myPLV™ Getting Started Guide

myPLV™ is a chilled water system annualized energy modeling tool.  It uses a bin analysis methodology that is customized for the building type, an 8760 hour building load profile as well as location specific 8760 hour weather data.  The use of job specific customized bin data provides an accurate comparative basis for different types and efficiency chillers.  This is something that the standard chiller only Integrated Part Load Value (IPLV) rating cannot and is not intended to provide.  The job specific bin operating points produced by myPLV also provide excellent performance verification points for use in the bid, submittal and factory performance testing process. 

See the FAQs for more information on IPLV and its limitations.

A project specific whole building analysis conducted by an experienced modeler is the best method for evaluating chiller plant options, however this level of analysis may not be planned or budgeted for all projects.  This tool can be used as a simplified alternative to a building model analysis for evaluating chiller energy usage considering rudimentary aspects of the plant design, location, and operation.

This tool has 3 main purposes:

1. Condenser Water Design Flow Optimization (New).  This worksheet assists the designer in determining the optimal condenser flow for the specific system in question based on a customers needs.  Numerous, peer reviewed, industry publications as well as other industry experts recommend designing condenser water systems with lower flows, (aka higher full load design Delta Ts), to optimize system power.  These recommendations range from 12°F to 18°F depending on the system and chiller type.  This worksheet can help zero-in on the optimal flow and Delta T for a specific project at the crucial time when chillers are being selected. For further reading on this topic we suggest the ASHRAE GreenGuide (available for purchase), ASHRAE 50% Advanced Energy Design Guides (free to download) and CoolTools™ Chilled Water Plant Design Guide (archive free to download from Taylor Engineering).

2. myPLV Calculator.  Develops four chiller operating points that are then integrated into an application specific metric called myPLV™ which can be used as a means to predict chiller energy consumption specific to job location and application.  The myPLV™ metric has a format similar to IPLV but is generated for a given project by using the criteria specified by the user in combination with load profiles from industry standard building models. 

3. myPLV Bid Form.  Provides a worksheet using the four myPLV™ submittal points and their calculated weighting to enable a quick and simple energy economic comparison between chiller performance alternatives.  Because the economic impact of peak demand charges are also considered, the design operating point of the chiller(s) is also required for this analysis.

For detailed explanation of the inputs and calculation methodology scroll down to the “Assumptions and Explanations” section of this document or see the FAQ worksheet.

The following are the basic steps for using the myPLV™ tool.  The user is encouraged to “click around” and experiment with the worksheets’ operation. 

1. Open the myPLV™ application and save as a new file name for the specific job.
   
   
2. Determine the desired Condenser Water Design Flow.
   • On the “Cond Water Flow Optimization” worksheet enter:
     • Location data
     • Chiller plant and chiller sizing information
     • Tower / condenser water base data.
     • Cost assumptions for anciallary equipment
   • Press the “Run Flow Optimizer” button.
   • Examine the system "Annualized kW/ton Performance trends" and energy and costs savings data. Note, there are three scenarios to examine based on a customers requirements. Those are Energy optimized. This is the scenario where no equipment is downsized except the condenser pumps and will yield the highest energy savings, but the lowest first cost savings. The next is a lowest first cost scenario. This will yield the highest first cost savings and the lowest energy savings by reducing the size of the cooling towers, pumps, and the condenser piping. The last scenario is a balanced approach. This is a combination of the other two approaches. The cooling towers and pumps are reduced in size to save capital cost, but the condenser pipes are assumed to be the same size at each varying flowrate. By keeping the pipes the same size, the pressure drop and pump energy in the system is reduced significantly.
     
   • Select the radio button under the desired system Condenser flow (gpm/ton). The input data for that selection will be transferred to the other calculation worksheets.
      
3. Select the “myPLV™ Calculator” worksheet.  If the “Condenser Water Design Flow” worksheet was used the input data for this sheet should be pre-entered.  If not the user will have to enter the building, chilled water system and tower design information.
   • Press the “Calculate myPLV™ Conditions” button.
      
4. Select the “myPLV™ Bid Form” worksheet. 
   • Enter the electrical utility rates - $/kWh for consumption and $/kW and Ratchet Rate for demand
   • Press the “Save and Send” button.  This creates a copy of the myPLV™ Bid Form worksheet that can be sent to all qualified chiller vendors to fill out with base and alternate chiller performance and cost data for submittal back to the design engineer.
   • When data is returned from all vendors it can be combined on the master sheet for analysis.
   • The chiller load and ECWT points can be included in the chiller factory test schedule to create “myTEST” points for chiller test approval.
      
