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RTU Comparison Calculator

RTUCC: Revision History

Version 4.3 (10/09/2014)

  • Revised and updated the calculation engine to support the assessment of three specific high-performance RTU systems (each of these options corresponds to a commercially available add-on control system or a specific rooftop unit).
    • A new calculator control, "Specific Candidate Unit,"  allows the user to select one of these specific systems for evaluation. Selecting a system activates specific algorithms to represent the system:
      • Specific fan-control strategies from the manufacturer of the advanced-control system.
      • Specific performance curves from the manufacturers of the three-stage unit and the variable-speed unit.
    • Extended general algorithms for modeling staged systems to represent units having three or more stages.
    • Extended algorithms for modeling variations in evaporator fan speed, condenser fan speed, and condenser cooling capacity to better represent staged and variable-capacity RTUs. This yields better estimates of sensible capacity and better modeling of row-split evaporators as affected by flow and capacity variations. This also improves the calculator's estimates of evaporator and condenser fan-energy consumption and better estimates fan energy when a system is in economizer mode.
    • Added a control to specify the n value used in applying fan-affinity laws in fan-energy calculations.
    • Promoted the "Number of Stages" control so it can be specified differently for the candidate and standard units.
    • Updated and added content to the help and methods pages. Added a new quick-start page that is accessible off the main menu. Added an overview page to the spreadsheet and additional annotation.
  • Added functionality to support the calculation of savings related to variable-speed condenser fans. Split out the condenser fan in the controls, calculation engine, and results reporting. Added a "Condenser Fan" help topic and updated related help topics and methods pages.
  • Updated the spreadsheet to include example calculated values in the "EER" and "EER-Cond" part-load tables on the "Part-load Performance" sheet. This is done with Visual Basic code embedded in the workbook. The code includes regression models for capacity and condenser power for an example RTU represented by the spreadsheet. This code also accounts for common part-load behaviors: (1) variable-speed condenser and evaporator fans, and (2) cycling degradation.
  • User interface improved:
    • Added page-state capture and re-load (from local storage) capability on the controls page to support use of WebKit-type browsers like Google Chrome and Apple Safari. This is necessary to preserve the state of several of the controls when returning to the controls page from the results page.
    • Added a feature to preserve the vertical scroll position of the controls page. This preserves the scroll position when using controls that invoke a resend (regeneration) of the controls page from the server.
    • Modified the custom-load model controls to enable viewing the load-model parameters for each of the standard building types. Modified the lock-the-load-line feature so that locking now acts to disable the building-type control and two of the custom model controls. Added a help topic for the custom-load model controls.
    • Changed the humidity control to be visible whenever the advanced controls are displayed. This is consistent with the approach used with the custom-load model controls.
    • Resolved several CSS (style-sheet) formatting issues to improve rendering behavior and speed in the Internet Explorer and Google Chrome browsers.
    • Added an input parameter-summary table that is displayed after the results table for each run.
    • Combined the two discounting-related controls into one row.
    • Removed the S&I control which has been replaced by the "Building Type" control. The S&I fraction is now reported in the design table as the "Internal Loads Fraction" in the "Design Conditions" table.
    • Made each gray-divider row on the controls page to act like a submission button.
    • Improved the y-axis scaling on the "Loads and Hours" and "Equipment Performance" charts so that all four similar charts (standard, candidate, occupied, and unoccupied) are scaled similarly.

Version 4.2 (9/26/2012)

  • Corrected a problem in the fan energy calculations. This issue occurred when the "Fan and Compressor" control was set to "V-Spd Always ON" or "2-Spd Always ON" and the "Setback" control was set to "Cond. Off." For these two cases, the calculator overestimated fan energy consumption during unoccupied hours.

Version 4.1 (9/21/2011)

  • Implemented the use of EnergyPlus load models to calculate S&I factors and ventilation levels based on the selection of a building type.
  • Changed the web application name from UAC Cost Estimator to RTU Comparison Calculator (RTUCC).
  • Updated the help and methods pages. Added new content on latent-load calculations and the building-type feature.

