COMFORT AND PSYCHROMETRICS

COMFORT AND PSYCHROMETRICS

General Objective

To understand the psychrometric processes of heating, cooling, humidification and dehumidification.

INTRODUCTION

The air that surrounds us has to be maintained at the correct conditions for us to be comfortable. The air in our homes is treated by the processes of heating, cooling, dehumidifying, humidifying, and cleaning it so that our bodies will give off the correct amount of heat for comfort.

THERMAL COMFORT

Certain combinations of air temperature, relative humidity, air movement and mean radiant temperature (MRT) will result in what most people consider thermal comfort. When these combinations are plotted in a psychrometric chart, they define an area as the comfort zone. This chart also indicates the type of discomfort one experiences outside of the comfort zone. The comfort zone should be the goal of the thermal design of a building because it defines those conditions that most people in our society find comfortable.

PSYCHROMETRICS

The study of air and its properties is called psychrometrics. When we move through a room, we are not aware of the air inside the room, but the air has weight and occupies space like water in swimming pool. Air weights 0.075 lb/ft³ at standard atmospheric conditions and the density of water is 62.4 lb/ft³ . Air like water, offers resistance to movement. For example invert an empty glass and push it down in water. The air in the glass resists the water going up into the glass. Cooling, heating, humidifying, dehumidifying and cleaning out air describes the air conditioning profession. Air consists of approximately 78% nitrogen, 21% oxygen and 1% other gasses. Water in the form of lower-pressure vapor is suspended in the air and called humidity.

HUMIDITY

The moisture content in air (humidity) is measured by weight, expressed in pounds or grains (7000gr/lb). Relative humidity is the most practical and most used for field measurements. It is based on the weight of water vapor in a pound of air compared to the weight of water vapor that a pound of air could hold if it were 100% saturated.

DRY BULB AND WET BULB TEMPERATURES

The moisture content of air can be checked by using a combination of dry-bulb (DB) and wet-bulb (WB) temperatures. DB temperature is the sensible-heat level of air and is taken with an ordinary thermometer. WB temperature is taken with a thermometer with a wick on the end that is soaked with distilled water. The reading from a WB thermometer takes into account the moisture content of the

air. It reflects the total heat content of air. Note that a 100 % relative humidity, DB and WB are equal.

DEW POINT TEMPERATURE (DP)

The dew-point temperature is the temperature at which moisture begins to condense out of the air. For example, if you were to set a glass of warm water in a room with temperature of 75^oF and 50% RH, the water in the glass would evaporate slowly to the room. If you gradually cool the glass with ice, when the glass surface temperature becomes 55.5^oF, water will begin to form on the surface of the glass. This temperature at which water forms is called the dew point temperature of the air. Air can be dehumidified by passing it over a surface that is below the dew point temperature of the air, moisture will collect on the cold surface for example, an air conditioning coil. The condensed moisture is drained and this is the moisture that we see running out of the condensate line of an air conditioner.

ENTHALPY

Enthalpy is the total heat contained in lb of a substance. It is measured from a reference (datum) point. This reference point is 0^oF (-17.8^oC) for dry air, 32^oF (0^oC) for water vapor and –40^oF (-40^oC) for refrigerants. A psychrometric chart in metric units uses 0^oC as a reference base.

PSYCHROMETRIC PROCESS

The following examples show how different processes of air conditioning are plotted.

1. Air is heated. Movement through the heating equipment can be followed as a sensible-heat direction on the chart
   
2. Air is cooled. There is no moisture. This shows a sensible-heat direction on the chart.
   
3. Air is humidified. No heat is added or removed. An increase in moisture content and dew point temperature shows
   
4. Air is dehumidified. No heat is added or removed. A decrease in moisture content and dew point temperature shows
   
5. Air is cooled and humidified using an evaporative cooler.
   
6. Air is heated and humidified is the most common in the winter application. This will show both a rise in temperature and an increase in moisture and dew point temperature .
   
7. Air is cooled and dehumidified is the most common in the summer application. A decrease in temperature, moisture content and dew point temperature will occur
   It is important to notice that any change in heat content or moisture content of air will cause a change in the wet bulb reading and therefore, a change in the total heat content.
   

Psychrometrics

Psychrometrics or psychrometry are terms used to describe the field of engineering concerned with the determination of physical and thermodynamic properties of gas-vapor mixtures.

The principles of psychrometry apply to any physical system consisting of gas-vapor mixtures. The most common system of interest, however, are mixtures of water vapor and air because of its application in heating, ventilating, and air-conditioning and meteorology.

