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Basic Air Conditioning and

Refrigeration

Mark Broomfield (PhD)

January 9, 2012



Definition: Refrigeration and Air conditioning

Refrigeration :

is the process of removing heat from an enclosed space, (substance), and

moving it to a place where it is unobjectionable.

- primary purpose: lowering the temperature of the enclosed space and

maintaining that lower temperature.

C ooling: cooling generally refers to any natural or artificial process by

which heat is dissipated. The process of artificially producing extreme cold

temperatures is referred to as cryogenics.

Air conditioning:

simultaneous control of temperature, humidity, composition, motion, and

distribution of air for the purpose of human comfort or for industrial

utilization.



Applications: Refrigeration and Air conditioning

Heating, humidity, and

control of air quality

Industrial refrigeration

including food

preservation, chemicaland process industries

Air conditioning Refrigeration

Cooling and

dehumidifying

operations in air

conditioning



Assignment # 1

List the factors involved in providing comfort forzone space.

State the need for air conditioning such as:

- comfort zone and its parameters- humidity control.

Specify the temperature ranges for differentapplications.

Explain the need or importance of control of temperatures within the comfort zones.

Due Date : Today



Basic Psychrometry

Definition: is the study (science and practice) of the properties of mixtures of air and water vapor.

or Psychrometry is the study of moist air and of the changes in itsconditions.

Importance in air conditioning:

Atmospheric air is not completely dry (mixture of air and water).

Water is added and removed depending on process.

Application:

- load calculations- air conditioning systems

- cooling and dehumidifying coils

- cooling towers and evaporative condensers



Basic Psychrometry : psychrometer

A psychrometer is an instrument (commonly used in laboratories )to measure

relative humidity. Also referred to as a wet- and dry-bulb thermometer.

Dry bulb: bulb exposed

to the air.

Wet bulb : wrapped in an absorbent material such as muslin,

which is immersed in water and serves as a wick.

Operation :

When the web bulb is taken out of the water, itcools by evaporation of the water. If the bulb is

whirled around to hasten evaporation, it is

called a sling psychrometer. If air is forced past

the bulb, it is referred to as an aspirated or

ventilated psychrometer.

Note :The amount of evaporation, and consequent cooling of

the thermometer, depends on the humidity of the

atmospherethe drier the atmosphere, the faster the

water evaporates. Using this data and humidity tables or

calculations, the dew point (the temperature to which air

would have to be cooled for saturation to occur) can be

determined, and from it, the relative humidity.



Basic Psychrometry : Important terms

Absolute humidity : (AH) is the vapor content of air, given in grammes or kg of water vapor per kg

of air, i.e. g/kg or kg/kg. It is also known as moisture content or humidity ratio. Air at a given

temperature can support only a certain amount of moisture and no more. This is referred to as the

saturation humidity.

Relative humidity :( RH) is an expression of the moisture content of a given atmosphere as a

percentage of the saturation humidity at the same temperature.

Wet-bulb temperature : (WBT) is measured by a hygrometer or a sling psychrometer and is shown

as sloping lines on the psychrometric chart. A status point on the psychrometric chart can be indicatedby a pair of dry-bulb temperature (DBT) and WBT.

Specific volume : (Spv) , in m3/kg, is the reciprocal of density and is indicated by a set of slightly

sloping lines on the psychrometric chart.

Enthalpy : (H) is the heat content of unit mass of the atmosphere, in kJ/kg, relative to the heat

content of 0 deg ?C dry air. It is indicated on the psychrometric chart by a third set of sloping lines, near

to, but not quite the same as the web-bulb lines. In order to avoid confusion, there are no lines shown,

but external scales are given on two sides. Sensible heat : (Qsen) is the heat content causing an increase in dry-bulb temperature. Latent heat

(Qlat) is the heat content due to the presence of water vapor in the atmosphere. It is the heat required

to evaporate a given amount of moisture.

Saturated vapor : Vapor which is in equilibrium with its liquid phase at the specified temperature

and pressure.

Dew Point : Temperature below which water vapor in the air will start to condense.



Basic Psychrometry : psychrometric chart

Locating air properties on a psychrometric chart.




Wet bulb

temperature




Development



REVISION : psychrometric chart

Wet bulb

temperaturescale

dry-bulb

temperature

scale

humidity ratio(moisture

content)

saturated air (or 100 percentmoisture holding capacity)



DIAGRAMMED CHART

Chart presents physical and thermal properties of moist air in a graphical form.

Chart isstandard for

atmospheric

pressure (14.7

psi) and

temperatures

of 30o to 120 oF.

Adequate for

most

greenhouse or

livestock

housingapplications.



Psychrometric Chart Use

Presents physical and thermal properties of moist air in a

graphical form. helpful in troubleshooting greenhouse or livestock building

environmental problems and in determining solutions.

diagnosing air temperature and humidity problems.

Importance of understanding psychrometric chartshelps visualization of environmental control concepts

such as:

why heated air can hold more moisture, and

conversely, how allowing moist air to cool will result in

condensation.



Psychrometric Chart UseNote:

Properties of

moist air on a

psychrometric

chart. Wet-

bulb

temperature

and enthalpy

use the samechart line but

values are

read off

separate

scales.

Note:

cooler air (locatedalong lower, left

region of chart)

will not hold as

much moisture (as

seen on the y-axis'

humidity ratio) aswarm air (located

along right side of

chart).

As rule of thumbInside typical greenhouses or animal buildings during winter conditions, a

10oF rise in air temperature can decrease relative humidity 20 percent.

