Component 151: Evaporative Cooler

In This Topic

Specifications

Line connections
1	Air/gas inlet
2	Air/gas outlet
3	Water inlet
4	Water outlet
5	Logic inlet for NTU (M5)

General User Input Values Characteristic Lines Physics Used References Example

General

Component 151 Evaporative Cooler can be used to model a device which reduces the temperature of an air or gas stream by means of evaporating liquid water. The amount of heat that can be removed from the air/gas stream equals the heat of evaporation consumed by the phase change of the water. Since this process is controlled by the rate of mass transfer from the liquid to the gaseous phase, there are two key factors to the effectiveness of the cooler: the mass transfer coefficient beta (primarily driven by flow turbulence), and the surface area of the liquid (which can be maximized by appropriate shaping of the metal sheets/packing through which the water flows in cross-low relative to the air/gas stream). If - as typically applied in industrial applications - the surplus water is recirculated to the water inlet, the wet bulb temperature of the air/gas stream represents the minimum for the outlet temperature, and the maximum temperature change that is achievable through this process is limited by the saturation of the air/gas stream.

User Input Values

FMODE	Flag to set the calculation mode Design / Off-design Like in Parent Profile (Sub profile option only) Expression =0: Global =1: local off-design (i.e. always off-design mode, even when a design calculation has been done globally) =-1: local design (i.e. always design mode, even when a off-design calculation has been done globally)
FFU	Switch ON/OFF Like in Parent Profile (Sub profile option only) Expression =0: OFF (air/gas: outlet = inlet; water flow: M = 0) =1: ON
FDES	Flag to set the design mode Like in Parent Profile (Sub profile option only) Expression =1: Use EFF (with effectiveness EFF = (T1 - T2)/(T1 - T_{wet bulb})) =2: Use RHUM (specify exit relative humidity) =3: Desired outlet temperature (T2) =4: Outlet temperature set externally
EFF	Desired effectiveness (T1 - T2)/(T1 - T_{wet bulb})
RHUM	Desired exit relative humidity
T2	Desired air/gas outlet temperature
COC	Cycles of concentration (= ratio of concentration of salt/minerals in recirculated water to concentration in make-up water)
NTU	Number of transfer units (NTU = beta*A/M1_dry) (RESERVED for later editions, not used)
HLR	Height to length ratio of channel/packing (uniform distribution of water along width) (RESERVED for later editions, not used)
DP12	Pressure drop between air/gas inlet and outlet
FEFFOD	Flag for the off-design effectiveness calculation mode, where EFF = (T1 - T2)/(T1 - T_{wet bulb}), and EFFX = (delta x_{H2O actual}/delta x_{H2O max}) with x_{H2O max} humidity ratio at saturation Like in Parent Profile (Sub profile option only) Expression =0: Constant EFF = EFFN (effectiveness as designed) =1: EFF = CEFFEFFN (use characteristic line CEFF(VM1/VM1N) for effectiveness correction factor) =2: EFFX = 1 - exp(-BA/M1_dry); BA = CFBABAN(VM1/VM1N)^BAEXP (i.e. betaA correlates with volume flow; use characteristic line CFBA(VM1/VM1N) for betaA function correction factor) =3: EFFX = 1 - exp(-BA/M1_dry); BA = CBABAN (use characteristic line CBA(VM1/VM1N) for beta*A correction factor)
BAEXP	Off-design exponent for beta*A as a function of (VM1/VM1N)
FDP12OD	Flag for the off-design pressure drop calculation Like in Parent Profile (Sub profile option only) Expression =0: Constant DP12 = DP12N =1: Depending on volume flow and density (~rho*VM^2)
EFFN	Nominal effectiveness (T1 - T2)/(T1 - T_{wet bulb})
VM1N	Nominal volumetric inlet flow
V1N	Nominal specific volume at air/gas inlet
DP12N	Nominal air/gas side pressure drop
BAN	Nominal betaA (mass transfer coefficient surface area)
NTUN	Nominal number of transfer units (NTU) (not used)

The parameters marked in blue are reference quantities for the off-design mode. The actual off-design values refer to these quantities in the equations used.

Generally, all inputs that are visible are required. But, often default values are provided.

