CoolingTower:VariableSpeed:Merkel
Used in:
- Condenser loop, supply side
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CoolingTower:VariableSpeed:Merkel |
Used in:
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The variable speed (Merkel) tower model is based on Merkel’s theory and is similar to the single-speed and two-speed tower models. The open wet cooling tower is modelled as a counter flow heat exchanger with a variable-speed fan drawing air through the tower (induced-draft configuration). The model also includes a “free convection” regime where cooling tower performance modelled with the fan off.
For this model, Merkel’s theory is modified to include adjustments developed by Scheier to alter the heat transfer effectiveness based on current wetbulb, air flow rates, and water flow rates. The input requires performance curves or lookup tables to describe these three adjustment factors.
For multi-cell towers, capacity and air/water flow rate inputs are for the entire tower.
This is the name that you assign to the cooling tower which should be unique. If the supplied name is not unique, the software will automatically adjust the name to avoid duplicate names.
Your can select from 4 different types of cooling tower:
This field is used to choose which method is used to model the amount of water evaporated by the cooling tower. There are two options:
The default is 1-Saturated exit. The user-defined loss factor is entered under Evaporation loss factor, below. By assuming that the air leaving the tower is saturated, the evaporation can be directly calculated using moist air engineering calculations with data available within the cooling tower model (and does not require additional user input).
This field is used to specify the rate of water evaporated from the cooling tower and lost to the outside air (in %/K or %/F). It is only available if the Evaporation loss mode (above) is set to 2-Loss factor. The evaporation loss is then calculated as a fraction of the circulating condenser water flow and varies with the temperature change in the condenser water. The evaporation rate will equal this value times each degree Kelvin of temperature drop in the condenser water. Typical values are from 0.15 to 0.27 (%/K). The default is 0.2.
This field is used to specify the rate of water lost to the exiting air as entrained droplets (%). The drift loss is a percent of the condenser water flow. Typical values for towers with efficient drift eliminators are between 0.002 and 0.2% of the condenser water flow rate. The default value is 0.008%.
This field allows you to specify a sizing factor for this component to be applied when the component design inputs are autosized. The autosizing calculations are performed as usual and the results are multiplied by the sizing factor. This allows you to size a component to meet part of the design load while continuing to use the autosizing feature.
See also the section on Autosizing HVAC Components
This is the design air flow rate induced by the tower fan at full fan speed (in m3/s or ft3/min). The value can be autocalculated, in which case the air flow rate is based on the scalable sizing factor in the following input field
This is the sizing factor to use when auto-calculating the design air flow rate from the nominal capacity, in units of m3/s/W. The default is 2.76316*105.
The tower fan is allowed to operate between the minimum air flow rate ratio defined here and a maximum air flow rate ratio of 1.0 (which corresponds to the design [maximum] tower air flow rate). Below this value the tower is assumed to operate in the “free convection” regime with the tower fan off. The minimum air flow rate ratio must be greater than or equal to 0.1 and less than or equal to 0.5.
This setting allows the method by which the user will specify tower performance to be selected. There are two options:
This is the design water flow rate through the tower (in m3/s or gal/min). This value is the flow rate of the condenser loop water being cooled by the tower (not the flow rate of water being sprayed on the outside of the heat exchange coil). If the Performance input method is specified as 2-UA and design water flow rate, then a water flow rate greater than zero must be defined or the item can be autosized. If autosized, the design water flow rate is derived from the design load to be rejected by the condenser loop and the design loop temperature difference. If Performance input method is specified as 1-Nominal capacity, then the model automatically assumes a water flow rate of 5.382E-8 m3/s per W of tower capacity.
This is a scalable sizing factor for design water flow rate that scales with nominal capacity, in units of (m3/s/W or (gal/min)/(Btu/h)). The default value is 5.382*108 m3/s/W. This field is only used if the Design air flow rate is set to Autosize and If Performance input method is specified as 1-Nominal capacity. If the Performance input method is specified as 2-UA and design water flow rate then the design water flow rate is obtained from the plant sizing result.
This is the heat transfer coefficient-area product (UA) (in W/K or Btu/h-F) corresponding to the design air and water flow rates specified above. If the Performance Input Method is specified as 2-UA and design water flow rate, then a UA value greater than zero but less than or equal to 300,000 W/K must be defined, or the setting can be autosized. If autosized, the design tower UA value is derived from the design load to be rejected by the condenser loop and the design loop delta T, assuming a tower water inlet temperature of 35°C and tower inlet air at 35°C dry-bulb/25.6°C wet-bulb.
