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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On the next post, I will look at how to represent air with a particular set of properties on the chart. As has been illustrated in previous CPDs, the psychrometric chart is a useful tool to examine the properties and energy flows associated with air processes. So, for example, if we were selecting an external dry-bulb bin temperature of -3 o C, the total frequency of temperature below that bin is 0. There may also be some concerns as to the effect of climate change on future, predicted values based on historic data.

This combined process will produce an off coil temperature somewhat higher than that of the actual temperature of the coil known as the Apparatus Dew Point, ADP and shown as point X on Figure 5. The outline of the chart is as shown in Figure 1 and it may be used for individual rooms or processes as well as describing the changes to properties psychrimetric take place in a complete air handling system.

These particular data in Figure 2 have been laid out in terms of the percentage occurrence of coincident pairs of dry-bulb temperature and moisture content. 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 Chagt over the average year.

Dry bulb temperature and wet bulb depression tables or calculations can be used to determine the air moisture content. The actual chwrt data as collected by the weather station would be based on the same sets chxrt hourly averages data; however, the longer averaging period will moderate out the high and low values.

cibse guide Psychrometric Chart.pdf

Figure 2 is indicative of a type of weather data that can be sourced. 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.

Figure 2 — Paychrometric of Sensible Heating.

It is a graphical representation that shows properties of charh 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. Sensible heat is that added to, or taken frommoist air without a change in moisture content and latent heat is that added to, or taken frommoist air without a change of dry bulb temperature.


cibse guide Psychrometric

There is also a scale showing the specific enthalpy of ppsychrometric air below the temperature scale. 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.

Figure 4 — Psychrometry of Steam Humidification. The use of computer modelling tools enables designers to evaluate options of building design and plant selections at the press of a button. Figure 1 — Outline Psychrometric Chart. The atmospheric pressure will alter with both weather and location and so the relative effect of the vapour pressure will also change.

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.

The energy contained in the air as indicated by Enthalpy, h relating to both the sensible and latent energy increases towards the right and the top of the chart as each of dry-bulb temperature and moisture content increase.

If any two properties are known, then the others may be read from the chart — so, for example, in the field this would typically be wet bulb and dry bulb temperature measured with a sling psychrometer.

Figure 3 — Psychrometry of Sensible Cooling. Figure 5 — Psychrometry of Sensible and Latent Cooling. This is indicated in Figure 7 as air stream A mixing with air stream B to produce mixed air at condition M. The data can also be used to establish how frequently the selected design values are likely to be exceeded.

Fibse photo-enlarged chart or a specific area from the chart produced by computer software will improve the resolution. These calculations may be reorganised to determine unknown variables for any of the processes illustrated in this article.

Figure 8 — Example Psychrometric Process. Cibsw calculating room heating loads using winter design conditions from the CIBSE Guide chatr the selected external temperature would be based on historic weather data as well as a knowledge of the building thermal inertia.

But, more importantly, this data or, if available, the numeric data used to create this graph, may be used to see how many hour periods have a hourly averaged temperature below the bin of -3 o C. So, to take the yellow highlighted data as an example, over a year it might be expected that the dry bulb temperature of between 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.


Please note that the options for this question are: Looking at the height of the yellow column, it occurs on the equivalent of just under one daily period per year.

Module 3: The properties of 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 cigse external heat being added or taken away this is known as an adiabatic process. The data have been taken from Figure 2 and added to the outline chart as shown in Figure 3.

The historic data are available in tables that include amongst other things coincident wet-bulb and dry-bulb temperatures, and are ideally suited to act as a basis for the analysis of design conditions. If meteorological frequency data is available in terms of coincident dry bulb temperature and wet bulb temperature, then this could have been as simply added to a chart, but using a grid based on the dry-bulb temperature and wet bulb temperature axes.

The first step is to become familiar with the chart. Figure 6 — Psychrometry of Adiabatic Humidification. This equation allows calculation of the properties psychromefric gases under different pressures and temperatures. If the underlying data is available explicitly linked to each of the 8, hours charrt a year, then refined subsets of the data may be simply developed.

In this case it can be readily seen that by increasing the design wet bulb by 1K to 21 o C maintaining the dry bulb design at 28 psychromerric Cthe system would cahrt within its capabilities for about an extra 10 data points above that of the current condition.

Cibee thanks to Dr Roger Legg for his publications, which provided underlying material for this article. The psychrometric values were based on the on-coil and off-coil dry-bulb and wet-bulb temperatures psychromettic indicated in the diagram. A couple of the data points appear to exist in the impossible area to the left of the saturation curve; this is due to the approximated sketch of the psychrometric chart and should not distract from the bigger overall picture. Through the incorporation of appropriate weather data, a psychrometric chart can provide the tool to undertake such checks.