RACA Guides September 2011asdasd

0,020

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Mo

istu

re c

on

ten

t kg

/kg

Dry

Air

Barometric Pressure 101,325 kPa

SEA LEVEL

40

4

5

50

55

6

0

65

7

0

75 80 85 90

Ent

halp

y at

sat

urat

ion

kJ/

kg D

ry A

ir

Volume m /kg dry air3

Dry Bulb Temperature °C

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15

2

0

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25

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40

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5

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–0,4

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25

C° erutarepmeT noitarutaS ro bluB teW

90%

80%

70%

60%

50%

Rel

ativ

e Hum

idity

40%

30%

20%

10%

–10 –5 0 5 10 15 20 25 30 35

LIMITED RANGE

PSYCHROMETRIC CHARTBASED ON “CARRIER”

Volume 1 Chapter 1September 2011

Daily reference guide to Psychrometrics

A RACA Journal initiative

guide

Sponsored by:

PSYCHROMETRICSChapter 1 of 9

Changes for the better

As a worldwide organisation, Mitsubishi is committed to training and the upliftment of knowledge and therefore it’s a great pleasure for us to sponsor this issue of the RACA Guide on Psychrometrics.

The determination of physical and thermodynamic properties of gas vapour is the basis of our understanding of the business of HVAC & R.

It is this fundamental that must be used on a daily basis for designers, engineers and contractors to gain the optimum benefit from a system design and for the ultimate benefit of the customer.

Geoff Alder has not only excelled himself in the compilation of this guide, he has provided an electronic workbook that further contributes to the understanding, in a practical way, of psychrometrics.

Mitsubishi believes in access to knowledge and providing such knowledge, especially to your young people. These young people, a country’s future, as well as our experienced folk, the mentors, we urge you to be willing to learn and mentor and become sponges, taking in all the reliable knowledge, to the benefit of this industry of which you are a proud member.

– Marco Ferdinandi, Marketing Director, MS Airconditioning

Message froM the sponsor

Daily reference to installation techniques & mandatory standards guide

pUBLIsher prInt: Rory Macnamara | [email protected] onLIne: Gary Macnamara | [email protected]: Glyniss Bone | [email protected]

fInancIaL Manager: René Oliver | [email protected]

DIscLaIMer: While every reasonable precaution has been taken to ensure the accuracy of the advice and information given, neither the editor, publisher, proprietor, staff, nor any official body represented or published in this issue, will accept responsibility for any damages, loss, injuries or false claims that may arise or be made in the content. We subscribe to the Codes of Conduct of the Advertising Standards Authority and the Press Ombudsman (refer separate explanations in this issue). Disclaimers by individual companies are hereby overridden by this disclaimer. Reproduction of any of the content is expressly forbidden in terms of the Copyright Act of 1987 with all amendments. All requests to reproduce must be made in writing to the publisher and such confirmation must be given in writing before proceeding. A copy of where the reproduction was published must be supplied to the publisher at the above address. No reasonable request will be refused provided all conditions are met. All Publication and Exhibi-tion titles are registered as trade marks in terms of the Trade Marks Act of 1993 and are held by Pipe Trades Media Group (Pty) Ltd, Reg No 2001/011401/07. The right to use these titles is granted to Interact Media Defined (Pty) Ltd, Reg No 1994/07015/07.

ManagIng DIrectorSean Macnamara, CA (SA)

heaD offIce13 A Riley Road, Bedfordview, 2008, South AfricaBox 695, Edenvale, 1610, South AfricaTel: +27 (0) 11 + 579 4940Fax: +27 (0)861 112 478Int: +27 (0)11 579 4940E-mail: [email protected]

RACA Guide l September 2011 3

What do you see half a metre in front of your eyes? Well, you can't see it, because it is air, and air is invisible—of course! But the final outcome of all air conditioning and most refrigeration systems is to add or remove energy from this invisible air. So, while this air is invisible in itself, its psychrometric properties assume great importance to us. HANDLING THE ENERGY WE CANNOT SEE, CONTAINED IN THIS AIR WE ALSO CANNOT SEE IS THE VERY REASON FOR THE EXISTENCE OF ALL AIR CONDITIONING INSTALLATIONS AND MOST REFRIGERATION SYSTEMS! As people who depend on this Industry for our incomes, it would be foolish for us to walk round blindfolded. We have a great need to UNDERSTAND this key subject!

Definition: Psychrometrics or psychrometry are terms used to describe the field of engineering concerned with the determination of physical and thermodynamic properties of gas-vapour mixtures.This energy comprises a mix of two forms—sensible and latent heat.

Explanation: Sensible heat: That aspect which can be sensed. i.e. As on a thermometer.Latent heat: Dictionary definition is 'hidden heat'. This is heat involved with a change in state (i.e. Evaporation or condensation of a liquid—in our present case this refers to the evaporation or condensation of water. i.e. Atmospheric humidity.) This latent heat aspect is NOT directly associated with any temperature shift.

The mixture that is air. Air consists of

�78% nitrogen�21% oxygen�1% other substances (Including water vapour)The water vapour contained in the air is termed humidity.

Understanding humidity: The understanding of humidity is the key to the understanding of the psychrometric chart. We are going to move in that direction.Fig 01 presents a highly theoretical impression of what we would see, were we to view a sample of BONE DRY AIR under an extremely powerful microscope. In fact, it may have to be more powerful than any microscope in existence. A molecule of O2 (oxygen) comprises two atoms of oxygen (O) which have firmly attached to one another. Equally, a molecule of N2 (nitrogen)

UNDERSTANDING AND USING PSYCHROMETRICS.Written and compiled by Geoff Alder.

WORKBOOK There is a downloadable Workbook to support this material available on the RACA Website. The intention of this is to make available some simple exercises for you to help embed “Psychrometrics” into your mind as you pursue a study of this interesting subject. This download also contains certain additional explanatory material. The pdf file containing this Workbook may be found at this address, www.refrigerationandaircon.co.za

Nitrogen molecules.