5. Select the “myPLV™ Charts” worksheet tab.
   • Evaluate the various charts to understand the modeled chiller performance as desired.

The myPLV™ tool is fast so the designer can perform various “What If” analyses for different plant and chiller design concepts.

Building Load and Weather Profiles

The building load profiles included in this program were generated from the public domain EnergyPlus^TM files developed by Pacific Northwest National Labs (PNNL) for energy analysis work in conjunction with and with oversight from ASHRAE, for the development of ASHRAE/IES 90.1-2010 Energy Standard for Buildings Except Low-Rise Residential Buildings.  The web site for the PNNL files can be found at https://www.energycodes.gov/development/commercial/prototype_models.  PNNL used the EnergyPlus simulation program for various building types in the seventeen different ASHRAE Standard 169 climate zones.

For purposes of the myPLV analysis, all chiller loads in the building profiles of less than 1% max plant capacity have been set to zero.  This is due to an issue in some of the PNNL simulations that resulted in a large number of run hours for central chiller plants at extremely low loads. Typically these loads were composed of pump heat, when normal building and/or chiller controls are expected to inhibit chiller plant operation. Deleting these very low loads results in a more typical run time for the chiller plant, as validated by comparing the modeled to actual building profiles. It is important to note that the user entry for "City" is provided for the user’s convenience to look up the appropriate climate zone. The PNNL models use the most representative weather data to represent the entire climate zone, so that the building load profile can be generalized to the entire zone and limit the scope to a reasonable number of simulations.  In addition, the building load profile is scaled to the Building Peak Load entry as specified by the user on myPLV™ worksheet.  The four performance points listed on the myPLV worksheet as the “myPLV™ Test and Submittal Points” are developed by grouping the load data into (4) chiller plant load ranges - 0 to 37.5%, 37.5 to 62.5%, 62.5 to 87.5%, and 87.5 to 100%.  These groupings are the bins where the center weighted test points are 25%, 50%, 75%, and 94%.  Note that the 100% design point is not included in the performance weighting criteria since the top most bin of the 94% grouping includes the performance at the 100% load point.  

However, the 100% design point is required in the submittal entry with the user specifying the entering condenser temperature.  It is critical to specify this point since the chiller is selected to provide operation at this design point and meet the energy codes. Certified performance at this point is also critical for confidently sizing the electrical wiring, circuit ampacity, breakers and other protection devices, power factor correction, starters, controls, safeties, filling out the nameplate on the equipment, as well as for complying with UL and other agency listings and codes.

Tower Control Method selections (water-cooled only)

The myPLV™ tool develops four submittal points for the evaluated chillers.  For water cooled chillers, each submittal point is specified as a % load at an entering condenser water temperature (ECWT).  The myPLV™ tool uses the cooling tower design condition performance and tower control method to determine the hour by hour ECWT for the chillers in order to ultimately calculate a ton-hour weighted ECWT for each submittal point.

Note:  All calculations assume that a tower cell is sequenced with each chiller.  Chillers are allowed to operate up to 100% of their design capacity before an additional chiller is enabled.

Tower Control Method selections:

Full Tower Fan Flow - This selection will determine the entering condenser water temperature that results from the cooling tower running at full fan speed at all conditions unless the resulting entering condenser water temperature is less than the minimum specified by the user.  If full fan power results in a temperature less than the minimum value specified, the entering condenser water temperature will be set to the minimum value specified.  The tower performance is assumed to have a tower approach (T_leaving - T_{ambient wb}) that is equal to the user entry for the cell labeled Tower Full Load Design Performance - Tower Wet-Bulb Approach (F).  This tower approach linearly degrades to zero at no heat rejection (0 chiller plant load).  The tower approach also changes with the outdoor wet-bulb temperature as the heat capacity of the moist air stream changes with ambient wet bulb conditions. 

Fixed Temperature - This selection is typical of many installations that run the cooling towers to a constant temperature set point.  This selection requires a temperature set point entry by the user.  The computations assume this temperature set point value will be the entering condenser water temperature for the chillers unless the tower capacity at the specific conditions encountered cannot achieve the set point temperature.  In this case the leaving cooling tower temperature will be equal to a value at full tower fan flow conditions. 

Fixed Tower Approach - This selection will compute an entering condenser water temperature equal to the outdoor wet-bulb temperature value plus the tower approach entered by the user.  If the cooling tower cannot achieve this temperature value, the temperature will be returned as that achieved by the full tower fan flow condition.  If this method would result in a temperature value less than the minimum condenser water temperature specified, the minimum temperature set point value will be used.

