AN 04 3 1 Development and Testing of the Characteristic Curve Fan Model Jeff Stein P E Mark M Hydeman P E Member ASHRAE Member ASHRAE ABSTRACT This paper describes the development and testing of the characteristic curve fan model a gray box model This model produces fan efficiency as a function of airflow and fan static pressure It is accurate relatively easy to calibrate and could be easily incorporated into commercial simulation programs Also presented is an application of an existing model to predict fan speed from airflow and fan static pressure These models were developed as a part of a larger research project to develop design guidelines for built up variable air volume fan systems The models have been successfully employed in comparative analysis of fan types wheel diameters fan staging and analysis of supply pressure reset INTRODUCTION The authors were part of a publicly funded energy efficiency research team developing design guidelines for builtup fan systems in commercial buildings According to previous research fan energy in new construction for commercial buildings in California accounts for 1 terawatt hour of electric energy usage per year representing approximately half of all HVAC energy usage CALMAC 2003 The authors research demonstrates that up to half of that fan energy is avoidable through cost effective design practices including fan selection size and type fan sizing fan staging and static pressure control Hydeman and Stein 2003 Five monitoring sites provided field data on which to test the alternative fan system designs and design techniques These sites were selected to represent a range of climates occupancies and fan system configurations Kolderup et al 2002 As part of this work a simulation model of a fan system was sought that had all of the following characteristics Accurate at predicting fan system energy over the full range of actual or anticipated operating conditions Applicable for the full range of fan types and sizes Easy to calibrate from manufacturer s or field monitored data Ability to identify operation in the surge region Relatively simple to integrate into existing simulation tools Ability to independently model the performance of the fan system components including the motor the mechanical drive components the unloading mechanism e g VSD and the fan The purpose of this model is to evaluate design alternatives for fan selection and control through simulation Optimally simulation tools would directly utilize the manufacturers fan curves to evaluate fan system operation at each discrete step of evaluation Since this is not currently available the authors sought models that simulation tools could easily incorporate that replicated fan performance MAIN BODY Literature on component models for fans was reviewed including the models used in the DOE 2 simulation program DOE 1980 and in the ASHRAE Secondary Toolkit Brandemuehl et al 1993 Clark 1985 We also looked briefly at the models embedded in commercial simulation software such as Trace and HAP but found these suffered from the same problems as the model in DOE 2 DOE 2 uses a black box regression model that produces the fan system power draw as a function of percent design airflow using a second order equation as follows Jeff Stein is a senior engineer and Mark Hydeman is a principal at Taylor Engineering LLC Alameda Calif 2004 ASHRAE 347 2 CFM CFM P a b c CFM design CFM design P design 1 This model is implicitly built on several assumptions 1 Each fan operates on a single system curve that uniquely maps airflow to static pressure 2 Fan system efficiency is directly a function of airflow 3 A second order equation sufficiently models both of these effects The DOE fan model implicitly combines the operating system curve with the models for each of the fan system components Power is directly produced as a function of airflow only and there is no opportunity to have different conditions of fan static pressure at a given airflow Real VAV systems do not remain on a fixed system curve System pressure as a function of airflow behaves differently depending on the location of the boxes that are modulating the location of the static pressure sensor s and the static pressure control algorithm Although this model is simple to use it does not allow the user to independently model and evaluate each of the fansystem components Thus if designers wanted to evaluate the impact of motor oversizing they would have to independently assemble fan and motor models to develop the DOE 2 performance curve that represented the combination of the two together This model also does not directly account for the variation in fan system component efficiencies as the fan unloads nor does it allow for evaluation of a multiple fan system where fan staging will change both the operating efficiency and potentially the individual fan static as they are staged on and off The model in the ASHRAE Secondary Toolkit is a graybox fan component model that uses the perfect fan laws through application of dimensionless flow and pressure coefficients This model uses a fourth order equation to predict fan efficiency from the dimensionless flow parameter CFM c 1 3N D 2 P c 2 2 2 N D 3 2 3 fan a b c d e where CFM N D P C1 and C2 348 4 airflow fan speed fan diameter average air density fan static pressure and constants that make the coefficients dimensionless 4 This model allows the user to calibrate an entire family of fan curves with data from a single model Unfortunately this model does not permit the direct calculation of fan efficiency from airflow and pressure rather it correlates efficiency to the dimensionless flow term which requires both airflow and fan speed as inputs As elaborated below a designer and most simulation tools will use airflow and fan pressure as inputs to the fan system model in order to calculate fan speed and efficiency A second problem is that this model assumes a fixed peak efficiency for fans of all sizes This simplification reduces the applicability of the fan model for comparative analysis of fan options as peak efficiency tends to increase with fan diameter As a result of these shortcomings the authors set out to develop a new component model that could directly be driven by airflow and pressure Based on the fan laws ASHRAE 2000 the core assumption of this new characteristic curve fan model is that the efficiency of a fan is constant as the fan rides up and down on a particular characteristic system curve
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