Oxygen molecules.

DRY AIR (GREATLY MAGNIFIED.)

Fig. 01

September 2011 l RACA Guide4

comprises two atoms of nitrogen (N) which have attached to one another equally tightly. These are natural ‘default’ situations within the chemistry of Nature. It is completely normal for air to contain water vapour. (Humidity.) Even in the driest desert, there would be at least 5% or so of relative humidity. In fact, the only place you would find bone dry air is in a laboratory. In Fig. 02 we have allowed an amount of water vapour to mix in with the ‘bone dry’ air sample of Fig. 01. Fig. 03 shows how we get from ‘dry air’ to ordinary room air. When made available, the molecules of water vapour simply ‘slot into’ the relatively enormous gaps between the ‘air’ molecules. (Which is how we could reasonably view the mix of oxygen and nitrogen molecules which constitute ‘dry air’.)

Water vapour molecules.

“ORDINARY” HUMID AIR.Fig. 02

“NORMAL” HUMID AIR.

DRY AIR

WATER VAPOUR

THE MIXTURE THAT IS“NORMAL AIR”

Fig. 03

Mix.

Mix.

2

If you have printed out your Workbook to obtain maximum benefit from this material, please work through Exercise 1. You will find in several cases that additional assistance is given in the Workbook.

We require to take a look at DALTON’S LAW:

In chemistry and physics, Dalton's law (also called Dalton's law of partial pressures) states that the total pressure exerted by a gaseous mixture is equal to the sum of the partial pressures of each individual component in a gas mixture.

What we glean from Dalton’s Law is that each gas of a mixture ‘does its own thing’, just as it would do if it was on its own, but under the same temperature conditions, as we shall attempt to explain. So the water vapour of the lower left block of Fig. 03, where it stands on its own, will follow precisely the same rules of pressure versus temperature at saturation as will do the same water vapour, but now ‘slotted in’ with the ‘air’ molecules, to the right of Fig. 03. If the water vapour of the lower left block of Fig. 03 was transferred exactly as it is to the right block (and if we happened to be at sea level, where atmospheric pressure is 101 kPa, or 101,325 kPa exact) then the presence of that 3 kPa of water vapour would contribute 3 kPa to the total 101 kPa of the sample. The contribution of oxygen + nitrogen in this case would be 98


CHANGE OF STATE. We need to focus on the energy implications of a change of state. While we can produce liquid oxygen or nitrogen, these two can only be converted into their liquid states at temperatures which are vastly too cold for human survival. Therefore in psychrometrics we only need consider those two as being gases. Any heating or cooling processes regarding them can only be sensible. (A simple temperature change.) Atmospheric humidity is completely different. In many cases it changes state, entering or exiting the system of our psychrometric study as it does so. Herein lie considerable energy (enthalpy) implications.For example, note the enthalpy value ringed in in Table 1. This is the enthalpy of one kg of saturated water at 20º. (Saturated water is any water in isolation that is on the threshold of evaporating.) Enthalpy of a kilogram of saturated vapour is ringed in . The difference between these two (i.e. The amount of heat that must be added or abstracted to bring about many of the changes involving humidity is considerable. The enthalpy to be added to evaporate one kg of water at 20º is boxed in . This should convince you of the great energy-related importance of humidity in ‘air’ calculations. The psychrometric chart has been designed to handle such matters with ease.

blue

red

green

kPa. If vapour pressure of the moisture was only 2 kPa, then the pressure of the oxygen + nitrogen molecules of the mix would total 99 kPa. (Assume we are making these imaginary changes in the air of a normal ‘leaky’ room, which is far from airtight, and are thereby avoiding any awkward ‘squeezing in’ questions.)

Please work through 2 in your Workbook.Exercise

TABLE 1: ENTHALPY OF WATER

Abs

olut

e pr

essu

re(k

Pa)

3


A REFRIGERANT AT SATURATION. If you come from a refrigeration background you may have some understanding of just what we are talking about if we refer to “A refrigerant at saturation”. In case you do not, we will provide a swift although rather scanty explanation. If we had a peep-hole into a refrigerant cylinder which was half-filled with a refrigerant, we would see something as appears on Fig. 04. Before charging the refrigerant into the cylinder, we would first EVACUATE this vessel. As an outcome, as liquid later is allowed to be sucked in, some vapour would be formed from evaporation of part of the liquid, and would rush to fill the void above the liquid. The whole will settle itself to adopt the SATURATED CONDITION. The saturation condition is a variable– the higher the temperature, the greater the number of water molecules in the vaporous state that would be driven from the liquid phase to occupy a unit of volumetric measurement within the vapour. This drives up the pressure, following a precise

Saturated vapour.

Saturated liquid.(In this case, WATER.)

THE SATURATED CASE.

Fig. 04


relationship. This relationship of temperature versus vapour pressure is unswerving and absolute never changing for any given pure refrigerant. Please note that WATER is a recognised refrigerant, (Water is termed R-718. By way of comparison, ammonia is R-717.) Any pure refrigerant has its own unique pressure-temperature relationship. Apart from water enjoying its own unique pressure-temperature relationship, the saturation situation surrounding water is as for any other refrigerant. In pressure terms, its saturation curve is very much lower than that for any of that of the ‘usual’ refrigerants. A narrow band of such data for water ( Range 0º to 50º) appears in Fig. 05.

0 50 100 150

250

200

150

100

50

0

Temperature (ºC)

Pre

ssure

(kP

a A

bso

lute

)

PRESSURE VERSUS TEMPERATURE OF WATER AT SATURATION.

Atmospheric pressure (Sea level.)

Atm pressure (Gauteng.)

Fig. 05

Tem

pe

ratu

re (

ºC)

AB

SO

LU

TE

pre

ssu

re (

kPa

)

TABULAR. GRAPHED.