Version 4.0 (1/15/2011)

  • Apparatus Dew Point / By-Pass Factor (ADP/BPF) modeling of sensible capacities (S/T ratios):
    • Full implementation of iterative solution methods. Robust approach for web application. New module with shared psychrometric routines and ADP/BPF functionality.
  • Dynamic web-based regression capability:
    • Implemented commercial module in the calculator. Applied this for a dynamic regression of detailed user data and ADP/BPF demo page.
  • Restructuring of software to better support double runs of the engine (full comparisons) and promotion of general controls to the specific control groups (standard and candidate):
    • Classes and procedures for structuring of bin data and parsing and modeling of spreadsheet data.
  • Fan controls and algorithms:
    • Collected new supporting fan data for fan power (vs. capacity) for setting power defaults.
    • Converted to more appropriate (industry standard) curve shaping methods where operations are done using gross capacities (not net).
    • Controls for user specification of evaporator fan, condenser unit, and auxiliary electronics power draw.
    • Choices for user-control strategies of the fans and compressors including simulation of variable-speed fans and variable-capacity compressors.
    • Fan-affinity laws are used in calculating fan power at part-load conditions.
  • Spreadsheet interface:
    • Added feature for importing detailed full-load performance data from a spreadsheet into the web application. Built data processing capability in the web application for modeling (regressing) and using the detailed performance data from the specification spreadsheet. This is a mechanism to feed data from a manufacturer’s detailed specification into the calculation engine of the web application.
    • Added feature for importing and modeling condenser part-load performance based on a spreadsheet table of part-load EER values. This table is a superset of the part-load EERs that are required to calculate an IEER aggregate.
  • Night set-back:
    • This features supports an automated double run of the calculation engine. The double run splits operation into occupied and unoccupied bin hours. Unoccupied hours are analyzed at a set-back temperature reduced from the setpoint used during occupied hours.
  • Oversizing feature:
    • Changed to a more intuitive user interface and supporting algorithms.
  • Payback calculations:
    • Improved the calculation approach and corresponding messaging to the user.
  • Larger range of capacities (3-30 Tons) and corresponding additional DOE-2 shaping curves added for units less than 5 tons.
  • User specification of system performance degradation as a function of load fraction.
  • Features and algorithmic support for demand (peak power) and corresponding cost calculations.
  • Promoted some existing controls so they can be configured separately for each unit: economizer, S/T ratio, degradation factor.
  • Improved mixed-air routines for better accounting of fan heat (mixing based on mass flows).
  • Correction of load-matching problem at design conditions.
  • Indoor humidity limits.
  • Various improvements in the report page to support new features.

Version 3.1   (11/16/2004)

  • Added hover-help tips for when mouse cursor is over the defining table cells of the Design Conditions and Bin Calculations tables.  This includes any table cell with a cryptic definition in the bin-calcs report.  Made some minor changes to the definitions on the bin-calcs report page.

  • Added restriction to inside humidity ratio calculation (when in auto mode) so that its corresponding relative humidity cannot exceed 100%.

  • Removed feature which automatically fed manufacturer-specific cost data to the cost controls.

Version 3.0   (04/10/2003)

  • Changed economics algorithms
    • Discounting: Changed the uniform present value equation to be dependent on the effective discount rate (as opposed to using the nominal discount rate, an assumed inflation rate, and an escalation rate as was done in the original release).
    • Rate of Return: Changed from an approximate closed-form equation to an exact iterative method.
    • Payback: Added feature to calculate and present an exact iterative result.
  • Added methods pages and context-sensitive help topics.
  • Added capability to represent special performance characteristics of manufacturers' units.
  • Changed default value of S/T ratio (an advanced control) from 0.75 to 0.72.  
    Note that this change slightly affects the results as compared to Version 2.0.  To reproduce Version 2.0 results, simply set the S/T control back to 0.75.
  • Added a "retail" schedule:  M-Fri 9am-10pm, Sun 11-6.
  • Added feature to track if control values differ from the defaults (background color of the default table cell turns yellow if selection differs from default value).
  • Changed the definition of the OCF factor to include normalization by the S/T ratio at ARI test conditions.

Version 2.0   (10/17/2002)

  • Converted the calculation engine from a "total" to a "sensible" energy analysis (see help topic on advanced controls).
    Note that this change significantly affects the results from the UAC Cost Estimator.  Refer to the discussions in the help topics for additional information on qualitative behaviors associated with various controls (see especially: ventilation, humidity, lock-load line, and enthalpy controls)
  • Added advanced controls (see help topic on advanced controls
  • Added mixed-air calculations to properly determine the conditions entering the evaporator coil.
  • Added a system performance calculation page that serves to illustrate the correction methods (see methods pages: system power draw, and sensible capacity).  It also demonstrates the iterative apparatus dew-point methods that were used in developing the generalization to the sensible capacity correction.  This generalization serves to extend the DOE-2 curves to entering temperatures other than 80F.
  • Corrected formulation error related to over-sizing factor.
  • Corrected formulation error related to staging.

Version 1.0   (09/28/2001)

  • Initial release


Rooftop Air Conditioner