Accuracy tests for simulations of VAV dual-duct, single-zone, four-pipe fan-coil, and four-pipe induction air-handling systems

ASHRAE Transactions, July, 2006 by G.K. Yuill, J.S. Haberl, J.S. Caldwell

ABSTRACT

This paper provides a method for verifying the accuracy of computer models that simulate the performance of the air-handling components of four types of heating, ventilating, and air-conditioning systems: the dual-fan VAV dual-duct system, the single-zone system, the four-pipe fan-coil system, and the four-pipe induction system. To accomplish this, a detailed description of each system and its operating parameters was developed and a set of eight test conditions was generated, consisting of carefully chosen space loads and weather conditions. Each of the systems was simulated at each of the defined conditions. The thermodynamic properties at each point in the system, as well as the heating and cooling coil loads, were calculated. The calculations were then repeated by a second analyst using an independently developed program. The maximum deviation in the total coil loads was 1.8%, and the average of the absolute deviations was 0.4%. These validated results can be used by program developers or users to test the performance of the air-side simulations contained in most building energy analysis computer programs.

INTRODUCTION

It is difficult for the user of a building energy analysis (BEA) computer program to know that the program is providing accurate answers. BEA programs are complex and carry out a large number of calculations. They also use empirical models to simulate many building components. Even a long and detailed hand calculation would check only one program path.

One part of a BEA program is easier to check than the others. This is the simulation of the air side of the air-handling system. Most of the analysis of the air side of the air-handling system involves only fundamental equations such as the first law of thermodynamics and the continuity equation. Also, most BEA programs do not take into account transients when simulating air-handling systems. Therefore, it is possible to perform a simple manual analysis to verify a BEA program's simulation of an operating condition for an air-handling system. By repeating this analysis for several different systems and for several different sets of load and weather conditions, a set of test conditions can be generated that will make it possible to validate the fundamental elements of a BEA program that simulates the air-handling system.

The objective of the work reported here was to generate such a set of data, to enable the user to test the accuracy of the simulation of air-handling systems by a building energy analysis computer program. The data presented here have been produced by analysis of the performance of four different types of air-handling systems (the dual-fan VAV dual-duct system, the single-zone system, the four-pipe fan-coil system, and the four-pipe induction system) for six different sets of outdoor and room conditions, using spreadsheets. All of the calculations involved are based on fundamental principles (the laws of conservation of mass and energy) and on the properties of air and water presented in the 1993 ASHRAE Handbook--Fundamentals (ASHRAE 1993) and the 1995 ASHRAE Handbook--HVAC Applications (ASHRAE 1995). To check the accuracy of the results, two independent analyses were performed.

All deviations were checked by hand by a third analyst and the results reconciled. These results are reported in detail in the RP-865 final report to ASHRAE (Yuill and Haberl 2002). Because of the extensive validation checks that were carried out, any deviation of the results of a building energy analysis computer program from the results presented here should be suspected to be an error or an approximation in the program that is being tested.

In spite of the effort that was expended to validate the set of results presented here, it is possible that not every case was analyzed correctly. Therefore, the thermodynamic properties (dry-bulb temperature, humidity ratio, specific volume, and enthalpy) and the mass flow rate are listed for each point of interest along the airflow path through the air-handling system. These data are presented in the RP-865 final report to ASHRAE (Yuill and Haberl 2002). Using these data, a user who finds a deviation between the program being checked and the present results can easily check the calculations by hand and establish whether an error has occurred in the validation data.

The methodology described in this paper was used to develop a set of test cases for three other types of systems. However, space limitations did not permit the presentation of all of the results in a single paper. A previous companion paper (Yuill et al. 2005) provided similar test cases for a constant volume terminal reheat system, a variable air volume system, and a single-fan constant volume dual-duct system.

The next section of this report contains a brief description of each of the systems analyzed and a schematic diagram of each system. Following that is a description of the six test cases to which the systems were subjected. A table summarizing and comparing the coil loads predicted by the two independent analyses of each system is contained in the next section.