4

In Fig. 06, we have played around with the content of Figs. 03, 04 and 05, and have mixed saturated water vapour at 20º in with an equal volume of bone dry air, also at 20º. This will result in a marginal increase of volume in the original dry air example. The pressure will equalise through room leakage. As previously stated, we have no need to distract ourselves by exploring that avenue.

20º20º

20ºC. Therefore this SATURATED water vapour is at 2,34 kPa (Abs.) as per Fig. 5.

Bone dry atmosphericair at 20º

Result is SATURATED air at 20º

Fig. 06

THE MIXING OF EQUAL VOLUMES OF BONE DRY AIR ANDSATURATED WATER VAPOUR.

(A REHASH OF FIG. 3 AND A THEORETICAL EXERCISE.)

20ºMix. Mix.

Water vapour.

Ga

s cy

lind

er.



SATURATED AIR The bone dry air sample we conjured up in Fig 06 had a matching volume of SATURATED water vapour merged into it, with the two sources, for the sake of consistency, both being at 20º. The situation indicated by the term ‘saturated’ is that the water vapour within its previous confines of the cylinder, at that temperature, would be carrying all the water molecules in the vaporous state that it was capable of carrying. (If it had arrived at that state by way of a fall in temperature, there would at this arrival have briefly been present an excess of molecules in the vapour state. That excess would CONDENSE, and drift and dribble down to join the liquid water volume. Conditions would normalise to saturation.) If we recall the ‘Dalton’s Law’ discourse, and we realise that the water vapour, although now thoroughly mixed in with the dry air sample, will continue to behave exactly as it did when in isolation inside that cylinder. This is a key and critically important understanding. What we particularly need to grasp at this moment is that our air sample presently is at saturation. In other words, it is presently standing at 100% relative humidity. It is carrying all the moisture in vapour form that it is capable of carrying WHEN AT THAT TEMPERATURE. If the sample’s temperature were reduced, it would be compelled to lose some of its previous content of water vapour. The mechanics of this would be for a mist of liquid droplets to develop, and these would slowly settle to form a layer of liquid. (As such, that part would remove itself from the psychrometric chart,and the heat of condensation, produced by this change would be reflected on the psychrometric chart as a cooling requirement that we would . This happens at a cooling coil when this is actively cooling and dehumidifying. ) Conversely, if the air sample was heated, it would be capable of carrying MORE water as a vapour. (It would be up to us to introduce that water, by way of one or another psychrometric process. See Fig. 23 / 24.) If evaporation were to take place in connection with this opportunity, heat would be required to bring this about. The psychrometric plot would make possible calculation of the exact amount of necessary heat.

“DRY AIR” ON THE PSYCHROMETRIC CHART. As we become involved with the psychrometric chart, we will see that it has been configured to provide the user with many different properties of 1 kg of DRY air at any intersect point of conditions. As we are aware, air in nature always contains a percentage of moisture. Therefore this description of dry air containing moisture strikes many as being a conflict of terms. But, in the foregoing illustrations we have seen bone dry air, then with moisture added, thus to become HUMID air. The amount of air considered on the psychrometric chart comprises sufficient “air” molecules (i.e. Oxygen and nitrogen) for that alone to have a mass of 1 kilogram. To this dry air ‘vehicle’ is added, as a ‘passenger’, a VARYING amount of water vapour.

BASIC STRUCTURE OF THE PSYCHROMETRIC CHART. Fig. 07 illustrates the basic grid upon which our customary South African version of the psychrometric chart is constructed. (Also for USA, UK, etc.)The x-axis is a scale of DRY BULB TEMPERATURE. (See Fig. 6.) This is limited in extent to cover the intended use of the chart. (e.g. The range shown here would suit normal air conditioning. A low temperature chart would cover anticipated cold and freezer room temperatures. A chart intended for use in connection with kiln drying of timber and other materials would cover the high temperature range of such an activity. The y-axis has been displaced to the right, from the customary y-axis position on the left of the chart. This has been done so as to rationalise the functionings of the completed chart.

ALTITUDE Altitude is of great significance. We will require to select for use a chart which has been prepared for the ALTITUDE OF THE APPLICATION, and also one which will embrace the entire thermodynamic process within its bounds.

5


6

SATURATION LINE. The SATURATION line completes the boundaries of the chart. Thus the working boundaries of the chart are as depicted in Fig. 08. The significance of the saturation line as a boundary is that water features on the chart solely in the vaporous condition. If vapour is converted to a liquid as part of the process, it will move to the left, beyond the saturation line, and will disappear from off that chart. For example, in the extremely common process of cooling and dehumidifying, liquid water will form on the fins of the cooling coil, and will run down into the condensate drain. From here, it usually is allowed to flow to waste. It is possible to do a physical measurement of the liquid run-off by measuring into a bucket, while timing the flow with a stop-watch, and converting to kg/s. This may be compared with a predicted run-off rate on a psychrometric chart as a verification procedure, accompanied by information which allows determination of the amount of refrigeration that must be committed to achieving this objective.

DRY BULB TEMPERATURE.

MO

IST

UR

E C

ON

TE

NT.

WORKING BOUNDARIESOF THE CHART. Fig. 08


1. Far from sufficient attention is paid to the fact that condensate runoff from a cooling coil is excellently suited as makeup feed water for a cooling tower or an evaporative condenser. It does not contain the dissolved solids of a regular water supply, and therefore will not cause significant scaling, if any, of heat exchange surfaces. This is of greatest importance in humid regions, where a considerable amount of condensate is yielded by the dehumidification process. Where water is costly and in short supply, it is practically inexcusable to not use this water runoff for non-potable applications, such as cooling tower makeup feed water. (An aside: NEVER use untreated ground water for this purpose. Doing that can introduce microbial corrosion on a horrific scale.)

0,028

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0,00

0,010

Mo

istu

re C

on

ten

t (k

g/k

g D

ry A

ir)

Saturation Te

mperat

ure

(°C)

0

5

10

15

20

25

30

Dry Bulb Temperature (°C)

0 5 10 15 20 25 30 35 40 45 50

Co

nst

an

t d

ry b

ulb Constant moisture content.

