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Basics of PSYCHROMETRICS

Psychrometrics is a branch of thermodynamics which looks at the properties of moist air – essentially, how air and water vapour interact with each other. Psychrometrics is fundamental to the operation of all refrigeration and air conditioning systems.

But how well do we all understand it?Air is made up of a mixture of gases – mostly nitrogen and oxygen, but also carbon dioxide and some other inert gases.

Air also contains a small amount of water vapour. In temperate climates, the amount of water vapour in air is usually more than 1% by weight. In extreme conditions (such as in the tropics), however, it can approach 3%.

There are two important temperature concepts that need to be understood in psychrometrics – those of dry bulb and wet bulb temperatures.

Dry bulb temperature is the temperature as measured by a regular thermometer, expressed in degrees Celsius (°C). This is the most commonly used measure of temperature – it’s usually assumed that a temperature is a dry bulb temperature unless stated otherwise.

Wet bulb temperature is the temperature as measured by a thermometer with its bulb covered by a wet piece of cloth (known as a wick) and exposed to moving air. At relative humidities

below 100%, water evaporates from the wick, cooling the bulb of the thermometer below the dry bulb (or ambient) temperature. Wet bulb temperature is also expressed in degrees Celsius (°C).

Dry and wet bulb temperatures can be measured using thermometers, or specially designed digital instruments.

Figure 1 shows a device traditionally used to measure dry and wet bulb temperatures: a ‘sling psychrometer’, which features two thermometers that has been specifically designed for measuring dry and wet bulb temperatures simultaneously.

Notice the cloth covering the bulb of the bottom thermometer, and the flexible joint at the other end of the thermometers. The device works by wetting the cloth then spinning the two thermometers around to generate air movement.

From these two readings we can calculate the amount of water vapour in the air.

Air temperature and air pressure both have an effect on how much water vapour air can hold – generally, warmer and lower pressure air will be

Figure 1 – a sling psychrometer, used for measuring dry and wet bulb temperaturesThe basic science behind psychrometrics was established in the 1800s, and the psychrometric chart was first revealed to the public in a technical paper presented to the American Society of Mechanical Engineers by Willis H Carrier, who went on to found the Carrier company (which most in the industry today will still be familiar with).

Willis Haviland Carrier, 1876-1950 When going home by train in the evening

from the office, Carrier would so immerse himself in his problems that he frequently went through his home station. This happened so often that he was driven to moving his home to the terminal station on the line.

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able to hold more water vapour, and colder, higher pressure air will be able

to hold less water vapour.

When it’s referred to in refrigeration and

air conditioning calculations, water

vapour content in air is often

expressed in terms

of

‘relative humidity’. Relative humidity is the ratio of the amount of moisture in the air to the maximum amount of moisture the air could hold at the same temperature.

Relative humidity:(amount of moisture in the air) : (maximum amount of moisture the same air could hold)

The level of relative humidity can have a significant effect on human comfort and storage conditions inside refrigerated spaces.

Luckily, moist air follows known thermodynamic laws and behaves in a predictable way, so we can make accurate calculations about the effects of adding or removing heat from the air, or adding or removing moisture from it.

Calculations to do with air temperature and water vapour content are usually plotted on the psychrometric chart: (Figure 2).

Psychrometrics, and the psychrometric chart, can be used to identify air quantities and equipment capacities, and what needs to be done to the air (adding or removing heat or moisture) to meet particular conditions.

The lines on the chart are taken from data tables, which in turn have been calculated using the laws of thermodynamics.

The horizontal axis represents the dry bulb temperature of the air, while the vertical axis represents the moisture content of the air (expressed here as grams of moisture per kilogram of air, or g/kg).

The curved lines (or ‘saturation curves’) represent relative humidity (percentages up to 100% are marked for each line), and wet bulb temperatures can be plotted along each curve.

Dry bulb and wet bulb temperatures can be measured using thermometers as described above, or by using specially designed digital instruments.

Figure 2: the psychrometric chart

AIRAH PSYCHROMETRIC CHART

Barometric pressure 101.325kPa

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COLD FACTS

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Once the dry and wet bulb temperatures are known,

values for moisture content, relative humidity, specific

volume and enthalpy can be read off the chart (see

sidebar for more information on these terms).

The consequences of various changes to

the system can also be predicted using the

psychrometric chart.

If heat or moisture is added to or removed from

a system, a new point will be able to be plotted

on the chart. The chart can be used to assess

whether the new point will, for example, cause

condensation to form inside a space, or raise

humidity to uncomfortable levels.

Figure 3 gives an overview of the effects of adding or removing heat or moisture from a system.

What do we use it for?Where psychrometrics is most helpful is in the process of designing a system and selecting the right capacity equipment to use in it.

We usually know the conditions

we want to be able to

achieve inside the refrigerated or air

conditioned space. We can plot these points on the psychrometric chart.

We can also plot other points on the psychrometric

chart – a common example is the temperature of the air

outside, which needs to be considered when designing an

air conditioning system.

The distance between points on the psychrometric chart can tell us a lot about the requirements for a system, and designers can use this information to solve problems such as:

• Selecting the right equipment for a job

• Working out system capacity

• Checking comfort levels within an air conditioned space

• Calculating when an air conditioning system can benefit from using an economy cycle / full outside air

• Checking that a system that has been installed is working properly

• Verifying the performance of a piece of equipment

• Calculating where problems with condensation may occur

• Manually checking the output of computer programs or cooling load calculations s

Other important termsDew point – temperature at which water vapour starts to condense from the air

Sensible heat – heat due to temperature difference only

Latent heat – heat due to moisture difference only

Total heat – the sum of sensible heat and latent heat

Enthalpy – the ‘heat energy’ contained in moist air

Specific volume – the amount of space occupied by air at the given temperature and humidity

Barometric pressure, or air pressure,

changes depending on altitude. It is

lower at the peak of Mount Everest

than it is at sea level, for example.

Barometric pressure can also have an

effect on psychrometric calculations.

The standard psychrometric chart

is drawn with a standard sea level

barometric pressure of 101.325 kPa in

mind, though it is usable at pressures

ranging from 98.3 kPa to 105.1 kPa

without introducing significant errors.

Further readingAIRAH Building blocks of air conditioning – psychrometrics courses

AIRAH technical manual DA09 Air Conditioning Load Estimation

AIRAH building blocks courses are run around the country, or can be delivered in-house

to your company. Visit www.airah.org.au for more information.

Figure 3 – effect of various psychrometric processes

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