the psychrometric chart and air conditioning systems

bs newsapril 2008page 18

the psychrometric chart & air conditioning systems

BUILDING SERVICESdesigning

Fergus Dunphye: fergus.dunphy@pmg.ie

The purpose of this article is toexplain the psychrometricprocesses that occur during theheating, cooling and othercycles of a typical airconditioning system and howthese processes are representedon, and calculated, through theuse of a psychrometric chart.

The psychrometric chart is oneof the basic tools which shouldmake up the tool box of everyHVAC engineer, together withthose other essentials such asthe ductulator or and the pipesizing chart. Yet it would seemthat the workings of thepsychrometric chart remain amystery to many engineers whotend to shy away from its use.Also, with the availability ofcomputer software applicationsand excel spread sheets, manyengineers rely more and moreon these tools to carry out theirheating and cooling loadcalculations while not fullygrasping or understanding thebasics of psychrometrics.

The psychrometric chart and itsintricacies can seem a dauntingand complicated subject butlike all things, if it is explainedproperly and the elementaltheory understood, then just likeriding a bike, it will never beforgotten.

The Psychrometric ChartA psychrometric chart is simply a

graphical representation of theproperties of air which appear insteam or hygrometric tables. Atfirst sight it appears to beextremely complicated butconsider firstly the values for airat saturation conditions only.From steam tables orhygrometric data it is possible toplot a graph of moisture content(g/kg) against dry bulbtemperature. The result is asshown in Figure 1.

The curve obtained is known asthe saturation curve and itshows the quantity (g) ofmoisture (water) present in a unitquantity (kg) of dry air when theair is in a saturated condition.

Now let’s include in the graphthe moisture content values ofthe air at various conditions ofrelative humidity from 10% to90% in steps of 10% to producea family of curves as Figure 2.

Since the area of the graph tothe left hand side of thesaturation curve is not used it isnormal to plot the curves withthe moisture content axes onthe right hand side. So the chart

Figure 1 — Moisture content atsaturation

Figure 3 — Normal shape on thePsychrometric Chart

Figure 2 — Curves of relativehumidity

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the psychrometric chart & air conditioning systemsnow has the familiar shape asFigure 3.

So, basically the psychrometricchart enables us to find the drybulb temperature, moisturecontent and relative humidity ofair. In addition to these values,air at a given dry bulbtemperature and % RH will haveother known conditions of wetbulb temperature, specificvolume and enthalpy. Thesescales can be thereforesuperimposed on the graph togive the complete picture ofdata for air at any condition ascan be seen from Figure 4.

So, now that we have thecomplete psychrometric chart,let’s see how some basic airconditioning processes arerepresented on the chart.

Air MixingConsider two different airconditions A and B plotted onthe chart. If the air from twosuch sources is mixed theresultant moisture content, drybulb temperature and othervalues must be of values

between that of A and B.Drawing a line between points Aand B means that the mixedcondition is somewhere alongthat line.

Graphically, this mixed conditioncan be found by dividing theline A/B in direct ratio to themass flow rates of the two airstreams. Therefore, if there is 40%(A) and 60% (B), you canphysically divide the length ofline A/B into the ratio 2/3 andfind point M.

Note that because there is agreater quantity of air atcondition B, the condition of themixed air (M) will be nearer tothat of condition B.

The other method of finding themixed condition is to calculatethe dry bulb temperature ofcondition M by simplycalculating —tm = 0.4 tA + 0.6 tB— and plotting the intersectionof tm with the line A/B on thechart.

HeatingIf air is heated by an LPHWheater battery or an electricheater battery then the dry bulbtemperature of the air isincreased but the moisturecontent of the air remainsconstant. This process is calledsensible heating – as there isonly a change in the sensibleheat content of air.

Therefore, if we have air atcondition A and we know it isheated to condition B, we canread from the chart the increasein enthalpy and obtain theheating load in kW bymultiplying by the air mass flowrate.

CoolingIf air is sensibly cooled, i.e., nomoisture removed, then theprocess is the opposite to that ofthe heater and is as shown inFigure 7.