BASIC FRAME OF THE PSYCHROMETRIC CHART. Fig. 07

CHART IS FOR…1 kg of DRY AIRAnd is valid for a

SPECIFIED ALTITUDE.


MOISTURE CONTENT The scale of moisture content is shown vertically on the right side of the chart. Note that it is expressed in kg/kg of dry air. (This serves to give us the number of kilograms of moisture–a small decimal value–which is contained in any one kilogram of dry air that we might be examining.) We have stated kg/kg, as it is a rule of the SI system that multiples of units shall not be mixed in a single expression. However, you will find that use of the term ‘grams per kilogram’ is often encountered. This is similar to use of the pressure term: ‘bars’. Rules of SI prefer that decimal intervals between units and sub-units are in removes of THREE. Bars require a remove of two decimal places, when converting from kPa. This adds confusion. But the term is in widespread use. Like g/kg, it may be considered a confusion-setter, but is not viewed as being a totally serious misdemeanour. A second name for the moisture content scale is the SPECIFIC HUMIDITY SCALE. Units, as before, are kg/kg dry air. ‘Specific’ means, of course, that we have in mind the ACTUAL total amount if water vapour in that air sample. The term ‘specific humidity’ is less in every-day use than the term ‘relative humidity’. There is unfortunately widespread confusion between these two within the Industry. You will need to get your own thinking extremely clear, so that you can assist in eliminating this Industry-wide confusion.

RELATIVE HUMIDITY. Fig. 09 introduces us to the concept of relative humidity. The constant temperature line signifying 25º dry bulb temperature will provide a good, rounded example. Note that at 25º saturation, the moisture content is 0,020 kg/kg of dry air. An intersect point has been indicated half-way down the dry bulb line that originates at 25º saturation. You will see that, at this point, moisture content is shown as being 0,010 kg/kg. Exactly 50% of the 0,020 kg/kg it was at saturation. By the same token, the specific humidity of 20º saturated air is almost 0,015 kg/kg. Where 20º intersects the 50% rh, the moisture content is almost 0.0075 kg/kg. (I am well aware that almost

7

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Mo

istu

re C

on

ten

t (k

g/k

g D

ry A

ir)

Saturation Te

mper

atur

e (°

C)

0

5

10

15

20

25

30


0 5 10 15 20 25 30 35 40 45 50

ytidimu

H evitaleR %05

THE BASIS OF RELATIVE HUMIDITY. Fig. 09


8

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istu

re C

on

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t (k

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ry A

ir)

Saturation Te

mper

atur

e (°

C)

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ytidimu

H evitaleR %05

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ytidimu

H evitaleR %05

THE RELATIVE HUMIDITY LINES. Fig. 10


is not good enough in a psychrometrics exercise, but it is used here entirely to avoid ‘crazy’ decimal places in an attempt to illustrate a difficult point in uncomplicated terms. This thinking may be extended across the face of the chart, to provide a full family of relative humidity lines. (Fig. 10.)

This means the chart-maker must get down to very high levels of accuracy, but this all is stems from the rationale we originally provided for the derivation for 50% rh at 25ºC.

VAPOUR PRESSURE. You will recall that our original curve of Fig. 05 was based on vapour pressure. Things have now moved to moisture content. The outcome is very similar. But, to focus too much on this point (kPa absolute versus kg/kg dry air) would add unnecessary confusion. It will receive mention on our ‘Exercises’ download.

A SLING PSYCHROMETER (AKA “WHIRLING HYGROMETER”.)A sling psychrometer is described in Fig. 11. In application, it is whirled rapidly. (e.g. At about 200 rpm.) The bulbs of the two thermometers are thus well aspirated. The dry bulb thermometer (the uncovered bulb) will register the true (“dry bulb”, or “DB”) temperature, the same as it would were it not being spun. However, evaporation will be free to take place from the wetted ‘sock’. (Wet a finger in your mouth, and blow on it. You will notice a distinct cooling. This is due to your skin sensing the removal by evaporation of the latent heat of evaporation from the moisture.) Evaporative cooling by evaporation of bodily perspiration is a natural cooling mechanism of the human body. In part, it is one function of air conditioning to take care of this. The temperature measured under this wetted wick is termed “wet bulb, (or WB,) temperature”. Point here is, evaporation of water from the close-fitting wetted ‘sock’ will depress the temperature of its water content. This temperature depression will be registered by the


9

Water reservoir.

A close-fitting wettedcotton wick is fittedover this bulb.(For WB reading.)

Two identical, high qualitythermometers.

Frame carried on a bearing.

SLING PSYCHROMETER. Fig. 11

WET BULB READING.

DRY BULB READING.

Bulb directly exposedto airstream. (For DB reading.) (Use distilled water. OK to use water

from cooling coil condensate run-off)

appropriate thermometer bulb. If the air sample being measured contained water vapour at saturation, no evaporation would be possible. The wet and dry bulb temperatures would reflect this by both registering the same values. See on Fig. 12 the example as would be measured for saturated air at 25º. The further the air sample is from saturation, the greater the extent of evaporation, and therefore the greater will be the wet bulb depression. With regard to the use of a sling psychrometer (or, for that matter, for all your work in psychrometrics) it is necessary to work as close to an accuracy of 0,1K as possible.

DEW POINT.Consider the condensation on this bottle of cold water. (Fig. 13.) How does such condensation occur? We need to look at the humidity in the room, and something termed the ‘dew point’ to find that answer. If there is a cold surface inside a space (e.g. An uninsulated chilled water pipe running within the space), this will lower the air temperature at the contacting surfaces. This could cool the contacting air to the point of saturation. (Fig. 09, and associated discussion.) This will not yet bring about wetting down, or sweating. But the situation will be poised to bring about condensation of part of the moisture contained in the contacting air. This important point is termed the “dew point”.

