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Understanding Psychrometric Charts, Mechanical (HVAC) Courses from Mid Career College. MCC has been providing training for building services. The Psychrometric charts provide a graphical representation of the thermodynamic properties of moist air using the data contained in CIBSE Guide C. The. A practical introduction to psychrometry and the use of the psychrometric chart as a design tool. KS20 is essential reading for those involved in designing and.

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Air is normally considered to be a mix of dry air and water vapour. On the curved edge of the chart there are scales for wet bulb temperatures and specific volume with the specific enthalpy scale drawn in steps. This article will explain how to set this up, and explore some first examples of application.

The form of the psychrometric chart has changed little from the early versions — the most visually significant alteration being the changes in the units to SI for the CIBSE Chart, see Figure 2. These were taken from data produced by the Meteorological Office 3.

The effect of the time period used for averaging the temperature observations may clearly be seen by noting that the hour data is far less extreme than the hour data.

cibse guide Psychrometric

These measurements may be undertaken in a number of ways including the Sling Psychrometer see Figure 1 and the Aspirated Hygrometer. The atmospheric pressure will alter with both weather and location and so the relative effect of the vapour pressure will also change. The psychrometric values were based on the on-coil and off-coil dry-bulb and wet-bulb temperatures as indicated in the diagram.

The saturated vapour pressurePswill have been reached — the actual value of this is related to the dry bulb temperature of the air; air with a higher dry bulb temperature will have a greater potential to hold water vapour, and so will have a higher saturated vapour pressure. Figure 4 — Psychrometry of Steam Humidification. Some of the sprayed water will evaporate into the air — the heat to cause that evaporation being taken from the air itself and so the air dry bulb temperature will drop until the point where the air is saturated with water vapour.

Figure 3 — Psychrometry of Sensible Cooling. Also on the vertical axis is the continuation of the specific enthalpy scale. Figure 2 is indicative of a type of weather data that can be sourced. If the underlying data is available explicitly linked to each of the 8, hours in a year, then refined subsets of the data may be simply developed.

For a cooling coil to provide sensible cooling only and no latent cooling the coil surface temperature must be greater than the dew-point temperature of the air. The wet bulb temperature is the temperature that a sample of air would reach if it had a fine spray of water added to it the water being at the air dry bulb temperature without any external heat being added or taken away this is known as an adiabatic process.


Module 23: Travelling into time with psychrometry

These relationships can also be chartt employed to examine the properties of other gas mixtures — as might be found, for example, in controlled specialist environments. Adding up the hour columns to the left of the -3 o C column comes to about one occasion ie one hour period when the temperature falls below -3 o C over the average year. Figure 7 — Psychrometry of Mixing Air Streams. Figure 2 — Psychrometry of Sensible Heating.

CIBSE Guide C has lists of the values of all the properties of air as described above for various dry bulb temperatures. So-called Adiabatic constant energy Humidification as shown in Figure 6 practically characterises the processes of atomising spray and ultrasonic humidifiers as well as the rarely used recirculating spray and wetted pad humidifiers. Figure 6 If sending in the tear-out page from the magazine, just write in one of these options under question one.

This will relate such parameters as the number and shape of coil rows, the spacing and type of fins on the coil, and the air velocity. The parameters described so far are combined to produce the psychrometric chart, which dates pyschrometric to empirical work undertaken by James Glaisher in Greenwich in the first half of the 19th century.

cibse guide Psychrometric Chart.pdf

My thanks to Dr Roger Legg for his charg, which provided underlying material for this article. This moderating effect becomes more obvious as the averaging period is lengthened for example, if a monthly or seasonal average temperature is considered.

The actual underlying data as collected by the weather station would be based on the same sets of hourly averages data; however, the longer averaging psychromstric will moderate out the high and low values. Glaisher had fhart reliably determining the wet bulb temperature — these problems arose from the differences between screen and sling wet bulb temperatures.

It is a graphical representation that shows properties of air at 1 atmosphere pressure The chart provides a convenient way of examining the changes in the dependent parameters in air as the air undergoes a process in a space for example, heating and humidification from occupants or as it is conditioned in an HVAC system. The data have been taken from Figure 2 and added to the outline chart as shown in Figure 3. If cigse frequency data is available in terms of coincident dry bulb temperature and wet bulb temperature, then this could have psychromettric as simply added to a chart, but using a grid based on the dry-bulb temperature and wet bulb temperature axes.


This vapour pressurePv measured in Pa vhart mbaris related to the mass of water vapour in the air — the more molecules present, the higher the vapour pressure. There may also be some concerns as to the effect of climate change on future, predicted values based on historic data.

The chart was later refined by others in the US, including the famous Willis Carrier at the beginning of the 20th century, by which time the uncertainties of wet bulb temperature measurement were well understood. When using the Psychrometric Chart for plotting cycles that include both latent and sensible change the individual processes are simplified to straight lines.

The two values are likely to vary slightly from each other. These calculations may be reorganised to determine unknown variables for any of the processes illustrated in this article. How close the dry-bulb temperature of point B approaches the ADP will depend on the effectiveness of the cooling coil.

The molecular mass of water vapour is 18 and so the specific gas constant may be obtained from This will not just be, as a standard psychrometric calculation, in terms of power, but by including the frequency information, and hence time, comparative energy use may be examined.

Over the next blogs, I will cover different aspects of the chart and show how it can be used in practice. Modelling software will use weather data as made available to it such as the CIBSE Test Reference Year 1 and will attempt to calculate loads across the operational period. The graphical representation provides, at the least, a swift visual check on the value, but also can inform and communicate the effect of alternative values on the consequent plant operation.

But these data are limited to just temperature: Pwychrometric is also a scale showing the specific enthalpy of the air below the temperature scale. If molecules of water vapour were to be continually added to a volume of static air for example, from a room humidifier then at some point the air would not be able to support any more molecules — the air is said to be saturated at that point and any further water molecules added will simply condense out.

On the graph itself, the curved lines show relative humidity and the various straight lines show dry bulb and wet bulb temperatures, moisture content and specific volume. And, of course, is most frequently sensed using an electrical or electronic device.