Volume 3 No.2, Spring 1999
Simulating the performance of an Air Handling Unit on a Psychrometric Chart
David B. Meredith
Mark W Meredith
School of Engineering Technology Department of
and Commonwealth Engineering The Pennsylvania State University
The Pennsylvania State University
When teaching the principles of Heating, Ventilating and Air-conditioning, it is usually difficult to get students to visualize the interaction among all of the inter-dependent variables as they pass through an Air Handling Unit (AHU). Similarly, it is difficult to explain how these processes would appear on a psychrometric chart. The Web-based software package (JAVA) was developed to assist faculty who teach in these disciplines, and to demonstrate to students the various functions and relationships involved. The URL for this web page is currently: http://www2.fe.psu.edu/~dxm15/refrig/psych/AHU.html
There are three basic fields to this page. In the upper left corner is an animated and interactive model of the airflow through the AHU. The right side of the page shows an animated and interactive psychrometric chart including a sensible heat factor protractor. The bottom left corner contains an interactive control panel for adjusting parameters in the simulation. Each of these sections will be discussed in detail below.
The Air Handling Unit
The user can observe the representative air molecule as it passes through the unit . As the air molecule passes through this loop, its color changes to represent the air temperature on a color spectrum scale (red is relatively hot and blue is relatively cool). The flow rate and fraction of outside air can be adjusted by the user.
A fraction of the conditioned return air is exhausted to the outside (EA) and replaced with outside air (OA). This flow of fresh air into the building is required to maintain good indoor air quality (IAQ), but the quantity of outside air (as a percentage of the total airflow) can be adjusted by the user. The mixed air (MA) then passes through the cooling coil where both sensible energy (temperature) and latent energy (moisture) are removed from the air stream to become supply air (SA). The cooled and dehumidified air is then ducted into the building to absorb energy and moisture from the thermal load.
The Psychrometric Chart
The right side of the page displays the animated and interactive psychrometric chart. The horizontal axis represents dry bulb temperature (sensible energy) and the vertical axis represents absolute humidity ratio (latent energy). The saturation line is also indicated on this chart. Superimposed on this chart is a rectangle representing the nominal comfort zone used in the HVAC&R industry. The boundaries of this zone range from 70 to 80F on the horizontal scale and from 0.005 to 0.010 pounds of water vapor per pound of dry air on the vertical axis. Also functioning in the psychrometric chart window is the Sensible Heat Factor (SHF) protractor with two animated vectors. One vector represents the cooling load required by the building and the other vector represents the cooling supplied by the cooling system. Equilibrium exists when these two vectors are concentric.
Control Panel and Limitations
The lower left window on the screen contains the control panel where the user can modify various parameters. These are grouped into several related functions. The first three parameters are usually controlled by the building mechanical manager. The second group represent the outside air conditions, and the third group represent the building load parameters. The bottom line of the control panel allows the user to adjust the Simulation Speed. The speed can be varied from very slow (to watch the processes occur simultaneously in the AHU and on the psychrometric chart) to fast (to observe how changes are reflected in the location of the room air within the comfort zone).
Changing the Percent of Outside Air moves the location of the Mixed Air condition on the psychrometric chart, and also affects the fraction of the air molecule in the AHU that exits as exhaust air and returns as supply air. While changing this parameter can have a major effect on the thermal load on the cooling coil and the overall cost of cooling a building, this program is not designed to address those issues. The Supply Air flow rate can be adjusted to match the load requirements. This action demonstrates the use of Variable Air Volume (VAV) systems commonly found in commercial projects today. The second method of modifying the cooling rate is to change the Supply Air Temperature (commonly referred to as reset). The interaction between these two parameters has strong implications for the operating cost of the overall system, but that issue is beyond the scope of this project.
The Outside Air Temperature and the outside Wet Bulb Temperature represent the ambient weather conditions. The primary effect in this simulation is to alter the location of the OA point on the psychrometric chart. In a real sense, the outside temperature affects the building load. However to simplify the modeling of these processes, these two functions have not been coupled.
The building load is modeled as a Total Load (measured in Tons of refrigeration) and a Sensible Heat Fraction. These two parameters define the length and slope of the cooling load vector on the psychrometric chart's protractor. At equilibrium, the same slope is used on the psychrometric chart between the supply air condition and the return air condition. When not at equilibrium, the slope between these points represents the cooling rate being supplied. The difference between the cooling load and the cooling rate causes the room air condition to migrate to a new equilibrium point.
One of the main strengths of this learning tool is to help students visualize how room conditions are affected by changing building loads, and how the building operator attempts to maintain design conditions within the space by adjusting air flow and temperature.
In the message line at the bottom of the simulation, the user is flashed brief messages when parameters are out of range. For example, if the room temperature or humidity falls above or below reasonable preset limits, a message indicting that the occupants are unhappy is shown. If the user tries to increase the air flow above the nominal maximum of 40,000 cfm, they are warned about the noise generated at high duct velocities. These messages are to remind the novice user that the operation of a building mechanical system is even more complex than what is represented by this simulation.
Relationships and equations
The conditions for each state point of air are calculated from equations found in Chapter 6 of the 1997 ASHRAE Handbook of Fundamentals, Atlanta, GA. These equations relate the dry bulb temperature, wet bulb temperature, dew point temperature, relative humidity, absolute humidity ratio and enthalpy.
The user defines the dry bulb temperature and wet bulb temperature of the Outside Air via the control panel. The user also defines the Supply Air temperature via the control panel, and the relative humidity of the Supply air is assumed constant at 90%. The Mixed Air conditions are linearly interpolated from the user-defined percent of Outside Air and by the conditions of the return air and outside air. The return air conditions are allowed to float in response to the parameter values selected by the user.
The conditions at each point (Supply Air, Return Air, Outside Air and Mixed Air) are calculated at each time step. The animation of the points is linked to where the air molecule is in the AHU model. The return air and outside air are simply proportional movements along the process line between them. The process between the mixed air condition and the supply air (i.e., through the cooling coil) is modeled to follow the profile of an ellipse in the second quadrant. This path demonstrates that mostly sensible cooling occurs in the first half of the coil rows, followed by a combination of sensible cooling and dehumidification in the last half of the coil rows. The process between the supply air condition and the return air (or room air) follows the process line defined by the sensible heat ratio in the protractor.
A simple energy and mass balance model is used to account for changes in the condition of the room air. Constant sensible and latent capacitance values have been assumed to make it easy for the user to observe changes but for the system to return to equilibrium within a few minutes.
The cooling load of the building and cooling supply rates are determined using a total load and Sensible Heat Factor (SHF) concept. The total load (Btu/hr) is the sum of the sensible load and the latent load. The SHF is the ratio of the sensible load to the total load. Typical office buildings are represented by an SHF = 0.9. The loads are measured in Tons of refrigeration, where one Ton equals 12,000 Btu/hr. The flow rate of Supply air is measured in cubic feet per minute (cfm).
A web-based tool to assist in the study of building system operation has been introduced. The simulation allows the student to control various parameters, and to visualize the effect those parameter changes have on the operation of the system. It also allows the student to compare what is happening in the physical system represented by the building schematic with how the air conditions are changing on the psychrometric chart. Finally, this software package allows the students to grasp the time-dependent nature of building mechanical systems. By acting as the building mechanical manager, they have the opportunity to try to maintain acceptable comfort conditions in the safe (and inexpensive) environment of a simulation. Finally, it should be noted that this simulation is for educational purposes only, and should never be used as a design tool.
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