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Moi

stur

e C

onte

nt (k

g/kg

Dry

Air

)

Saturation Temperature (°C)

0

5

10

15

20

25

30

Dry Bulb Temperature (°C)0 5 10 15 20 30 35 40 45 50

90%

80%

70%

60%

40%

30%

20%

10%

ytidimu

H evitaleR %05

Wet Bulb Temperature

THE WET BULB LINES.

25

25º

DB

.

25º WB.

Wet bulb and dry bulb linesfor the same value alwaysintersect at saturation.

Fig. 12

100% rh.

Bottle contains ‘fridge water’.

Note condensation formationon its outer surface.

THE FORMATION OF CONDENSATION.

Fig. 13


10

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Mo

istu

re C

on

ten

t (k

g/k

g D

ry A

ir)

Saturation Te

mper

atur

e (°

C)

0

5

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15

20

25

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0 5 10 15 20 25 30 35 40 45 50

ytidimu

H evitaleR %05

90%

80%

70%

60%

40%

30%

20%

10%

ytidimu

H evitaleR %05

ESTABLISHING THE DEW POINT AND SPECIFIC HUMIDITY OF AN AIR SAMPLE.

Fig. 14

50%

rh

29,7º DB.

Dewpoint =18º C.

To establish DEW POINT,plot horizontally from theroom condition to thesaturation line.

To establish the SPECIFIC HUMIDITY,plot horizontally from the room conditionto the moisture content scale.

Sp

eci

fic

hu

mid

ity

= 0

,01

32

kg

/kg

of

dry

air.

Room condition.

This tells us that any surface which might be cooler than the dew point of the room (18º in this case) has the potential to sweat. At 18º it will be borderline. If the bottle contained water at 15º, you certainly could expect it to sweat. If its contents were at 12º, you could expect considerable sweating to take place, possibly to the point of run-off and nuisance. There can be no problem in this instance with any surface that is higher than 18º.This procedure may be followed to determine (e.g.) whether a duct, containing air at a certain temperature, and passing through a room at a given set of conditions is likely to sweat if it is not insulated.

ENTHALPY A further major situation covered by the psychrometric chart is that of providing ENTHALPY values. (Enthalpy is assigned the symbol ‘h’.) While the full engineering definition of enthalpy is far reaching and complex, for our purpose in psychrometrics it is sufficient to consider ‘enthalpy’ as being ‘heat content’. In the psychrometrics case, as the chart refers to the properties of 1 kg of dry air, enthalpy would be expressed as being ‘per kilogram of dry air’. The unit of energy (in heat, or other form) is the joule. (J.) As we are working with kilograms of air as our unit, and not with grams, our heat content value would be expressed as ‘kilojoules’. (kJ) Thus the unit of heat (energy) within psychrometrics is expressed in full as kJ/kg of dry air. (Once we are amid a discussion, the ‘of dry air’ rider is usually accepted “as read”.) In obtaining the enthalpy value of the plotted chart condition above, a CONSTANT WET BULB line has been followed. While very near the mark, this is not wholly true. The lines of constant enthalpy fall very close to being parallel with lines of constant wet bulb temperature. But not absolutely so. (See Fig.16.) Our charts thus far have been much simplified of detail, and this minute deviation is not highly obvious. Fig. 16 has been extracted from a “full” chart. Fig. 16 shows this cut from a CSIR (Actually the old NMERI) psychrometric chart with a black line, representing a line of constant enthalpy, having been overscored. A glance at Fig. 16 will convince you as to why psychrometric charts, other than those printed in multicolour and in an unusually large format, cannot carry both wet bulb and enthalpy lines. The Industry has two solutions. Many charts, including those published by the CSIR, have a


11

perimeter scale for enthalpy. Once you have identified a point on the chart, for which the enthalpy value is known, you will place a straight-edge directly over that point, and will adjust it until it intersects the same enthalpy value on both scales. That, without further ado, is the enthalpy value of interest. Fig 17 illustrates this case for a check that produced an enthalpy reading of 61,0 kJ/kg. Willis H Carrier (widely recognised as being the Father of Air Conditioning) was the inventor of the psychrometric chart. His approach was different from that just explained. I personally find his method to be considerably easier to use. Dr Carrier has preliminary enthalpy lines running parallel with wet bulb lines. His enthalpy scale has been adjusted to be correct at saturation. His chart has a few curves which indicate subtractions to be applied for correction purposes.

0,028

0,025

0,020

0,015

0,005

0,00

0,010

Mo

istu

re C

on

ten

t (k

g/k

g D

ry A

ir)

1015

2025

2530

3540

4045

5055

6065

7075

8085

9095

100

Saturation Temperature (°C)

0

5

10

15

20

25

30

0,08

0,85 0,90

3Volume (m /kg Dry Air)


0 5 10 15 20 25 30 35 40 45 50

PSYCHROMETRIC CHARTProperties of 1 kg of dry air.

Sea Level — 101,325 kPa

90%

80%

70%

60%

40%

30%

20%

10%

ytidimu

H evitaleR %05

Enth

alpy

at s

atur

atio

n (k

J/kg

Dry

Air)

Wet Bulb Temperature

The enthalpy scale.

A known set of conditions.

h = 50 kJ/kg dry air.

THE ENTHALPY SCALE. Fig. 15

18º WB. (Green.)

h = 51 kJ/kg. (Black.)

CLOSENESS OF WET BULB AND ENTHALPY LINES. Fig. 16


12

Fig. 17READING AN ENTHALPY VALUE ON A PERIMETER-SCALED CHART.

A STRAIGHT EDGE.

Straight-edge alignmentmust be with BOTH the left and right sides scales.


The sea level chart which appears as Fig. 18 will be too tight to read, as it should occupy at least a full A4 page. (Preferably A3.) But the prime objective now is to illustrate the enthalpy deviation lines and the correction values.


