THE BENEFITS OF SYSTEM-BASED DESIGN
INTRODUCTION
With the introduction of the Hourly Analysis Program v3, Carrier is incorporating system-based design features into its load estimating software. System-based design is a new concept in computerized system design that allows the computer to do a more complete and accurate job of sizing equipment than traditional system design methods. This approach yields significant benefits to HVAC system designers because of the productivity advantage it offers.
This paper explains system-based design and its benefits. First the paper discusses how traditional system design methods work and the shortcomings of the traditional approach. Next, the concept of system-based design is explained, and the benefits it offers are explored.
HOW TRADITIONAL SYSTEM DESIGN METHODS WORK
Most computer programs used for HVAC system design are based on a traditional approach that manual methods use. First, the engineer inputs weather data, information about the building construction, internal loads and layout, and HVAC sizing parameters. The latter includes such things as thermostat setpoints, the required supply temperature and the required outdoor air ventilation rate. Using this data the program then:
- Computes zone sensible cooling loads for all zones for a series of design cooling months.
- Identifies the maximum zone sensible load for each zone in order to calculate required zone airflow rates and the required supply fan airflow rate.
- Calculates central cooling coil loads for the months being considered in order to identify the maximum cooling coil load.
- If the system also provides heating, calculations are performed to determine the maximum heating coil load.
This procedure yields data useful for sizing terminal diffusers, the supply fan, the central cooling coil, and the central heating coil.
SHORTCOMINGS OF THE TRADITIONAL APPROACH
It is important to note the traditional approach does not explicitly consider the type of HVAC system being designed. This approach is acceptable when designing simple CAV or VAV systems. However, when an HVAC system with special features, components or aspects of operation is involved, the traditional approach has two important flaws.
First, it leaves a gap between what the engineer needs to design the system fully, and what the program provides as sizing data. This gap results because the engineer ultimately needs to size all components of the HVAC system, but different HVAC systems include different components. Because the traditional method does not consider system specifics, it cannot account for the unique features of certain systems.
As an example, consider a Series Fan Powered Mixing Box system. This system uses a central air handling unit to provide a variable volume of constant temperature chilled air to zone terminals. Each zone terminal is a mixing box containing a fan and a heating coil. Mixing box terminals mix cold primary air with warm plenum return air to provide a constant volume of air to each zone. Terminal heating coils operate when necessary for reheat and for heating duty.
The components and operation of the Series Fan Powered Mixing Box system are quite different from simple CAV and VAV systems. For one thing, the terminal mixing boxes must be sized both for airflow and heating capacity. Further, fan heat gain in the mixing boxes and the use of return plenum air cause the mixing boxes to have an effect on the required sizes of the primary supply fan and the central cooling coil. Because the traditional computerized approach to system design does not consider system specifics, it cannot consider these factors. Computer programs using the traditional approach provide peak zone sensible cooling loads, required zone airflow rates, the required supply fan airflow rate and peak cooling coil load data. Among these results, the peak zone sensible cooling loads and zone airflow rates are the only usable values for designing the Series Fan Powered Mixing Box system. The other results are not usable because the special operating features of the mixing box system were not considered in their calculation. Consequently, the engineer must use the zone sensible cooling loads as raw data for hand calculations to determine the required primary supply fan airflow rate and the primary cooling coil capacity considering the special operating aspects of the mixing box system. These hand calculations are often not trivial.
The Series Mixing Box system example is just one of many similar situations in which a gap between design needs and sizing data provided exists. A 2-Fan Dual Duct VAV system is another example. In this case, two supply fans must each be sized. In addition, supplemental system components such as preheat coils and precooling coils often need to be sized as well.
The second problem with the traditional approach involves accuracy. Because hand calculations needed to account for unique features of specific HVAC system design can be difficult, the results of traditional method calculations are often used as an approximation instead. Using the previous example for a Series Fan Powered Mixing Box system, results from the traditional approach might be used to size the primary supply fan and primary cooling coil even though the terminal fan heat gains and special operating features of the mixing box system were not considered. Because the traditional approach does not consider aspects of operation for a specific HVAC system, the results it provides are often less accurate than if specific system attributes were considered.
SYSTEM-BASED DESIGN AND HOW IT WORKS
The system-based design approach considers the unique features of the HVAC system being designed and then tailors the load estimating and sizing procedures to that system. It can therefore provide specific, accurate sizing information for each component of the system.
If a Series Fan Powered Mixing Box system is being designed, for example, the system-based approach will provide the information necessary to size the terminal mixing boxes, their fans and heating coils. It will also consider the special operating features of the system to determine accurate primary supply fan and primary cooling coil sizes. In this way sizing methods and output data are tailored to each specific system type.
By providing system-specific sizing data, the system-based design approach can bridge the gap between whan an engineer needs and what a computerized system design program provides.
How It Works. The information a designer must supply to initiate the design process is similar to the traditional approach. The engineer must:
- Input weather data.
- Input building construction, internal load and layout information.
- Define the HVAC system. In addition to thermostat setpoints and sizing parameters such as the required supply air temperature and required outdoor air ventilation rate, the engineer specifies exactly what type of HVAC system is involved and its attributes. For example, it could be VAV Reheat, VAV with baseboard heat, Series Fan Powered Mixing Box, Dual Duct VAV, etc...
Next, the system-based design computer program calculates loads and sizes system components:
1. Zone Load Calculation. The program first calculates hourly zone sensible cooling loads for all zones for the design cooling months being considered.
2. Zone Airflow Sizing. The program then identifies maximum zone sensible loads in order to determine required zone supply airflow rates and required central fan airflow rates. For some systems, such as fan powered mixing box systems, special aspects of system operation may influence the required airflow rates.
3. System Simulation. Once system airflows have been determined, the program simulates the hour-by-hour operation of the HVAC system and all its components to determine loads for all coils in the system. This mathematical simulation considers the interplay of component operation for the specific system being studied. Simulations are performed for the range of design cooling months specified by the designer and for the heating design condition.
4. Coil Sizing. Finally, the program searches results of system simulation to determine maximum required size for each component coil in the system.
BENEFITS OF SYSTEM-BASED DESIGN
The major benefit of the system-based design approach, of course, is that it gives the engineer exactly what is needed to design a system. Specific sizing data is provided instead of raw material for further hand calculations. The result is increased productivity for the designer because the computer is being put more effectively to work. The computer does a complete job of system sizing, not a partial job.
A related benefit is that the system-based approach does a more accurate and therefore reliable job of generating sizing data. This is because sizing calculations consider the specific operating nature of the system, not the features of a simple, generic system. Further, the approach can evaluate more operating conditions than can be checked by hand, so that the approach is more through and comprehensive.
Finally, because detailed, dynamic system simulations are part of this approach, the method has the potential to be used to investigate the effect on sizing of such devices and controls as:
- Outdoor air ventilation heat reclaim devices.
- Outdoor air economizers.
- Active dehumidification and humification controls.
- Night-time free cooling controls.
Previously, such controls have only been evaluated in energy analysis simulations to determine effects on operating costs. But each can also have an effect on sizing which in turn can have a major effect the first cost of the system.
CONCLUSION
System-based design represents a promising advance in the field of HVAC system design. It offers improvements in productivity and accuracy, and opens new avenues of investigation to the designer in the pursuit of the optimal design.