Ashrae Psychrometric Analysis Software Free Download
Waltruda Monie
ASHRAE cooling design data for Cape Town is 31.2C db, 19.3 wb (or at least it was in the 2013 of Fundamentals and I don't have access to more up to date versions) and this would put the RH in the region of 30%
It looks like the calculation is using the correct data based on the bottom table in your screenshot but maybe there is a presentational error in the summary which is pulling the design DB into the WB field and leaving the DB field blank??
There will be days/months when you might have higher DB and there may be days/months when you have higher WB but the "design day" is based on statistical analysis of DB & WB conditions which occur simultaneously and therefore give the greatest enthalpy values. That is what the MCWB tag means, Mean Coincident Wet Bulb.
There is then a built in acceptable failure rate - in the case of cooling design usually taken as 0.4% - so 0.4% of the hours over the whole year might exceed that design condition in a "typical" year.
0.4% is 35 hours, probably not sequential, spread over the whole year. For human comfort that is unlikely to be significant (other failure rates are also quoted for less critical cooling applications and extreme values are quoted for more critical (process) cooling applications).
The Relative humidity (ratio of how much water is actually in the air versus the maximum value which could be absorbed at that dry bulb temperature) varies with DB so even though you may have a high Absolute humidity (moisture content) the Relative humidity could be quite low due to the extreme DB.
You need to plot the coincident DB and WB values on the psychrometric chart to see the RH values and lower down the page for Cape Town you will see design values for each month so you should be able to track the RH over the year.
The latent load on your cooling/dehum coil will be the difference between the Absolute humidity outdoors and the Absolute humidity required in the supply air to offset latent gains in the room/zone. The Relative humidity is not used in this calculation, it is only an indication of how "dry" the air feels for human comfort and how much the evaporation rate from skin is likely to be.
It also seems that the absolute humidity in your zone is almost precisely the same as that for the supply air condition which would imply that you have near zero latent gain in the zone - does this seem reasonable to you? Have you accounted for all gains including people and infiltration air which might add moisture to the spaces?
As per your recent snip on the chart for Cape Town, I submit an additional use case for a more extreme climate (closer to the equator) for discussion. As it would have it, both Cameroon and Cape Town have February as their hottest Annual Cooling design condition.
Plotting both the Annual design point and corresponding Monthly Design Wet Bulb + MCDB data for February results in the graphs attached. If I look at the enthalpies, I get the following, which is supported by your own analysis, considering the two design points are fairly close from an enthalpy point of view:
Using Q=mc(Tf-Ti) as a reference, I would expect the overall cooling load to be higher in the monthly design condition versus the annual cooling design point? Below is the tabulated data from the monthly design wet bulb section of the ASHRAE weather report if it helps.
The correct method is to use Q=mΔh where Δh is enthalpy difference between the air entering the coil and the air leaving the coil. That way you take account of both the sensible and the latent components.
I live in a temperate climate in UK (station 031400) where we tend to favour temperature control rather than humidity control (except for critical applications) so we would not normally use design conditions biased towards maximum WB, although in our case it makes little difference, but if that is the practice in Africa then the chart that I did earlier maybe understates the required dehum.
This produced more reasonable results as you could override the weather data values to account for the higher WB temperatures, but this method only reported peak loads for a month, day and hour and not a diversified load across the year. Output data is fairly limited and only good for peak capacity sizing.
Option 2 is the new shiny method based on the EnergyPlus engine that prints out the nice .html report with full EnergyPlus data that shows basically everything you could need. The reports can also be customized if you are tech-savvy enough to code in your own workflow. (Of which, I do not count myself amongst)
The only issue is that despite weather data being reenabled, from my experiments, Option 2 does not use these manual values, it will always default to the weather data obtained from the Location tab (Internet Mapping Service) despite editing the weather tab. This presents a larger problem as these weather data values are often decades old and not indicative of current climatic conditions, even if you ignore the high humidity issue we were exploring in the previous posts.
Regarding " these weather data values are often decades old and not indicative of current climatic conditions", as far as I can see the data that you are using corresponds to the ASHRAE Fundamentals weather station data which should as far as I know be updated every time the Fundamentals chapter is reissued (every four or five years?).
The data is however smoothed by averaging over something like a 20 year period so that it is not unduly affected by short term events such as sunspots, solar flares and the El Nino which is happening just now etc.
