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Daemons>Heating, Ventillation and AC> Manual

Three HVAC daemons.
a. HVAC Daemons: The HVAC (Heating, Ventilation and Air Conditioning) or the psychrometrics daemons appear both under the Generic and Specific branches in TEST. In the Generic branch, HVAC systems or states are treated just like any other generic problem with moist air, modeled as a mixture of two perfect gases, is the working fluid. 

For evaluation of states involving moist air, follow the Daemons.States.Volume or the Daemons.States.Surface 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.Surface.MoistAir  page. More such screen shots can be found 
in the Slide Show. 

 

 
 
 
 
 
 
 
 
 
 
 
Thermodynamic, and system properties may have different units for moist air.

 

b. Moist Air: Launch a moist air state daemon, Daemons.States. Surface.MoistAir, for instance, on the main window (see Fig. 1). Although the daemon appears very similar to any other volume state daemons in its look and feel, there are some important differences. 

Thermodynamic Properties: The daemon displays 11 thermodynamic properties (intrinsic variables that are independent of observers speed or location, or system information), 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 the Message Panel. Moist air being a binary mixture (of dry air and superheated water vapor), three independent properties must be specified to determine a state. 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 Condensed Water or Saturated Steam.

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. For instance, a Vel1=50 m/s produces j1=40 kJ/kg, slightly higher than h.

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 choose Condensed Water or Saturated Steam as working fluids. Some of the state variables, such as p_v, p_g, phi etc., loose their meaning except when  moist air is the working fluid. In most psychrometric problems, temperature and pressure of water or steam are generally given.


 

 
 

Open and steady
HVAC problems are plentiful.
 
 
 
 

Governing equations.

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 chapter 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. 2b. 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 steam or liquid water. In dehumidification problems, one of the exit states can, similarly handle the condensate.
 



 

 
 

Closed processes in psychrometry may involve non-uniform b-State and f-State.
 
 

 

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.
 



Examples
You will find a number of HVAC examples on the Applications  page, Slide Show and the Archive .


(Each section above is divided into two sub-sections - Manual and Applications.)
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