White Paper: The Benefits Of System-Based Design

by James Pegues
Senior HVAC Systems Engineer, Carrier Software Systems

May, 1997

Introduction

With the introduction of the Hourly Analysis Program v3, Carrier has incorporated 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:

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. Different types of HVAC systems contain different components which each need to be sized. Further, different types of HVAC systems require different sizing procedures. Therefore defining the system type is necessary to determine the components to be sized and the procedures to be used. The following examples illustrate this point:

The second problem with the traditional approach involves accuracy. If the traditional approach is used to size a system such as VAV Mixing Box or 2-Fan Dual Duct, additional hand calculations will be required needed to size components not addressed by the calculation. These additional hand calculations make the design more difficult, more time consuming and prone to error. In more complex situations, sizing is often approximated to save time. Thus, the traditional approach plus hand calculations is often less accurate than a computerized approach that considers system type and does a complete job.

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 customized to each specific system type.

By providing system-specific sizing data, the system-based design approach can bridge the gap between what 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:

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 to work more effectively. 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 thorough and comprehensive.

Finally, because detailed, dynamic system simulations are part of this approach, the method can potentially be used to investigate the effect on sizing of such devices and controls as:

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 significant 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.

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