Select the curve that describes how the UA value varies as a function of air flow rate ratio (air flow rate/design air flow rate). The result of this curve is multiplied by the design UA value to adjust for air flow rate, along with the two other modifiers discussed below. The curve must be for one independent variable and should be normalized to 1.0 at an air flow rate ratio of 1.0.
Select the curve that describes how the UA value varies as a function of the current wetbulb temperature. The result of this curve is multiplied by the design UA value to adjust for wetbulb temperatures that differ from design conditions at 25.56°C (78°F), along with the two other modifiers discussed in the previous and following fields. The independent variable is the design wetbulb minus the current outdoor air wetbulb, in units of degrees Celsius. The curve must be for one independent variable and should be normalized to 1.0 at a wetbulb temperature difference of 0.0.
Select the curve that describes how the UA value varies as a function of the current water flow rate ratio (water flow rate/design water flow rate). The result of this curve is multiplied by the design UA value to adjust for water flow rates that differ from design level, along with the other two modifiers discussed above. The curve must be for one independent variable and should be normalized to 1.0 at a water flow ratio of 1.0.
Used when the Performance input method is specified as 1-Nominal capacity, this is the “nominal” heat rejection capacity of the cooling tower (in W or Btu/h), with entering water at 35°C (95°F), leaving water at 29.4°C (85°F), entering air at 25.6°C (78°F) wet-bulb and 35°C (95°F) dry-bulb temperatures.
The Heat rejection capacity and nominal capacity sizing ratio (below) is applied to this nominal tower capacity to give the actual tower heat rejection at these operating conditions. This field can be autosized.
Used when the Performance input method is specified as 1-Nominal capacity, this is the ratio of actual tower heat rejection to nominal capacity. The ratio is defined at entering water at 35C (95F), leaving water at 29.4C (85F), entering air at 25.6C (78F) wetbulb and 35C (95F) drybulb temperatures. Historically this ratio has been set at 1.25 based on the assumption that the tower must dissipate 0.25 W of compressor power for every what of heat removed at the chiller evaporator. The default is 1.25.
This numeric field contains the fan power (in W) at the design (maximum) air flow rate through the tower. A value greater than zero must be specified or this field can be autocalculated. When the field is autocalculated the following input field is used to size the fan power based on nominal capacity.
This numeric field contains the sizing factor to use when calculating the design fan power from the nominal capacity, in units of W/W. This field is only used if the previous is set to autocalculate. The default values is 0.0105.
Select the curve that describes fan power ratio (fan power/design fan power) as a function of air flow rate ratio (air flow rate/design air flow rate). The curve must be for one independent variable, typically a cubic, and should be normalized to 1.0 at an air flow rate ratio of 1.0.
This numeric field specifies the inlet air dry-bulb temperature (in ˚C or °F) at design conditions. This design temperature should correspond with the design values for range temperature, approach temperature, water flow rate, and air flow rate specified on this dialog. The minimum value for this field is 20˚C and the default value is 35˚C.
This numeric field specifies the inlet air wet-bulb temperature (in ˚C or °F) at design conditions. This design temperature should correspond with the design values for range temperature, approach temperature, water flow rate, and air flow rate specified on this dialog. The minimum value for this field is 20˚C and the default value is 25.6˚C.
This numeric field specifies the tower approach temperature (in ˚C or °F) at design conditions. The approach temperature is the outlet water temperature minus the inlet air wet-bulb temperature. The design approach temperature should correspond with the design values for inlet air wet-bulb temperature, range temperature, water flow rate, and air flow rate specified for this tower. The value for this field must be greater than 0˚C and the default value is 3.9˚C.
This numeric field specifies the range temperature (in ˚C or °F) at design conditions. The range temperature is defined as the inlet water temperature minus the outlet water temperature. The design range temperature should correspond with the design values for inlet air wet-bulb temperature, approach temperature, water flow rate, and air flow rate specified for this tower. The value for this field must be greater than 0˚C and the default value is 5.6˚C.
This setting contains the “nominal” heat rejection capacity of the cooling tower (in W or Btu/h) when the tower is in the “free convection” regime (water flow exists but tower fan is turned off), with entering water at 35C (95F), leaving water at 29.4C (85F), entering air at 25.6C (78F) wetbulb and 35C (95F) drybulb temperatures. The Heat Rejection Capacity and Nominal Capacity Sizing Ratio is applied to this free convection tower capacity to give the actual tower heat rejection at these operating conditions.
If you do not wish to model “free convection”, then this capacity should be set to 0.0. If you set a value greater than zero, then the Air flow rate in free convection regime must also have a value greater than zero. This field may be autocalculated, in which case it is set to a fraction of the Nominal capacity determined using the following field.