What would you do to decrease relative humidity in a winter greenhouse

during a critical time period??????????



Psychrometric Chart Use

Example1:Find air properties A sling psychrometer gives a dry-bulb temperature of 78oF and a wet-bulb

temperature of 65oF. Determine other moist air properties from this information.

NOTE

Two useful air properties for environmental analysis in agricultural buildings would

be relative humidity and dew point temperature.

REMINDER

Relative humidity is an indicator of how much moisture is in the air

compared to desirable moisture conditions, and dew point temperatureindicates when condensation problems would occur should the (dry-

bulb) temperature drop.



CALCULATIONS

Find the intersection of the two known properties, dry-bulb andwet-bulb temperatures, on the psychrometric chart,

The dry-bulb temperature is located along the bottom horizontalaxis.

Find the line for 78oF, which runs vertically through the chart.Wet-bulb temperature is located along diagonal dotted linesleading to scale readings at the upper, curved boundary marked"saturation temperature".

The intersection of the vertical 78oF dry-bulb line and thediagonal 65oF wet-bulb line has now established a "state point"for the measured air.

Read relative humidity as 50 percent (curving line running fromleft to right up through the chart) and dew point temperature as58oF (follow horizontal line, moving left, toward the curvedupper boundary of saturation temperatures).



Example 2 Winter ventilation

Often air is heated before it is introduced into greenhouse oryoung-livestock building environments. Consider an

application where outdoor air at 40oF (dry-bulb) temperature

and 80 percent relative humidity is heated to 65oF (dry-bulb)

before it is distributed throughout the building.

Find the state point for the incoming cool air on the lower left

portion of the psychrometric chart .



BASIC REFRIGERATION SYSTEM

Specific heat capacity:

Find the amount of heat (in kJ) that must be removed to

cool



BASIC REFRIGERATION SYSTEM

Properties of heat :Unit : Btu, amount of heat required to raise the temperature of 1 lb. of water 1oF. ( SI;

unit is J or kJ) heat required to raise the temp. of 1kg of water 1oC.

Example 1 : Cal. the quantity of heat required to raise the temp. of 62.4 lb.

(1 cu. Ft.) of water from 40oF to 80oF.

Solution:

Btu = wt. in lb. x temp. change in oF

Btu = 62.4 x 40 = 2496 Btu.

Example 2 : Determine the amount of heat removed to cool 50 lb. of water from 80oFto 35oF.

Sensible heat : is dry heat causing change in temperature but not in the moisture

content.latent heat : is the amount of energy released or absorbed by a chemical

substance during a change of state (changing from solid to liquid, or from liquid togas), or a phase transition.

Specific heat capacity : measure of the heat energy required to increase the temp. of aunit quantity(1kg) of a substance by a certain temperature interval(K)./ heatcapacity per unit mass, C/ c. Unit Btu.(R/AC), J/kg. K (SI unit).



REFRIGERATION SYSTEM COMPONENTS/ Basic

Evaporator : remove unwanted heat from the product, via the liquid refrigerant

(boiling at a low-pressure). Level of pressure is determined by two factors:

- rate at which the heat is absorbed from the product to the liquid refrigerant in theevaporator.

- rate at which the low-pressure vapor is removed from the evaporator by thecompressor.

Compressor: draw the low-temperature, low-pressure vapor from the evaporator viathe suction line.

Condenser: extract heat from the refrigerant to the outside air.

Expansion Valve/capillary tube:-located at the end of the liquid line, before the evaporator.

-high-pressure liquid reaches the expansion valve ( coming from the condenser).-valve reduces the pressure of the refrigerant as it passes through the orifice

-reducing the pressure, also decreases the temperature of the refrigerant to a level belowthe surrounding air. This low-pressure, low-temperature liquid is then pumped in to theevaporator.

Refrigerant; to conduct the heat from the product



Basic Refrigeration Cycle



REFRIGERATION SYSTEM COMPONENTS/ Safety devices

Accumulator: designed to protect the compressor by preventing liquid refrigerant

(knocking and damage) from flowing into the compressor. An accumulator should beincluded on all systems subjected to varying load conditions or frequent compressor cycling.

Liquid line strainer/drier: prevents plugging of the valve or tube by retaining

scale, dirt, and moisture. Moisture can freeze in the refrigerant control flow or mix with oilto form slug and acids.

Oil separator : Placed between the compressor exhaust and the condenser, separates the oil

from the compressed vapor. Too much oil entering the condenser, refrigerant controls, evaporator, andfilters interferes with operation.

Liquid receiver: storage tank for liquid refrigerant (especially during servicing). Usually on

systems which use low-side float control. Capillary systems do not use liquid receivers.

Other devices : evaporator pressure regulators, hot gas bypass regulators,

electric solenoid valves, suction pressure regulators, condenser pressure regulators,low-side or high-side float refrigerant controllers, service valves, etc.



Quick quiz

Reminders

To melt 1 ib. of ice 144 Btu are required (to melt 1 ton of ice 288,000

Btu are required).

To melt 1 kg of ice 335 kJ are required.

Questions

1. How many Btu will be absorbed in changing 25 lb. of ice at 50F to

water at 400 F.

Givens:

specific heat capacity of ice is 0.50btu/lb

Latent heat of fusion of ice is 144 Btu/lb.

2. With the aid of a sketch show the relationship between pressure

and temperature and explain how it is applicable to the AC &

refrigeration system.

Only those who prepare to pass at all times will escape failure.

baisc refrig and air con

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