HVAC Software - Analyze HVAC Problems - Manual for the Web-Based Psychrometric Analyzer

Tutorial (TOC) > Daemons > HVAC > Manual

a. HVAC Daemons: The HVAC (Heating, Ventilation and Air Conditioning) or the psychrometrics daemons appear as state daemons, specific closed-process daemons, and, most commonly, as specific open-steady daemons.,

For evaluation of states involving moist air, follow the Daemons.States.System or the Daemons.States.Flow branch. Problems involving closed processes are handled under the Closed.Process.Specific.HVAC branch, and problems involving open and steady systems are handled under the Open.Steady.Specific.HVAC branch. The following discussion assumes that you have already visited the States, Closed Process and Open Steady chapters of this tutorial.

Fig. 1 Image of the Daemons.States.Flow.MoistAir page. More such screen shots can be found
in the Slide Show.

b. Moist Air (Moist Gas): Launch the moist air state daemon, Daemons.States. Flow.MoistAir on the main browser window (see Fig. 1). Although the daemon appears very similar to any other flow state daemons in its look and feel, there are some important differences. Although the following discussion involves only the moist air model, it also applies to any moist gas (such as MoistCO2).

Thermodynamic Properties: The daemon displays 11 thermodynamic properties (intrinsic variables in blue symbols that are independent of observers speed or location), p, T, v, u, h, p_v, p_g, phi, omega, T_dp, and T_wb. Others such as p_a can be deduced from simple equation (p_a=p-p_v, for instance). As in any other state daemon, moving the pointer over a variable brings up its definition on Message Panel. Moist air being a binary mixture (of dry air and superheated water vapor), three independent properties must be specified to determine its thermodynamic state according to the state postulate.

You will notice that some of the units where the mass of dry air is involved are not precisely shown. For instance in SI units, the appropriate units of mass flow rate is kg of dry air/min, u, h, e or j is kJ/kg of dry air and v is m3/kg of dry air . The reason the units are deliberately left slightly ambiguous as kg/min , kJ/kg and m3/kg respectively, is to allow the same variables to be meaningful if the working fluid is changed to H2O or R-12.

Extrinsic Properties: The extrinsic properties, symbolized by the green variables, depend on the velocity (Vel) and location (z). The latter is assumed zero as change in potential energy can be ignored in most psychrometric problems. The remaining variables, Vel, e and j, can be evaluated if any one of those are known, provided all the blue variables have been already evaluated.

System Properties: There are four system variables, m, vol, m_v and m_g. Knowledge of any one of those is sufficient to evaluate the rest, provided all the blue variables have been already evaluated. Note that m is the mass of dry air and not the total mass of the system.

c. The Psychrometric Chart: The calculated states can be quickly plotted on a psychrometric chart by choosing it from the Diagram menu.

d. Liquid Water/Saturated Vapor: In addition to moist air, you can select a phase-change (PC) fluid - H2O, R-12, etc. - as the working fluid. When water is sprayed, steam is injected, condensate is removed, or heat exchanged with a flow of a refrigerant, these working fluids become important. To accommodate the PC model, properties x (quality)and y (volumetric quality) are included in the state panel. Some of the state variables, such as p_v, p_g, phi etc., loose their meaning when a PC fluid is selected. Recall that to set the PC fluid as a saturated liquid or vapor the quality x (or y) must be set to 0 or 1 respectively.

Fig. 2 The device panel shown above is for generic devices. It can be switched to Cooling Tower configuration.

e. Open Steady Daemon: Most HVAC devices involving simple heating, simple cooling, cooling with dehumidification, heating with humidification or wet cooling towers. All these devices can be handled by this daemon which is located at the ..Open.SteadyState.Specific.HVAC . The following discussion assumes that you have already visited the Open Steady pages in this tutorial.

The analysis panel for the steady HVAC daemon is shown in Fig. 2, with the Generic option is selected in Fig. 2a and the Cooling Tower option in Fig. 2. The device variables, Qdot and Wdot_ext, remain unchanged from the open steady daemons. However, the balance equations now contain a separate mass balance equation for H2O.

The daemon allows for up to two inlets, i1 and i2 , and two exits, e1 and e2. In a simple heating problem, only one inlet and one exit are used with moist air as the only working fluid. On the other hand, in humidification problem one of the inlet states can be saturated or superheated steam or liquid water. In dehumidification problems, one of the exit states can, similarly handle the condensate.

f. Closed Process Daemon: Although problems involving a closed process are not very common in psychrometry, such problems are adequately covered by the ..Closed.Process. Specific.HVAC daemon. The following discussion assumes that you have already visited the Closed Process chapter of this tutorial.

As an example, suppose a rigid tank containing some moist air is cooled so that a certain amount of liquid water condenses out. In the analysis of this problem, we have a closed system (starting with a single working fluid) undergoing a process ending up in two different working fluids. Another example of such a closed process could be humidifying a closed volume of air by spraying liquid water or saturated steam. In yet another scenario, two tanks containing moist air at different states can be connected together for the two air to mix. The final mixture may or may not be a single working fluid. There is a possibility of condensation.

All these examples can be generalized by saying that the b-State and the f-State in these processes are composed of non-uniform
mixing systems (see Fig. 2). That is to evaluate the b-State we will consider bA and bB states (states in two tanks, state of moist air and steam that is sprayed), and, similarly, the f-State will be made up of fA state (moist air) and fB state (condensed water). In a particular problem we may need only two or three states out of the four states available in the daemon.