Hopefully you are making some progress with your calculations and can figure out a way to get the results that you need and it might be worth reaching out to @Kevin.Lawson.PE for a bit of help - he has also created some free apps on the Ripple Engineering website which are intended to improve upon the built-in calculation tools (but I've never used them so you would need to explore them yourself).
Psychrometric Analysis is a handy tool that helps you to create psychrometric charts. The charts are drawn with unmatched accuracy. It allows for complete psychrometric and process analysis, all graphically displayed. It also provides a professional state point and process report detailing all of the psychrometric and process values. One can copy the output chart or calculation reports directly to the Windows clipboard so that it can be pasted right into proposals, presentations or spreadsheets.
The Elite Software PsyChart program displays the psychrometric chart on the computer screen, andallows the designer to carry out all operations and analysis normally done using a conventional psychrometric chart. However, PsyChart is more than an electronic display of the standard psych chart. For example, it can quickly display numerical values of all properties for any selected point on the psych chart. Points on the psych chart can be labeled for future reference, and reports of their properties can be displayed on the screen or printer. Additionally, PsyChart allows psychrometric processes to be analyzed. The lines for all standard psychrometric processes, such as heating and humidification, cooling and dehumidification, mixing, collecting, etc., are displayed on the chart with simple user commands. Although the initial display is the standard ASHRAE sea-level chart, this can be changed by the user to show a chart for any elevation or pressure, and for any range of dry bulb temperatures. The user can further customize the psych chart by specifying to show or omit any of the property lines which can be displayed with either English or SI metric units. In order to view sections of the psych chart in fine detail PsyChart allows the user to zoom in and create an expanded "window" around the selected region of the psych chart. Windows enhance the clarity of processes where state points tend to crowd together. PsyChart can print reports and plot charts on dot matrix and HP LaserJet printers. AutoCAD users can plot charts from the DXF files created.
If you would like to evaluate the PSYCHART program in further detail you can download ademonstration copy from our website, or order a copy, with complete documentation. The demonstration copies retain all the functionality of the full programs, they are justlimited on the size of the project data that can be entered. Please follow this link for description ofthe demonstration limits for the PSYCHART program.
The PsyChart program uses the psychrometric equations for moist air in Chapter 6 of the 1989 ASHRAE Handbook of Fundamentals. Rapidly converging iteration techniques have been employed to allow PsyChart to solve problems with virtually any combination of input parameters.
PsyChart can operate on any IBM-PC compatible computer using DOS 5.0 or higher with at least 512K of memory and a graphics monitor which can be color or monochrome. Hercules, CGA, EGA, andVGA graphics are supported. Only one floppy disk drive or one hard disk is required.
The PsyChart program is completely interactive in that the chart is continuously displayedwhile menu selections are presented at the bottom of the screen. As selections are made and values entered, the results are immediately displayed on the screen. Menu selections are provided to define, label, and erase state points. A single state point can be defined with anytwo of the following inputs: dry bulb temp, relative humidity, enthalpy, wet bulb temp, humidityratio, and dew point. Alternatively, a state point can be defined by using the cursor to locatethe position. Seven processes are allowed including: general linear processes, heating, coolingand dehumidification, collection, mixing, and various processes for air as it passes through a zone.
Besides the on-screen display of psych charts, there are five additional reports provided. Two of these reports are partial screen reports (not shown on this flyer) that list limited statepoint data and air volume data while the pscyh chart is still displayed on the screen. The remaining three reports require the full screen. They are the State Point Report, the ProcessEnergy Report, and the Process Input/Output Report. The State Point Report lists all six psychrometric parameters for all defined state points. The Process Energy Report lists air volume, sensible, latent, and total loads, along with water addition/removal rates for all defined processes. The Process Input/Output Report lists the air volume rates, percentages for mixing and collecting processes, and the input and output state conditions for the processes. These reports occupy the entire screen and provide complete data listings concerning state pointconditions, sensible and latent loads encountered from state point to state point, and psychrometric conditions created by each process. Displays of the psych charts can be printed onan Epson and HP Laser compatible printers. Psych Charts can also be plotted by AutoCAD since DXFfiles can be created. Shown to the left are examples of how the psych chart would appear on yourscreen as you begin defining state points and processes. Examples of three full screen reports are shown below.
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