For more information on colour of the input fields and their descriptions see Edit Component\Specification values

For more information on design vs. off-design and nominal values see General\Accept Nominal values

Characteristic Lines

Characteristic line 1: CEFF: Cooler effectiveness correction = f (VM1/VM1N)

                    1       Volumetric flow ratio                 1^st point
                    2        Volumetric flow ratio                2^nd point
                    .
                    N       Volumetric flow ratio                 last point

     Y-Axis    1       Effectiveness correction factor          1^st point
                    2        Effectiveness correction factor          2^nd point
                    .
                    N        Effectiveness correction factor         last point

Characteristic line 2: CFBA: beta*A*(VM1/VM1N)^BAEXP correction = f (VM1/VM1N)

     X-Axis     1       Volumetric flow ratio                 1^st point
                      2      Volumetric flow ratio                2^nd point
                    .
                    N       Volumetric flow ratio                 last point

     Y-Axis    1       beta*A function correction factor          1^st point
                    2        beta*A function correction factor          2^nd point
                    .
                    N       beta*A  function correction factor         last point

Characteristic line 3: CBA: beta*A correction = f (VM1/VM1N)

     X-Axis     1       Volumetric flow ratio                 1^st point
                      2      Volumetric flow ratio                2^nd point
                    .
                    N       Volumetric flow ratio                 last point

     Y-Axis    1       beta*A correction factor          1^st point
                    2        beta*A correction factor          2^nd point
                    .
                    N       beta*A  correction factor         last point

Physics Used

Energy Balance Model

The mass and energy balance of the evaporative cooler is solved taking account of the evaporation of the water stream that is recirculated in cross-flow to the air/gas stream. Since the recirculated stream of water is significantly larger than the make-up water that compensates for the blow-down and the amount of water that evaporates, it can be assumed that the recirculating water is at the phase equilibrium temperature (= wet bulb temperature). This temperature thus constitutes the minimum exit temperature that is achievable for the air/gas stream flowing through the device, and the cooling effectiveness can be quantified as the ratio of the actual temperature change of the air/gas stream to its maximum temperature change.

From the mass and energy balance, the respective humidity ratios for the actual exit conditions and the ideal state of saturation at the air/gas exit can be determined, and the mass transfer effectiveness EFFX can be calculated as follows:

Design Calculations

Mass Balance Equations:
	M1 + M3 = M2 + M4
	M2 = f(EFFX) = f(EFF) depending on method FDES in design and FEFFOD in off-design, respectively
	M4 = (M2 - M1)/(COC -1) ; COC= ratio of concentration of salt/minerals in recirculated water to concentration in make-up water; COC > 1

In design mode, the air/gas exit temperature T2 can be defined through various design methods FDES:

Effectiveness EFF

Exit relative humidity RHUM

Exit temperature T2

T2 set externally

In all of these cases, the balances for mass and energy can be closed with the respective input values, and the mass transfer effectiveness EFFX can be determined from the inlet, exit, and saturation conditions. Since the evaporation process can also be described by the laws of mass transfer with beta being the mass transfer coefficient accounting for the flow conditions of the process and A being the surface area available for the phase change, the mass transfer effectiveness can also be expressed with the following equation.

From this equation, the product beta*A can be derived as the nominal value BAN expressing the design characteristic of the evaporative cooler.

Off-Design Calculations

As the flow conditions on the water side do not change, the performance characteristics of the evaporative cooler are primarily affected by the flow conditions on the air/gas side of the equipment which can be expressed as the ratio of volumetric inlet flow at current conditions to the volumetric inlet flow at the design point (VM1/VM1N). The parameter FEFFOD offers several options to adjust the off-design performance characteristics of the evaporative cooler:

0: Constant EFF = EFFN

The cooling effectiveness as designed will be used under all operating conditions.

1: EFF = CEFF*EFFN

The cooling effectiveness will be adjusted by an effectiveness correction factor defined in characteristic line CEFF(VM1/VM1N).

2: EFFX = 1 - exp(-BA/M1_dry); BA = CFBA*BAN*(VM1/VM1N)^BAEXP

The beta*A value of the design case will be adjusted with an exponential function of (VM1/VM1N) with user defined exponent BAEXP and a correction factor defined in characteristic line CFBA(VM1/VM1N).

3: EFFX = 1 - exp(-BA/M1_dry); BA = CBA*BAN

The beta*A value of the design case will be adjusted with a correction factor defined in characteristic line CBA(VM1/VM1N).

References

Kloppers J.C., Kröger D.G., A Critical Investigation Into the Heat and Mass Transfer Analysis of Cross-Flow Wet Cooling Towers, Numerical Heat Transfer, Part A, 46: 785-806, 2004

Component Displays

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Example

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