In order for only sensible coolingto occur, the operatingtemperature of the cooling coilmust be greater than the dewpoint temperature of the air. Inthis case the reduction inenthalpy can be read from the

Figure 4 — Data present on thePscychrometric chart

Figure 5 — Air mixing process

Figure 6 — Heating process

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the psychrometric chart & air conditioning systems

chart and the cooling load inkW calculated by multiplying bythe air mass flow rate.

Cooling and DehumidifyingIf the cooling coil temperature isbelow the dew-pointtemperature of the air, then thewater vapour in the air willcondense on the tubes of thecoil. The amount of moisturewhich is removed from the airstream will depend on thedifference in temperaturebetween the air dew pointtemperature and the coolingcoil operating temperature. Thiscooling coil temperature isknown as the “apparatus dew-point”. This cooling and moistureremoval process is shown asfollows on the psychometricchart, Figure 8.While the above shows the

process occurring in twoseparate stages, it would bemore normal to indicate theprocess as one straight line fromA – B on the psychrometricchart.In this instance the total cooling

coil load (sensible and latent)can be calculated bymultiplying the reduction inenthalpy by the air mass flowrate. Similarly, the amount ofmoisture removed (g/hr) can becalculated by multiplying thereduction in moisture content(g) by the air mass flow rate.

HumidificationHumidification or an increase inthe moisture content of an airstream can be accomplished ina number of different ways,each of which can be shown onthe psychrometric chart. In eachprocess there must be intimatecontact between the air andwater and this is achieved byusing, for example, atomisingnozzles, an evaporativemembrane, or steam injection.The psychrometric processwhich results depends on therelative enthalpy andtemperature of the water withrespect to that of the air steam.If water is re-circulated over awetted surface or membranethrough which the air passes,

the water will assume the wetbulb temperature of theincoming air. In this case thehumidification process underthese conditions will be one ofconstant wet bulb. There is noexternal heat exchange in theprocess and it is known as an”Adiabatic Process“, as shown inFigure 10, ... the increase inmoisture content results in areduction in sensible heat (i.e., afall in temperature).

(The process is one of constantenthalpy and in reality the wetbulb line deviates slightly fromthe constant enthalpy line onthe chart). If the water on awetted surface is heated to thesame temperature as the air drybulb temperature then theprocess is ‘Isothermalhumidification’ i.e. constant drybulb temperature, as shown inFigure 11. If the water is heatedfurther, or if steam is used thenthere is an increase in air drybulb temperature during thehumidification process.

Air Conditioning CyclesWe will finish the article byputting together all of theprocess which we havechartered on the psychrometricchart in order to illustrate acomplete air conditioning cycle

Figure 7 — Sensible cooling process

Figure 8 — Cooling & moistureremoval (dehumidification)

Figure 9 — Dehumidification

Figure 10 — Adiabatic humidification

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the psychrometric chart & air conditioning systems

for a clean room application,where it is required to controltemperature and humidity inboth winter and summer. Seethe summer air conditioningcycle as shown in Figure 12.

1. Return air heat gain (sensibleheating)2. Mixed air 40% outside air.60%return air3. Cooling and dehumidification4. Re-heating to supply ducttemperature (sensible heating)5. Room re-heat (sensibleheating)

I hope this article has beensome help in explaining andclarifying the basics of thepsychometric processes whichoccurs in all air conditioningsystem cycles, and that it hasgone some way towards makingthe psychrometric chart moreuser-friendly.

Fergus Dunphy is an AssociateDirector of PM Group and, asDepartment Manager, isresponsible for the technicaloutput and administration of themechanical building servicesdepartment. Fergus has over 25years experience in the buildingindustry, both in the contractingand consultancy fields in Ireland,the UK, the Middle East, Germanyand Turkey. He has extensiveexperience in mechanicalbuilding services design for awide variety of clients andsectors includingpharmaceutical, web hosting,commercial and medicaldevices.

Figure 11 — Isothermalhumidification A — BSteam Humidification A — C

Figure 12 — Summer air conditioning cycle