14, Dec. 16)
Thus, it can be said that entire procedure of air conditioning involves:
Control of temperature
Control of humidity
Control of air movements
Air purification

11. Thermodynamics of Human Body
(Dec. 16)
The average temp. of human body tb depends upon net heat produced by body and exchanged with the environment.
Fundamental thermodynamic equation of the heat exchange between human body and environment is given as :
M = W + E + (R+C)
Where,
M = energy produced by metabolism depending on activity (i.e. from food),
W = Mechanical work done by body,
E = Evaporated heat (evaporation of body fluids)
R+C = Heat exchange between body and environment by radiation and convection.

12. Thermodynamics of Human Body
(Dec. 16)
M = W + E + (R+C)
The evaporative heat loss E depends on partial pressure of water vapour in air surrounding body i.e. upon humidity of space.
Radiation and convection heat loss (R+C) depends upon space temp. as body temp. is constant.
Air movement affects both E & C.
The matching of heat produced and heat dissipated during normal conditions would maintain core body temperature between 36 to 38°C.
The combined effect has given rise to the concept of effective temperature.

13. Effective Temperature
(Dec. 11, Dec. 12, Dec. 13)
People feel comfortable only when all three parameters, say DBT, RH and air velocity are within certain limits.
Effective temperature is a single parameter at which people feel comfortable. Effective temp. is an index of measure of comfort i.e. it is a measure of feeling warmth or cold by human body in response to air temp., RH and air motion.
This factor combines the effects of dry bulb temperature and air humidity into a single factor.
It is defined as the temperature of the environment at 50% RH which results in the same total loss from the skin as in the actual environment.
This means effective temp. is DBT of a uniform enclosure at 50% RH in which people have same net heat exchange by radiation, convection and evaporation as they do in varying humidities of the test environment.

14. Human Comfort Chart
(Dec. 15, Dec. 16)
The concept of effective temp. enabled a graphical representation of human comfort conditions in the form of human comfort chart.
ASHRAE after an extensive survey involving large number of people arrived at conditions giving same degree of comfort and represented them on a graph.
A correction for hot climate of India may be in order as the subject under survey was used to North American cooler conditions.
Assumptions for designing chart:
Subject is in normal indoor clothing 0.6 clo for summer and 0.9 clo for winter.
Radiation effect is neglected.
Air velocity is less than 0.15 m/s for winter and 0.25 m/s for summer.
Subject is engaged in light activity <= 1.2 met.

15. Human Comfort Chart
(Dec. 15, Dec. 16)
Summer comfort zone has higher temp. as compared to winter zone.
There is a small overlap between the two zones.
The chart allows engineers to choose or suggest to users most economical room conditions without jeopardizing the comfort.

16. Indoor Air Quality Requirements (IAQ)
(Dec. 12, May 14, Dec. 15)
This is the requirement of user related to space to be air conditioned.
It is quality of air (temp., humidity, air purity and air movement) inside air conditioned space.
For Example:
Temp. range : 25°C ± 2°C
Humidity range : 50% ± 5%
Air purity gives the largest size of impurity permitted in the room, e.g. 10 microns
Air velocity: 8 to 15 m/min

17. Sources of Heat Load / Cooling Load
(Dec. 16)
The sources of heat load are classified as external and internal sources.
External Sources:
Solar Heat Gain through glass (SH Load)
Heat gain through opaque surface like wall, roof, floor, etc. (SH)
Infiltration: Heat load brought in by outside air entering through windows and doors (SH + LH)
Ventilation: Heat load of outside air supplied through system to bring freshness to room air (SH + LH)

18. Sources of Heat Load / Cooling Load
(Dec. 16)
2. Internal Sources:
Occupancy of people in room (SH + LH)
Lighting (SH)
Equipment (Some give out SH while some give out LH as well)
System heat gain:
i) Duct heat gain (SH)
ii) Duct air leakage (SH + LH)
iii) Blower power (SH)

19. RSHF, ESHF, GSHF
(Dec. 16)
The outside air after mixing with return air gives condition E as shown in figure. This is then passed over the cooling coil.
The cooling coil and dehumidification process then proceeds from the mixture condition ‘E’ to Adp as shown in figure.
The condition of air leaving the coil is ‘L’ on GSHF line.
The line joining L to R is RSHF line as shown in figure.
It could be seen that GSHF and RSHF lines are actual processes taking place in coil and room respectively.
The ESHF line is a conceptual line and hence represented as dotted line.

20. RSHF, ESHF, GSHF Room Sensible Heat Factor (RSHF): 𝑅𝑆𝐻𝐹= 𝑅𝑆𝐻 𝑅𝑆𝐻 + 𝑅𝐿𝐻
(Dec. 16)
Room Sensible Heat Factor (RSHF):
𝑅𝑆𝐻𝐹= 𝑅𝑆𝐻 𝑅𝑆𝐻 + 𝑅𝐿𝐻
Where,
RSHL = Room Sensible Heat Load,
RLHL = Room Latent Heat Load
Then,
Effective Room Sensible Heat Load (ERSH)
= RSH + (BF) x (Outside air SH)
Effective Room Latent Heat Load (ERLH)
= RLH + (BF) x (Outside air LH)
𝐸𝑆𝐻𝐹= 𝐸𝑅𝑆𝐻 𝐸𝑅𝑆𝐻 +𝐸𝑅𝐿𝐻