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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The data have been taken from Figure 2 and added to the outline chart as shown in Figure 3. A bounding envelope has been added to the psychrometric data in Figure 4 to emphasise the most frequently occurring psychrometric conditions. 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.

On the next post, I will look at how to represent air with a particular set of properties on the chart. 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 psychometric water molecules added will simply condense out.

Dry bulb temperature and wet bulb depression tables or calculations can be used to determine the air moisture content. For a cooling coil to provide sensible cooling only and no psycrhometric cooling the coil surface temperature must be greater than the dew-point temperature of the air. Figure cbse — Psychrometry of Mixing Air Streams. The process that would typically result from a steam humidifier almost pure latent heating is indicated in Figure 4 — it can reasonably be considered as a vertical line following a constant dry-bulb temperature.

The opposite process, sensible cooling, is shown in Figure 3. CIBSE Guide C has lists of the values of all the properties of air as described above for various dry bulb temperatures. Posted by The Gibbons Group at However, when evaluating the cihse operation of different possible solutions, it is important not only to take into account the values of the operational psychrometry in design and part psyhcrometric conditions, but also to consider how frequently this might occur.


Module 9: The basic psychrometric processes – CIBSE Journal

Figure 1 — Outline Psychrometric 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. These relationships can also be usefully employed to examine the properties of other gas mixtures — as might be found, for example, in controlled specialist environments.

When lines are drawn on the chart they reflect changes in sensible heat or latent heat or a combination of the two. The use of computer modelling tools enables designers to evaluate options of building design and plant selections at the press of a button.

Figure 5 — Psychrometry of Sensible and Latent Cooling. 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.

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

In this case it psychdometric be readily seen that by increasing the design wet bulb by 1K to 21 o C maintaining the dry bulb design at 28 o Cthe system would operate within its capabilities for about an extra 10 data points above that of the current condition. This is indicated in Figure 7 as air stream A mixing with air stream B to produce mixed air at condition M.

Module Travelling into time with psychrometry – CIBSE Journal

The designer whether aware of it or not will already be applying frequency-based weather data when undertaking any building load analysis, even when calculating a simple steady state building heat loss. The actual underlying data as collected by the weather station would be based on the same sets of hourly averages data; however, psychrometrjc longer averaging period will moderate out the high and low values.

As with any gas, the moving molecules of water vapour in the air exert a pressure. 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.


The atmospheric pressure will alter with both weather and location and so the relative effect of the vapour pressure will also change. It is a graphical representation that chqrt properties of air at 1 atmosphere pressure This moderating effect becomes more obvious as the averaging period is lengthened for example, if a monthly or seasonal average temperature is considered.

Module 23: Travelling into time with psychrometry

The guide also includes sets of equations that will allow determination of values for other gases that may be used in spreadsheets or computer programmes to the Characteristic Equation may be used in conjunction with the Gas Laws. Glaisher had problems reliably determining the wet psychronetric temperature — these problems arose from the differences between screen and sling wet bulb temperatures.

Over psyfhrometric next blogs, I will chagt different aspects of the chart and show how it can be used in practice. So, to take the yellow highlighted data as an example, over a year it might be expected that the dry bulb temperature of between 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. Inferred frequencies of occurrence of wet bulb temperature and specific enthalpy, together with any other air properties that are included in the base psychrometric chart, are now automatically available for analysis.

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

When calculating room heating loads using winter design conditions from the CIBSE Guide 2 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.

This psychromeetric allows calculation of the properties of gases under different pressures and temperatures. Figure 6 If sending in the tear-out page from cibbse magazine, just write in one of these options under question one.

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