THERMOPTIM®
Example
Air conditioning unit
JAVA VERSION 1.3
R. GICQUEL SEPTEMBER 2000
CONTENTS
EXAMPLE: AIR CONDITIONING OF A BUILDING.................................................................................... 3 SOLUTION OF THE PROBLEM ................................................................................................................................ 3 1 Supply conditions ......................................................................................................................................... 3 2 Properties of the mix .................................................................................................................................... 4 3 Air conditioning ........................................................................................................................................... 5 4 PLOTTING THE CYCLE ....................................................................................................................................... 7
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Example: Air conditioning of a building
One seeks to maintain the internal ambience of a building at a temperature of 24 °C (297.15 K) and a relative humidity equal to 50%. External climatic conditions are the following: temperature to equal 30 °C (303.15 K), and relative humidity of 80%. It is necessary to remove external and internal thermal loads of 162.6 kW, as well as a quantity of water equal to 60 kg/h, that is 0.01667 kg/s. Knowing that, for sanitary and comfort reasons, the supply temperature must not be less than 14 °C (287.15 K), and that the recycled air proportion must not exceed 70 %, the purpose of the exercise is to determine: - the supply conditions - a way of processing of the outdoor air / recycled air mix.
Solution of the problem
Begin by creating a new project, titled "air conditioning". As for the time being there are no components for the moist processes, you cannot use the diagram editor. In this exercise, all points will be of the same dry gas, air, and constant pressure, 1 atm.
1 Supply conditions
Begin by creating two points titled "indoor air" and "outdoor air" , and setting their respective temperatures and relative humidities, as well as a third point, titled "supply", with a temperature of 14 °C. For the first point, for example, you obtain the following result:
Then, create a supply water vapor / gas mixture process, connecting points "indoor air" and "supply". Enter the values of the thermal and water loads in the appropriate framework, with a negative sign because they are to be removed:
Finally calculate the process:
The process is calculated. The supply flow rate is determined, as well as the point "supply":
2 Properties of the mix
The first step of the air processing consists of calculating the properties of the mix (outdoor air / recycled air). The problem consists now in determining how to process mixed air to obtain the supply conditions (287.15 K or 14 °C, w = 0.0079 kg/kg).
A solution is to cool the mixed air until its specific humidity is equal to that of the supply air, then to reheat it to obtain the desired temperature. Create two new points, called "mixed air" and "cooled air", and connect them by a moist gas process of the cooling type named "cooling" ("mixed air" at the inlet and "cooled air" at the outlet). Now, to be able to calculate the mixed air properties, it is necessaray to create two "exchange" processes representing the flows of outdoor air and recycled air, named "indoor flow" and "outdoor flow", and to set their respective mass flow rates (0.7 and 0.3). Each of these processes is connected at a single point at both the inlet and the outlet. Create then a moist gas mixer whose branches are "indoor flow" and "outdoor flow", and whose main vein is "cooling". This mixer will automatically calculate the properties of the mix.
The mix properties are the following:
3 Air conditioning
To calculate the cooling process, choose a realistic cooling coil surface temperature (for example 7 °C), check the "Calculate the process, the outlet point's humidity being known" calculation mode, and set the outlet point ("cooled air") humidity ) to that of the supply conditions (w = 0.0079 kg/kg). Then click on the "Calculate" button, so that THERMOPTIM researches the required coil efficiency. You obtain the following results: the temperature of the cooled air is equal to 284.93 K or 11.78 °C, and the cooling coil efficiency is 75%.
To determine the required Delta H, one must connect the points "cooled air" and "supply" by a heating water vapor / gas mixture process of the same flow-rate as the "supply" process.
Save the project after having named if (for instance "aircond.prj").
4 Plotting the cycle
If you have access to the Interactive charts, the cycle can be plotted in the following way: i n the main frame,
select item Interactive Charts of menu Special.
Then double-click in the field facing the label "Chart" and select "Psychrometric". If the substance selected is not airselect it by clicking "load a gas from the data base" after selecting the item "parameters" in menu "Chart" ; similarly change the pressure if necessary. The interactive chart frame is shown:
Then go back to the "Interactive Chart" frame, and click on "Update the point table". All the project points are displayed in the table:
The two first column show the names and the substance of the points. When a point is part of the Thermoptim project, a "X" appears in the third column, and when it is pszrt of the Chart cycle points, a "X" appears in the fourth one. Here, there are only Thermoptim project points. The last table column titled "selected" shows the point status: if a "X" appears, the point is selected for being taken into account in the transfers between Thermoptim and the charts, otherwise not. To change a point status, double-click on the corresponding line. Here we want to plot all points, so we keep them all selected. Now, click on "Update the chart from Thermoptim" to have the points transferred to the chart, and select "Connected Points" in menu Cycle. If necessary, change the axis layout in menu "Chart". You get the following result:
The problem is that the points are not in the right order: it is necessary to edit the cycle points to get a nice plot. Open the cycle point editor by selecting "Edit a cycle" in menu "Cycle". You get the following frame:
By clicking on the little arrows located at the right of the screen, you can move up or down the different lines. This way you can move point "supply" above point "indoor air":
When you are done with your modifications, they can be transferred on the chart by clicking on the button "Validate". Automatically, the editor table points are then repositionned on the screen.