Psychrometric chart

EXTRACT SYSTEM FOR A KITCHEN

COMMON POLLUTANTS

The following is a further description of some of the more common pollutants :-
i. Radon – radon is a colorless, odorless and radioactive gas. The main health risk of breathing air polluted with radon is lung cancer.
ii. Environmental tobacco smoke – it is the mixture of smoke that comes from the burning end of a cigarette, pipe or cigar and smoke exhaled by the smoker.
iii. Biological contaminants – viruses can be transmitted by people and animals. Water damaged materials and wet surfaces provide opportunities for molds, mildew and bacteria to grow.
iv. Pollutants from combustible products - wood, stoves and unvented kerosene and gas space heaters may be the source of pollutants from combustion products. The pollutants release carbon monoxide, nitrogen dioxide and particles.
v. Asbestos – it may be found in pipe and furnace insulation materials, shingles, textured paints and floor tiles when materials have been disturbed by cutting, sanding or removing activities.
vi. Lead – human may be exposed to lead through air, drinking water, food, contaminated soil, deteriorating paint and dust. Lead particles can result from lead dust from outside door sources and indoor activities using lead solder.
vii. Pesticides – it is used in and around a residence and may be used to control common insects, termites, rodents or fungi.

ODOR REMOVAL
Air transferred from dining areas into kitchen keeps odors and heat out dining areas and cools the kitchen. Outdoor air intake and kitchen exhaust louvers should be located so that exhaust air is neither drawn back into the system, nor causes discomfort to passersby. Where odors may possibly be drawn back into dining areas, activated charcoal filters, air washers or ozonators may be used to remove odors. No kitchen, locker room, toilet or other odoriferous air should be re-circulated unless air purifiers are used.

KITCHEN AIR CONDITIONING
Kitchens can often be air conditioned effectively, without excessive cost, if planned in the initial design phases. The relatively large number of people and food loads in dining and kitchen areas produce a high latent load. Additional cooling required to eliminate excess moisture increases refrigeration plant, cooling coils and air handling equipment size.

The kitchen has greatest concentration of noise, heat load, smoke and odors so that ventilation is the chief means of removing them and preventing these objectionable elements from entering dining areas. Kitchen air pressure should be kept negative relative to other areas, to ensure odor control.

HOOD TYPES
The air quantity for proper ventilation is a function of kitchen equipment heat release and kitchen hood size. While the heat release factor is more important, canopy-type hoods do not operate at maximum efficiency unless entrance air face velocity is at least 75fpm. Face velocities of 75fpm to 100fpm should be used for design, with 60 fpm as an absolute minimum.

Slot-type exhaust hoods are more efficient than overhead hoods, but they are more costly and may diminish valuable work area unless properly applied. Slot hoods require 150cfm to 200cfm per linear foot for proper operation but many substantially reduce kitchen exhaust air requirements. Slot hoods may also reduce overall kitchen ventilation system cost by obviating an additional make up air system and related energy cost.

HOOD EXHAUST SYSTEM DESIGN

Ductwork should be designed for a velocity of 1800fpm to 2200fpm to minimize the settling of grease particles. All turns should be made with elbows that have a minimum centerline radius of 1.5 times duct dimension in the turning direction. When this is not possible, grease traps and cleanout panels may be provided in the ducts. Turning vanes should not be used.
The fan should be located at the discharge end of the duct run to minimize leaks, which could cause odor problems. Ductwork design must allow for sufficient expansion caused by high temperatures during a fire.

KOMPENEN PENTING DIDALAM "AIR COMPRESSOR"

1. Capillary Tiub
- alat penurun tekanan didalam condensor
- mengawal pengaliran cecair
2. Hot Gas Bypass Valve
- mengawal pengaliran cecair
- jika tekanan didalam capillary tiub rendah, injap ini akan terbuka dan gas yang dimampatkan didalam compressor akan terus masuk ke dalam Air Cooler dan tidak melalui condensor.
3. Fan Control Switch
- berfungsi secara automatik.
- ia bergantung kepada tekanan semasa bagi capillary tiub.
- jika tekanan tinggi, kipas akan berfungsi untuk tujuan menyejukkan condenser, supaya gas yang akan melalui capillary tiub akan bertekanan rendah.

Bahagian - Bahagian Air Dryer

1. Auto Drain Trap
2. Strainer
3. Injap
4. Accumulator
5. Pemampat
6. Control Box
7. Condenser
8. kipas Penyejuk
9. Arah Pembuangan Udara
10. Paip Masukan Udara
11. Paip Keluaran Udara
12. Instrumen Panel
13. Penukar Suhu
14. Injap Bypass Gas Panas

4) Proses Pengeluaran Udara

- ini fasa terakhir
- ia adalah proses dimana udara yang sudah dimampatkan berada dipenghujung rotor. Pada bahagian ini udara akan disalurkan pula ke bahagian lain manakala udara lain akan disedut masuk.
- proses ini adalah berterusan sehingga alat ini dimatikan. (OFF)

3) Proses Kemasukan Minyak

- minyak akan dimasukkan setelah proses mampatan udara hampir selesai.
- ia bertujuan untuk penyejukan rotor.
- komponen rotor pada pemampat akan mengalami pemanasan apabila proses mampatan udara berlaku.
- medium minyak di perlukan untuk penyejukan rotor pemampat.
- medium minyak dapat melancarkan proses mampatan udara.
- minyak dapat meminimumkan haus pada bahagian lurah rotor pemampat.