SPECIFIC VOLUME. Another (more limited) family of lines presented on the chart provide specific volume. Whenever you see the word “specific”, used in conjunction with SI material such as this, know that it means “per kilogram”. In this case, ‘specific volume’ tells us what volume a kilogram of air would occupy at chart altitude. Note that specific volume is one of the values that alters substantially for a change of altitude. Fig. 21 determines specific volume at sea level for the popular indoor conditions of 22º DB; 55% rh would be 0,85 m³/kg.

THE EFFECT OF ALTITUDE. Fig 20 will provide some idea of the effect of altitude upon any sample of air. As air, unlike water, is COMPRESSIBLE, the greater the atmospheric pressure, the more closely the molecules of ‘air’ (nitrogen and oxygen) will be bunched. In improved technical terms, as we go up in altitude from sea level, the air becomes less dense. A kilogram of air will become larger. At sea level, it will occupy about 0,83 cubic metres. At 1700 metres (Johannesburg), it will have expanded to occupy about one cubic metre. This is one reason why we must work initially in kilograms of air, not in volumetric terms, such as l /s, or as m³/s. That conversion can only come later. Also, note that it is necessary to work on a psychrometric chart which has been made for the altitude of the installation under consideration.


13

–10 –5 0 5 10 15 20 25 30 35 40 45 50 55

0,033

0,032

0,031

0,030

0,029

0,028

0,027

0,026

0,025

0,024

0,023

0,022

0,021

0,020

0,019

0,018

0,017

0,016

0,015

0,014

0,013

0,012

0,011

0,010

0,009

0,008

0,007

0,006

0,005

0,004

0,003

0,002

0,001

0,00

Mo

istu

re c

on

ten

t

kg

/kg

Dry

Air

0,36

0,40

0,45

0,50

0,55

0,60

0,65

0,70

0.75

0,80

0,85

0,90

0,951,00

Se

ns

ible

He

at

Fa

cto

r

Carrier

PSYCHROMETRIC CHARTNORMAL TEMPERATURES

SI METRIC UNITSBarometric Pressure 101,325 kPa

SEA LEVEL

40

4

5

50

55

6

0

65

7

0

75

80

8

5

90

95

1

00

1

05

110

1

15

Enth

alpy

at s

atur

atio

n k

J/kg

Dry

Air



Below 0°C Properties and Enthalpy Deviation Lines Are For Ice

10

15

2

0

25

25

30

3

5

40

–10

–5

0

5

10

120 125 130 135 140 145

35 40 45 50 55

En

t hal

p y d

evi a

t i on

kJ /

k g d

r y a

i r

0,95

0,75 0,80 0,85 0,90

–5

0

5

10

15

20

25

C° erutarepmeT niotaruta Sr olbu BteW

30 90%

80%

70%

60%

50%

40%

30%

ytidimuH evitaleR

–0,0

5–0

,1

–0,2

–0,4 –0,6

–0,8

–1,0

–1,2

Enthalpy values.(Correct at saturation.)



10

15

20

–0,0

5

–0,1

–0,2

–0,4

Pink box has beenenlarged below.

Fig. 18ENTHALPY DEVIATION LINES ON THE “CARRIER” CHART.

Measured condition:17,0º WB; 24,5º DB.

h = 47,9 – 0,2 = 47,7 kJ/kg.

47,9

Enthalpy deviation correction values.

20%

10%

17,0º WB; 24,5º DB.

The reason for this variation of the chart with altitude is that gravity pulls down on air. Fig. 20 will give you some idea of how this comes to be. As it is the MASS of air which is being handled by any air conditioning system, we will find that, for any air conditioning installation of (say) 1000 kW, an installation at the coast or in Gauteng would be designed to handle the SAME kg/s, or mass flow rate, of air. However, due to the air being more expanded in Gauteng, the air VOLUME flow rate would be GREATER. The ducts would be larger for the same capacity, and it might be necessary to select a larger fan. (Or to drive the fan faster.) However, the POWER required to drive the fan would remain unchanged.


14

–10 –5 0 5 10 15 20 25 30 35 40 45 50 55

0,033

0,032

0,031

0,030

0,029

0,028

0,027

0,026

0,025

0,024

0,023

0,022

0,021

0,020

0,019

0,018

0,017

0,016

0,015

0,014

0,013

0,012

0,011

0,010

0,009

0,008

0,007

0,006

0,005

0,004

0,003

0,002

0,001

0,00

Mo

istu

re c

on

ten

t

kg

/kg

Dry

Air

0,36

0,40

0,45

0,50

0,55

0,60

0,65

0,70

0.75

0,80

0,85

0,90

0,951,00

Se

ns

ible

He

at

Fa

cto

r

PSYCHROMETRIC CHARTNORMAL TEMPERATURES

SI METRIC UNITSBarometric Pressure 101,325 kPa

FOR 1 kg OF DRY AIR AT SEA LEVEL

40

4

5

50

55

6

0

65

7

0

75

80

8

5

90

95

1

00

1

05

110

1

15

Enth

alpy

at s

atur

atio

n k

J/kg

Dry

Air



Below 0°C Properties and Enthalpy Deviation Lines Are For Ice

10

15

2

0

25

25

30

3

5

40

–10

–5

0

5

10

120 125 130 135 140 145

35 40 45 50 55

En

t hal

p y d

evi a

t i on

kJ /

k g d

r y a

i r

0,95

0,75 0,80 0,85 0,90

–5

0

5

10

15

20

25

C° erutarepmeT niotaruta Sr olbu BteW

30 90%

80%

70%

60%

50%

40%

30%

ytidimuH evitaleR

–0,0

5

–0,1

–0,2

–0,4 –0,6

–0,8

–1,0

–1,2

20%

10%

Fig. 19

Popular indoor conditions:22,0º DB; 55% rh.

Specific volume ofthis air is 0,85 m /kg.3

SP

EC

IFIC

VO

LU

ME

LIN

ES

.

THE SPECIFIC VOLUME LINES.ON A SEA LEVEL CHART.