Warning: Due to a known bug in EnergyPlus 8.9, when using the 2-UA and Design water flow rate Performance input method option and UA at design air flow rate is set to "Autosize" you must set Free convection nominal capacity input to “Autocalculate”. You will need to change the Performance input method to 1-Nominal capacity to see it, but remember to change it back again to 2-UA and design water flow rate after checking.
Enter the sizing factor to use when calculating the free convection capacity. The default is 0.1.
This is the air flow rate (in m3/s or ft3/min) when the tower is in the “free convection” regime (water flow exists but tower fan is turned off). This value must be less than the value specified for the Design air flow rate. This field may be autocalculated, in which case it is set to a fraction of the Design air flow rate determined in the following input field.
If you do not wish to model “free convection” and are using the Performance input method 2-UA and Design water flow rate option, then this field should be set to 0.0. On the other hand, if you set the UA at free convection air flow rate or Free convection nominal capacity to a value greater than zero, then the free convection air flow rate must also be greater than 0.0.
This numeric field contains the sizing factor to use when calculating the free convection regime air flow rate. The default is 0.1.
This is the heat transfer coefficient-area product (W/K or Btu/h-F) when the tower is in the “free convection” regime (water flow exists but tower fan is turned off). If Performance input method is set to 2-UA and design water flow rate, then a UA value greater than zero but less than or equal to 300,000 must be defined, or the field can be autosized. If autosized the design tower UA value is derived from the design load to be rejected by the condenser loop and the design loop delta T, assuming a tower water inlet temperature of 35C and tower inlet air at 35C drybulb/25.6C wetbulb. If Performance input method is specified as 1-Nominal capacity, then EnergyPlus automatically calculates the tower UA based on the tower capacity specified under the setting Free convection capacity.
This numeric field contains the sizing factor to use when calculating the free convection regime UA value. The default is 0.1. This field is only used if the previous field is set to autocalculate and Performance input method is set to 2-UA and Design water flow rate.
This is the capacity of the tower’s electric basin heater (in W/K or Btu/h-F). It is used in conjunction with the Basin heater setpoint temperature (below). The basin heater electric power is equal to this setting multiplied by the difference between the basin heater set point temperature and the outdoor dry-bulb temperature. The basin heater only operates when the tower fan is off and water is not flowing through the tower, regardless of the basin heater schedule described below. The basin heater capacity must be greater than or equal to zero, with a default value of zero.
This is the set point temperature (˚C or ˚F) for the tower’s electric basin heater. The basin heater is active when the outdoor air dry-bulb temperature falls below this setpoint temperature, as long as the tower fan is off and water is not flowing through the tower. This set point temperature must be greater than or equal to 2˚C, and the default value is 2˚C.
This is the basin heater operating schedule. The basin heater operating schedule is assumed to be an on/off schedule and the heater is available to operate any time the schedule value is greater than 0. The basin heater operates when scheduled on and the outdoor air dry-bulb temperature is below the Basin Heater Setpoint Temperature. Regardless of this schedule, the basin heater may only operate when the cooling tower fan is off and water is not flowing through the tower.
This setting specifies which method is used to determine blow-down rates. There are two options:
The choice will determine which of the two models below is used. The default is 1‑Concentration ratio.
This is used to dynamically adjust the rate of blow-down in the cooling tower as a function of the rate of evaporation. Blow-down is water intentionally drained from the tower in order to offset the build up of solids in the water that would otherwise occur because of evaporation. The value entered here is dimensionless. It can be characterized as the ratio of solids in the blow-down water to solids in the make-up water. Typical values for tower operation are 3 to 5. The default value is 3.
This is the schedule used to define the amount of water (in m3/s only) flushed from the basin on a periodic basis to purge the tower of mineral scale build-up and other contaminants. This schedule is only used if the Blow-down Calculation Mode is set to 2‑Scheduled rate. The amount of water used due to blow-down depends on the makeup water quality and is specific to each geographical location. Typical values range from 0.0002 to 0.0013 m3/s (17.3 to 112.3 m3/day). This water usage is in addition to the amount of water lost to the atmosphere due to evaporation and/or drift. Since blow-down occurs when the basin water contaminant concentration is high, blow-down only occurs when the cooling tower is active and water is flowing through the tower (regardless of the water usage defined by this schedule).
This setting is used to enable a multi-cell tower to be defined.
This is the number of cells in the multi-cell cooling tower.
This specifies the method used to control the number of cells used to meet the load, the two choices are:
This is the allowable smallest fraction of the design water flow rate. Flows less than this value will commonly result in fluid distribution problems; the pressure at each nozzle will be too weak for the fluid to be sprayed out in the correct pattern, not all the fill would be wet. The default value is 0.33.
This numeric field specifies the allowable largest fraction of the design water flow rate. The default value is 2.5.