EvaporativeFluidCooler:TwoSpeed Low Fan Speed Error

asked 2016-08-01 17:44:48 -0500

DW gravatar image

I continue to encounter an error whenever I try and model a two-speed evaporative fluid cooler. I am trying to model a piece of real equipment: BAC VFL - 032 H. It is a 26-ton closed-circuit fluid cooler. It has a max airflow of 14,990 CFM, the spray pump has a capcity of 94 GPM. The fan is 5hp and the pump is 0.5 hp. It is located on a central plant loop whose flow is about 125 GPM. The design conditions are provided as:

EWT = 95 oF LWT = 85 oF at 78 oF ambient WB temp.

Whenever I run this either in my otherwise calibrated model or even just the EnergyPlus example file for this piece of equipment I receive the following error:

* Severe * EvaporativeFluidCooler:TwoSpeed "BAC VFL COOLING TOWER". Evaporative fluid cooler UA at low fan speed must be less than the evaporative fluid cooler UA at high fan speed.

I receive an error even if I autosize everything as soon as I my design capacity exceeds 9,000 W or about 3 tons. Has anyone else encountered this error before? I've checked my numbers several times and tried running this many times but can't seem to figure out what I'm doing wrong. Any help or advice is greatly appreciated:

Here is the object that I am using:

EvaporativeFluidCooler:TwoSpeed, !

BAC VFL cooling tower, ! Name

condenser splitter equipment outlet node, ! Water Inlet Node Name

cooling tower outlet node, ! Water Outlet Node

7.0744922, ! High Fan Speed Air Flow Rate 14990cfm = 7.07 m3/s

3728W , ! High Fan Speed Fan Power 5hp = 3728 W

3.53724, !Low Fan Speed Air Flow Rate = 1/2 high fan speeed air flow rate

0.5, ! Low Fan Speed Air Flow Rate Sizing Factor

autocalculate, ! Low Fan Speed Fan Power

0.16, ! Low Fan Speed Fan Power Sizing Factor

0.0059, ! Design Spray Water Flow Rate 94 gpm = 0.0059 m3/s

UserSpecifiedDesignCapacity, !Performance Input Method

cooling tower outdoor air node, ! Outdoor Air Inlet Node Name

1.25, !Heat Rejection Capacity and Nominal Capacity Sizing Ratio

, ! High Speed Standard Design Capacity

, ! Low Speed Standard Design Capacity

, ! Low Speed Standard Capacity Sizing Factor

, ! High Fan Speed U-factor Times Area Value

, ! Low Fan Speed U-factor Times Area Value

, ! Low Fan Speed U-Factor Times Area Sizing Factor

0.01, !Design Water Flow Rate: loop flow rate = 125 gpm = 0.01 m3/s

91438, ! High Speed User Specified Design Capacity 26 tons = 91438 W

45719, !Low Speed User Specified Design Capacity 16 tons = 45719 W

0.5, ! Low Speed User Specified Design Capacity Sizing Factor

35, ! Design Entering Water Temperature 95 F = 35C

29, ! Design Entering Air Temperature 85F = 29C

25.6; Design Entering Air Wet-bulb Temperature 78F = 25.5 C

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I spent more time with the EnergyPlus example file (EvaporativeFluidCooler_TwoSpeed.idf) and found that I can set almost every parameter I want except for two: I cannot change the design water flow rate away from autosize without an error, and I cannot size the high speed user specified design capacity above 16 tons (56270 W). As soon as I do either of those, the simulation fails.

DW gravatar imageDW ( 2016-08-02 10:20:42 -0500 )edit


Big EvaporativeFluidCooler, !

Condenser EvaporativeFluidcooler Inlet Node, !

Condenser EvaporativeFluidcooler Outlet Node, !

7.07, !H Fan Flow

3728, !H Fan {W}

3.5, !L Fan Flow

, !L Fan Flow sz fact

600, !L Fan {W}

, !L Fan {W} sz fact

0.0059, !Design Spray Flow Rate {m3/s}

UserSpecifiedDesignCapacity, ! Method

, !OA Node

, !HRej Cap

, !High Std Des

, !Low Std Des

, !Low sz fct

, !High UA

, !Low UA

, !Low sz

autosize, !Des Wtr flow

56270, !H Des Cap

25000, !L Des Cap

, !Low Des sz fact

25, !EWrT

29, !EAT

20, !AWB

DW gravatar imageDW ( 2016-08-02 10:35:10 -0500 )edit