CITY or AREA

All coastal regions.(i.e. Sea level)

PietermaritzburgNelspruitUpington

0

671670814

CITY or AREA

PretoriaBloemfonteinPolakwane

JohannesburgGermistonWindhoek

136914261293

175316651728

ALTITUDE(m)

ALTITUDE(m)

TABLE 2: ALTITUDE OF SOME SOUTH AFRICAN PLACES.

Fig 21 illustrates a plot for specific volume for air at these same conditions of 22º DB; 55% rh, but when occurring at the altitude of 1700 metres. This indicates that the 1 kg package of air would have expanded from occupying 0,85 m³ at sea level, to now occupying 1,02 m³.

Frequently it is necessary to establish the conditions of an air mixture on the psychrometric chart. For example, we could know outside air design conditions and intended return air conditions on an installation that is undergoing design. It is necessary to provide the potential suppliers’ of the cooling coil with the DB and WB conditions that will enter that coil. (Plus various other data.) An example follows. The relevant data are contained in the box top left of Fig. 22. Essentially, the mixture condition will be found upon a straight line which connects the two primary conditions. Its actual point will be determined by the proportions of (in this example) outside air and return air. If there was 50% outside air mixing with 50% return air, the mixture condition would be exactly at the midpoint of the line. But it seldom is that easy!


Fig. 21SPECIFIC VOLUME LINES ON A CHART FOR 1500m

–0,0

5

–0,1 –0

,2

–0,4

–0,6

5

10

+1,0



30%

20%

10%

PSYCHROMETRIC CHARTFOR ALT 1500m

1,02 m³/kg.0,95 1,00 1,05

The same indoor conditions:22,0º DB; 55% rh.

ABSOLUTE PRESSURE OF ATMOSPHERIC AIR DUE TO GRAVITY ACTING ON THE AIR COLUMN. Fig. 20

80,0 kPa Abs.

101,325 kPaAbsolute.

33 kPa Absolute.

15

On the next page you will be provided with an example of how this situation may be tackled.


1. We measure the line length. It is 69 mm.

2. We proportionalise. We choose between placing the amount of outside air over the total air flow, OR the amount of return air over that total. We have opted for working on the basis of outside air over total air.

16

AIR MIXTURES ON THE PSYCHROMETRIC CHART Fig. 22

24,8º DB

18,5º WB

Given conditions:

Experience has shown that the best .rule of thumb’ way to proceed from here is to apply the logic that the mixture point will be closest to the greater volume of source air. In this case, return air contributed the greatest source air volume flow. Therefore the mixture point will be closest to the return air entry point. The calculation showed 10 mm out of 69 mm. Therefore we will measure 10 mm UP from the return air entry to establish the mixture point. Our reading of this derived mixture condition is: 24,8º DB; 18,5º WB. These are the conditions which will appear as information to the potential suppliers who may be quoting for the required cooling coil or coils.

PSYCHROMETRIC PROCESSES A psychrometric process will move “from” a point “to” a point. Our study, in one way and another, will be determine “what is to happen” along the way, and the energy involvements and other technical requirements of establishing that process. To start out, we will consider a given ‘starting’ point, and will consider the nature of the processes which potentially could be followed. Our ‘starting point’ (not necessarily realistic for each process) will be 25,0º DB and 50% rh.

0,4(2,3 + 0,4)

× 69 mm = 10 mm.


N.B. While the starting point for these processes have all been shown starting from what is perhaps a ROOM CONDITION, in many cases actual start would be nearer the point marked with a star. Various processes might indeed have start points from many different points on the chart. In Fig. 24 we have indicated pictorially the type of equipment we could expect to find in connection with each discrete process.

17

50% rh

25º

DB

.AB

CE

F

G

H

D

DIRECTION FOLLOWED BY VARIOUS PSYCHROMETRIC PROCESSES. Fig. 23

There will be more explanation of these processes, along with questions and answers, in the printable Learner’s Workbook.

Changes for the better

Daily reference to installation techniques & mandatory standards guide

THERMODYNAMICSLook out for the next issue on

Sponsored by:


18

ACASE DESCRIPTION WHAT OCCURS

Sensible heating of air. There is an increase in DB; no change in moisture content. Therefore process will extend HORIZONTALLY to the right.

B

NO condensation.

Sensible cooling of air. There is a reduction of DB; no change in moisture content. Situation does not involve the formation of condensation. Therefore NO moisture leaves the system. Process moves HORIZONTALLY left. Does not reach the saturation line.

C Sensible AND latent cooling of air.Moisture is REMOVED from the system.

There is a reduction in DB and in moisture content. (Condensation is to be seen actively running off coil fins.) Net process line moves left AND curves downward.

D

HEATER BANK.

COOLING COIL.

COOLING COIL.

WATER SPRAY BANK. Evaporation of water. There is a REDUCTION in DB, but an equal (in energy terms) INCREASE in moisture content. Process follows a line of CONSTANT WET BULB to the left. This is an ADIABATIC process.

There is a INCREASE in DB, but an equal (in energy terms) REDUCTION in moisture content. Process follows a line of CONSTANT WET BULB to the left. This is an ADIABATIC process.

DESICCANT MATERIAL.

E Chemical drying of air.

Sensible and latent heat gain.

Typical process for heat flowing from diffuser in an application with both sensible and latent heat loads. Typical a/c process (given a lower starting point) for summer cooling in a humid region.

Sensible heat gain, latent heat loss.

FSimilar to , but in a dry area where there could be an overall moisture LOSS, due to moisture outflow through porous structure to desert-like outdoor air.

H

STEAM JET.

F

Humidification by steam injection.Moisture IS ADDED to the system.

A common supplementary process for close control of humidity in (e.g.) a computer room. (About 90% of the total heat being injected is LATENT heat. The sensible heat fraction is undesirable, but unavoidable).

DESCRIPTIONS OF THE PROCESSES OF FIG. 23. Fig. 24

Condensate run-off.

High pressure, finewater spray bank.

