How to Draw a Psychrometric Chart

Calculation of air conditioning processes in the Mollier h, x diagram


One of the essential ventilation calculations is the determination of the amount of heat that has to be expended in order to achieve an air condition in the room according to temperature and humidity. The air must be converted into a different state by means of suitable treatment such as mixing, heating, cooling, humidifying or dehumidifying. But how much heat output is required to bring an air volume flow to the desired temperature? How large must mixed volume flows be selected in order to obtain certain air temperatures? To what temperature does an air volume flow have to be cooled in order to reduce the water content?
The necessary calculations of changes in air condition can be created graphically with the Mollier h, x diagram, which was named after the Dresden professor for applied physics and mechanical engineering Richard Mollier. His publication "New Diagrams for Technical Heat Science" from 1904 has made it easier to calculate thermodynamic processes to this day. This is a diagram in which the enthalpy h (kJ / kg) is plotted against the absolute water content of the air x (g / kg dry air). It also contains the saturation states of moist air and its wet bulb temperatures. The air temperature (° C) and the relative humidity (% r.h.) are also important for describing and calculating the changes in the state of moist air. The properties and behavior of humid air depend on the barometric pressure. Therefore, a psychrometric diagram can only ever be recorded for a certain barometric pressure.

Today, a large number of software programs facilitate the necessary technical calculations. Some software solutions have also been developed for the representations and calculations in the h, x diagram. In the following we present the Excel tool of the ILK Dresden, with which up to 14 process status points can be displayed at the same time. The calculation of the state variables is pressure-dependent. Process representations in the range from -20 ° C to 100 ° C with a water content of up to 50 g / kg are possible. The entry can be made for each process step either as an absolute or as a relative water content. By linking status points, a simple process calculation is possible with little effort. The use of Excel's own target value search also allows, for example, the calculation of adiabatic changes in state. For each status point, additional informative parameters such as dew point temperature, wet bulb temperature (cooling limit temperature), enthalpy and also the effective power related to the previous status are displayed in tabular form.

Fig. 2: Up to 14 process status points can be displayed at the same time. By linking status points, a process calculation is possible with little effort.

The data is entered in the fields highlighted in yellow. Either the relative humidity (RH) or the water content (x) can be entered. Process states in the fog area are not permitted. Since Excel interprets empty input fields as "zero values", input fields that are not required must be filled with the variables of the previous air condition. The standard Excel functions (differences, references) can be used as usual. For the DEC system (Desiccant and Evaporative Cooling) shown in Figure 3 - i.e. a system with adsorptive cooling through drying and evaporation - the course of the air condition parameters is shown as an example in the h, x diagram:

Figure 3: Schematic representation of a DEC air conditioning system. The respective state variables of the air within the system are shown in the h, x diagram.

In the DEC process, outside air is passed through a sorption wheel (1) for dehumidification, in which the water vapor contained in the air is bound to the surface of the sorption material. This releases heat of condensation, which initially heats the supply air. It is pre-cooled in the subsequent heat recovery rotor (2). An aftercooler can also be used. In the cooling system, the washer-humidifier (3), the supply air is adiabatically cooled to the desired temperature level with a simultaneous increase in humidity. The air conditioned in this way is fed into the room via a fan. Warmed, moist air from the rooms is adiabatically cooled in the exhaust air humidifier (4), which results in a greater temperature difference for heat or cold recovery. In the rotary heat exchanger (5), the heat given off to the rotor in the supply air flow is absorbed by the exhaust air flow and preheated to regenerate the sorption material. The necessary temperature level of approx. 70 ° C for the regeneration of the sorption wheel is then reached in the heater (6). The heat energy for the regeneration process can, for example, be solar thermal or via a CHP. Finally, the heated air in the sorption rotor (7) absorbs the moisture bound in the silica gel and thus regenerates the sorption wheel. The now warm and humid exhaust air is blown outside by a fan.

Fig. 4: Mollier h, x diagram showing the course of the process in a DEC air conditioning unit. My software tool in the IKZ-FACHPLANER

The result is the process shown in Figure 4 in the h-x diagram. The graphics of the process courses created in this way can be pasted into a word processing program using the copy functions of Microsoft Excel.
The software tool is a free service from ILK Dresden. The program can be used free of charge and is available for download on the ILK Dresden website.

Pictures: Institute for Ventilation and Refrigeration Technology, Dresden

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