Air Conditioning

01/02/2010Short Course on Fundementals

of HVAC - Chapter 4 1

Fundamentals ofHVAC Systems

Conducted byDepartment of Mechanical Engineering,

University of Moratuwa

in collaboration withASHRAE Sri Lankan Chapter

Organized by UIIC, University of Moratuwa

July/August 2004



Concepts of Environmental Comfort - Chapter 4

IntroductionPhysiological Considerations for Human ComfortHeat Transfer Considerations of Human ComfortDescription of Environmental IndicesThe ASHRAE comfort envelopSkill Development



Introduction

Understanding of the Heat Balance of the Human Body for identifying requirements of human Thermal comfort.“Thermal Sensation of well being with the surroundings” – Definition.Body’s Internal heat flow and Generation rates that involves the mechanisms control of the body temperature.Heat transfer between body and surrounding is governed by conduction, radiation, evaporation and clothing insulation.



Physiological Consideration for Human Comfort

Human body generates 300 – 3000 Btu/h (88 –880 W) depending on the level of activity.

(sleeping to exercising)Heat is generated as a part of the metabolic process to keep the body function and allow to work. (efficiency is seldom better than 10%) To avoid discomfort, the body temperature should be maintained at a narrow rage.Few degrees high or low can cause serious danger.At constant body temperature the rate of heat generation is equal to rate of heat dissipation to surrounding.



Physiological Consideration for Human Comfort. Contd….

Dissipation takes place from the skin and respiratory systemThe body must carefully regulate heat generation and heat transferIf the body temperature drops, flow of blood to the skin is reduced by vaso-constrictionBeyond certain limit shivering will take placeIf the body temperature rises the flow of blood to the skin increases by vaso-dilationBeyond certain limit sweating and resulting evaporation will occur to keep the body cool.



Table 4-1. Physiological ResponsesUsed in Body Temperature Regulation

Neutral ZoneNo temperature regulation required by body

• Unclothed — Environment temperature 84° to 88°F (29° to 31°C)

• Lightly Clothed — Environment temperature 73° to 81°F (23° to

27°C)

Too Cold• Flow of blood to skin decreases

• Increased heat generation (shivering)

Too Hot• Flow of blood to skin increases

• Sweating occurs

• Accelerated respiration



Physiological Consideration for Human Comfort. Contd….

Heat generation and dissipation are NOT uniformTotal Metabolic energy M, Work W, leading to net energy (M-W)(M-W) must be stored or dissipated, since W is small (10%), M is importantUnit of Metabolic Generation is “Met”1 Met=18.43 Btu/h.ft2=58 W/m2Large persons will generate higher A/C loads.Average male = 19.6 ft2 (DuBois surface area)1 Met represents a load of 360 Btu/hRefer Table 4-2



Heat Transfer Considerations

Body dissipates heat and moisture involving conduction, convection, radiation & evaporation (diffusion of moisture is present)Body Sensible Heat: air drawn & exhaledBody Latent Heat: heat of vaporaizationBoth sensible & Latent transfers influence the A/C loadHeat transfer mechanism decides the balance between metabolic rate & dissipation from bodyThis imbalance determines the physiological response of the body



Heat Transfer Considerations.contd.

Main factor on meatbolic rate is ActivityFactors affecting dissipation are temp, humidity, air movement, radiation, clothingI.e, Personal Variables & Environmental VariablesClothing is a main factor and indicated by unit of insulation called “clo”1 clo = 0.88 ft2.h.F/Btu (resistance)Calculations are based on Dubios area and clothing resistance IcloTable 4-3



Table 4-3. Clothing Insulation Valuesfor Typical Ensembles

Ensemble Iclo(clo)

1. Briefs; knit, short-sleeve sport shirt; shorts; belt;

calf-length socks; hard-soled shoes 0.4

2. Panties; broadcloth, short-sleeve shirt;

A-line, knee-length skirt; pantyhose; sandals 0.5

3. Briefs; broadcloth, long-sleeve shirt; single-breasted

suit jacket; tie; long fitted trousers; calf-length

socks; hard-soled shoes 1.0

4. Panties; broadcloth, long-sleeved shirt; thick vest;

single-breasted suit jacket; thick A-line, knee-length

skirt; pantyhose; hard-soled shoes 1.0

5. Briefs; t-shirt; flannel, long-sleeve shirt; work jacket; belt;

work pants; calf-length socks; hard-soled shoes 1.2



Description of Envi. indices

Two types of indices: Direct & Empirical

Direct (those can be measured with instruments) : DBT, WBT, DPT, RH

Distinguish DBT,WBT,DPT

Difference between DBT & WBT is a measure of moisture content & evaporating force

Difference between RH & specific humidity

Air movement decides convection & evap

Air movement can reduce/increse discomfort



Description of Envi. indices. contd

Rationally Derived indices:Mean Radiative Temp (MRT)Operative temp (OT)

OT = (DBT + MRT)/2

Humid operative TempHeat stress indexOT incorporates temp for convection & radiationEffective Temp (ET) incorporates temp & evapET: Temp of envi. at 50% RH that results in the same heat loss from the skinET depends on clothind and skin wetedness, thus different for two different people.



ASHRAE comfort envelope

Specified in ASHRAE 55-1992

Two overlapping regions for light summer clothing slight heavy clothing

Envelope on the chart of OT & Humidity ratio

Regions shown are for people engaged in light office work

Based on 10% dissatisfaction criteria.



Figure 4-1. Standard Effective Temperature & ASHRAE Comfort Zones

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