High humidity.

G

Dry desert-like.

*

*See Fig. 22 for a more complete representation of this common process.


19

See Fig. 25. We will think of what occurs to the conditioned air from the time it leaves the coil face, and until it enters the return air grille. While passing along the route from to , it will pick up heat. This will be both gains of sensible heat, from the lights, computers, indirectly from the sun’s heat and sensible heat from the people themselves. Simultaneously, there will be gains of latent heat, from the breath and perspiration of the folks in the room, and also outdoor humidity, which presses in through any crack, such as round doors and windows, and also that amount of moisture driven inward by penetration of vapour through the fabric of the structure. (No moisture will press in through glass; moisture in vapour form will certainly be pressed in through brick and concrete.) These various gains will both drive the temperature upward, as well as driving upward the vapour content of the air as the air travels from to .

Supply air diffuser.

THEME: The cooled, dehumidified air gently flows through the conditioned space. As it travels, it “mops up” excesses of both sensible and latent heat.

Return air grille.A

B

Heat pickup occurs“along the way”.

PICKUP OF BOTH SENSIBLE AND LATENT HEAT. Fig. 25

A “real life” process.

The upshot of the foregoing would show on the psychrometric chart as in Fig. 26. Note how this forms an angle, which directly relates to the ratio of sensible versus latent heat of the process. Indeed, this is resolved on the chart as the ratio of sensible to total heat. Assume you were designing an application. For a conditioned area, your designs showed a sensible heat loading of 8 kW and a latent loading of 2 kW. Total heat in this case would be 8 + 2 = 10 kW. (Total heat is the amount of refrigeration that would have to be applied to accommodate the calculated loading of this specific space).

Air leaving cooling coil.

Room condition.

Sensible Heat Factor Ratio (SHFR.)

EXPLAINING S.H.F. RATIO. Fig. 26

The same on aPsychrometric chart.

A B

A B


18

In point of fact, the SHF is the ratio of SENSIBLE HEAT to TOTAL HEAT. So for our example we would have

810

SHF = = 0,8

0,033

0,032

0,031

0,030

0,029

0,028

0,027

0,026

0,025

0,024

0,023

0,022

0,021

0,020

0,019

0,018

0,017

0,016

0,015

0,014

0,013

0,012

0,011

0,010

0,009

0,008

0,36

0,40

0,45

0,50

0,55

0,60

0,65

0,70

0.75

0,80

0,85

0,90

0,951,00

Se

ns

ible

He

at

Fa

cto

r

40

4

5

50

55

6

0

65

7

0

75

80

8

5

90

95

1

00

1

05

110

1

15

Enth

alpy

at s

atur

atio

n k

J/kg

Dry

Air

120 125 130 135 140 145

35 40 45 50 55

–0,0

5

–0,1 –0

,2

–0,4 –

0,6

–0,8

–1,0

–1,2

En

thal

py d

evia

tion

kJ/

kg d

ry a

ir

0,95

10

15

20

25

30 90%

80%

70%

60%

50%

40%

30%

20%

ytidimuH evitaleR

Alignment point. SHF = 0,8

If an installation had a SHF of 1,00 the process would follow a line parallel with this.

Any process with a SHF of0,8 (as per our example)would follow a process lineparallel with this.

SH

F r

atio

= 8

,0Fig. 27PLOTTING THE SENSIBLE HEAT FACTOR RATIO LINE.

SH

F R

atio

sca

le.


Exercise 11 is a compound exercise, embracing many of the indicated processes indicated on Fig. 24. You will find more help in connection with use of the SHF ratio when you work through Exercise 11.

This brings us to the end of this learning material which describes in some detail the content, features and intentions of the Psychrometric Chart. I have available an Audiovisual Course on Psychrometrics, which delves far more deeply into this subject than we could hope to achieve in this

Booklet. Please visit my Website www.alder.co.za to learn more about this comprehensive Course which is on offer.

18

In point of fact, the SHF is the ratio of SENSIBLE HEAT to TOTAL HEAT. So for our example we would have

810

SHF = = 0,8

0,033

0,032

0,031

0,030

0,029

0,028

0,027

0,026

0,025

0,024

0,023

0,022

0,021

0,020

0,019

0,018

0,017

0,016

0,015

0,014

0,013

0,012

0,011

0,010

0,009

0,008

0,36

0,40

0,45

0,50

0,55

0,60

0,65

0,70

0.75

0,80

0,85

0,90

0,951,00

Se

ns

ible

He

at

Fa

cto

r

40

4

5

50

55

6

0

65

7

0

75

80

8

5

90

95

1

00

1

05

110

1

15

Enth

alpy

at s

atur

atio

n k

J/kg

Dry

Air

120 125 130 135 140 145

35 40 45 50 55

–0,0

5

–0,1 –0

,2

–0,4 –

0,6

–0,8

–1,0

–1,2

En

thal

py d

evia

tion

kJ/

kg d

ry a

ir

0,95

10

15

20

25

30 90%

80%

70%

60%

50%

40%

30%

20%

ytidimuH evitaleR

Alignment point. SHF = 0,8

If an installation had a SHF of 1,00 the process would follow a line parallel with this.

Any process with a SHF of0,8 (as per our example)would follow a process lineparallel with this.

SH

F r

atio

= 8

,0Fig. 27PLOTTING THE SENSIBLE HEAT FACTOR RATIO LINE.

SH

F R

atio

sca

le.


Exercise 11 is a compound exercise, embracing many of the indicated processes indicated on Fig. 24. You will find more help in connection with use of the SHF ratio when you work through Exercise 11.

This brings us to the end of this learning material which describes in some detail the content, features and intentions of the Psychrometric Chart. I have available an Audiovisual Course on Psychrometrics, which delves far more deeply into this subject than we could hope to achieve in this

Booklet. Please visit my Website www.alder.co.za to learn more about this comprehensive Course which is on offer.

Documents

RACA Guides September 2011asdasd