Chiller Tower Optimization - This selection simulates the behavior of Trane’s Chiller Tower Optimization control strategy which results in a dynamically changing entering condenser water temperature set point as a function of chiller loading and ambient wet-bulb temperature.  If the cooling tower cannot achieve the target set point value, the full tower fan flow leaving water temperature will be returned.  If this method would result in a temperature value less than the minimum condenser water temperature specified, the minimum temperature set point value will be used.

The myPLV™ tool has a new worksheet that provides energy impact calculation and trend charting to illustrate the benefits of optimized heat rejection system flow for water-cooled plant at specified operating conditions.  The Condenser Water Design Flow Optimization worksheet contains a water-cooled plant simulation engine allowing the user to do “What Ifs” on energy conservation measures.  As of this time this worksheet only works with “IP” units of measure.  This simple to use annualized simulation tool for user defined job specific water-cooled plants and its display of component energy use trend data provides the designer with the information enabling the choice, design and specification of higher efficiency chilled water systems.

Many of the user inputs on Condenser Water Design Flow Optimization worksheet are repeated and shared with the myPLV™ Calculator (weather location, plant sizing, tower control).  Beyond this basic system data, additional entries are required for the flow optimization simulation engine and can be modified within the selection range for “What if” analysis: 

• Chiller type: “Fixed Speed” or “Variable Speed”.  The code uses a representative performance model for centrifugal chillers
• A dataset describing tower performance at a 3 gpm/ton flow condition (design wet bulb, tower approach, condenser water pressure drop)
• Chilled Water temperature setpoint (range 36 to 55ºF)
• Chiller Design Efficiency at standard AHRI conditions (range 0.45 to 0.8 kW/ton)
• Tower Performance CTI Rating (range 30 to 100 gpm/hp)
• Cost assumptions for energy and anciallary equipment

Notes:

• The condenser water pump/motor combined efficiency is assumed to be 70% and the fixed pressure drop for the tower static lift is assumed to be 15 ft H2O.  The program assumes the piping that is sized for 3 gpm/ton is retained at the lower design flow values.  The condenser water pressure drop of the other flow elements, including the chiller, are assumed to reduce appropriately as the nominal design flow rate is reduced. Since the pressure drop is output in the results, the user can determine whether this pressure drop corresponds with their own hydronic system estimates.
• For the cooling tower, a CTI rating is used to determine the tower fan power at the tower design conditions.  A variable speed tower fan is assumed and the computations use a combined motor and inverter efficiency of 90%. It is assumed the tower size remains fixed for the condenser flow rates at 0.25 gpm/ton intervals.  It is assumed the manufacturer will make the appropriate tower nozzle or flow distribution modifications for each new design flow condition to optimize the selection.
• A radio button just underneath the gpm/ton value in the chart data is used to select new chiller and tower design data for use in the myPLV calculator worksheet.  The design data is automatically copied to the myPLV calculator based on the radio button selected.
• This program does not include a chilled water pumping power analysis.  Its focus is on the chiller and the heat rejection system and it assumes the same chilled water distribution system for all alternatives.

Here’s what to expect …

• Chilled water plants will show the ability to run at good annualized values.  Play around with the tower control scheme to get a feel for those control methods that seem to offer the best overall performance opportunity.  The advantage of optimizing condenser water design flow will be evident regardless of tower control methods.
• The amount of advantage is strongly influenced by the number of chillers in the plant as well as the condenser water system pressure drop.
• With today’s chiller, pump and fan efficiencies you will find that 2 gpm/ton condenser water flow (158F Delta T) serves as a good rule of thumb to maximize system efficiency. 
• The detailed output will show the energy impact and design point shifts for the chiller, condenser pump, and the cooling tower with varying condenser flow.  The user will need to evaluate their particular design to ensure similar behavior for tower approach and condenser water pressure drop.

Since the condenser water flow rate affects the chiller selection process, we anticipate the user will run this worksheet first.  After deciding on a condenser water flow rate, the chiller and tower design conditions are copied to the myPLV Calculator.  The myPLV calculator is then run to develop the myPLV Test and Submittal points with the relevant conditions moved to the myPLV Bid Form.   The myPLV Calculator and bid forms have been modified to include the chilled water design point temperature and the condenser water design flow rate so the data is available for the submittal process.

Version 3.2 - Condenser Water Design Flow Optimization

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