Component 79: Forced Cooling Tower

In This Topic

Component 79: Forced Draft Cooling Tower (with characteristic field)

Specifications

Line connections
1	Air inlet
2	Air outlet
3	Cooling water inlet
4	Cooling water outlet
5	Make-Up water inlet
6	Blow-Down
7	Fan power

General User Input Values Characteristic Lines Physics Used Displays Example

General

The component forced draft cooling tower simulates the performance of wet cooling towers with induced draft fans under nominal and off-design conditions. A characteristic diagram describes the cooling tower performance.

Deviating from earlier versions, an additional switch (FCHRX) allows using the inlet air wet bulb temperature for the look-up in the performance nomograph. This corresponds with the definitions set out in DIN 1947. For legacy reasons and compliance with earlier versions of EBSILON, it is possible to (incorrectly) use the air dry bulb temperature (T1).

Characteristic Diagram

The basis of the characteristic diagram is the German industrial standard DIN 1947 (version of May 1989). The detailed structure of the description is taken from page 11 of this standard. The main purpose of DIN 1947 is to support cooling tower acceptance tests; consequently, the characteristic diagram only covers a narrow range of operation.

The characteristic diagram defines the cooling range or cold water temperature as a function of

- air wet bulb temperature,

- relative fan power,

- relative water mass flow rate and

- warm water inlet temperature.

Ten (10) characteristic curves, organized in three (3) curve sets, describe the performance nomograph. All curve sets are plotted in the same x-y-diagram with the wet bulb temperature as x-axis and the warm water inlet temperature (TWARM) as y-axis. The following curves need to be defined:

- 2 characteristic curves for the relative fan power QRFAN_1 and QRFAN_2 each with 2 points (linear dependency)

- 2 characteristic curves for load-factors LOAD_1 and LOAD_2 each with 2 points (linear dependency)

- 6 characteristic curves for cooling range CR_1-CR_6 each with 6 points (non-linear dependency)

The actual values of the parameters: relative fan power, relative water flow rate and cooling range must be within the definition range of the curves. Thus

- Min (QRFAN_1, QRFAN_2) <= H7 <= Max (QRFAN_1, QRFAN_2)

- Min (LOAD_1, LOAD_2) <= M3/M3N <= Max (LOAD_1, LOAD_2)

- CR_1 <= Cooling Range <= CR_6

If inputs exceed these limits, the limits are taken as input parameters. Otherwise, a linear interpolation is done. No extrapolation will be done; all returning values will be kept between the MIN and MAX constraints.

FMODE

This switch permits the use of different off-design states.

FMODE = 2: Calculation of the cooling tower performance, using the performance nomograph

FMODE = 0: Design calculation by defining the cold water temperature (T4CN). The warm water temperature is set to T3 (T3N=T3). The air temperature is taken from line 1. The relative fan power is set to 100% and relative cooling water flow rate is set to 100%. The cooling range is calculated using inlet water temperature (T3) and the user input for the cold water temperature (T4CN). A characteristic cooling range is calculated, using the nomograph. CCR (Cooling range correction) is calculated and stored as the difference between the cooling range from the design calculation and the characteristic cooling range from the performance nomograph.

FMODE = 1: Off-design case using characteristic diagram and CCR-correction determined by the reference conditions. The cooling range is determined by the nomograph and is corrected with CCR (actual cooling range = cooling range from the characteristic diagram + CCR). This operation represents a parallel shift of the characteristic curve.

FMODE = -1 (identification mode): T4 is given from outside, the cooling range is calculated using T3 and T4. The characteristic diagram is not used.

FCHRX

This switch defines which temperature value of the incoming air will be used for the look-up procedure in the performance nomograph.

FCHRX = 0 Look-Up is carried out using the air dry bulb temperature (=T1).

FCHRX = 1 Look-Up is carried out using the air wet bulb temperature.

FSPECL

For this component, the physical behaviour is determined by a characteristic field. Outside this field, results may occur which are physically not reasonable. In Release 8.00, a mechanism was implemented that restricts the temperature to the range of the characteristic field. This action may yield to different results compared to Release 7.00. If this change has to be avoided, you can deactivate the restriction by deactivating the flag FSPECL in the property sheet of the cooling tower.

T4 specification

For the identification mode (T4 specification) there are result values T3CL and T3CLCCR indicating which hot water temperature would arise due to the characteristic field to the width of the cooling section specified from outside. Here T3CL takes the unchanged characteristic field as a basis, and T3CLCCR the characteristic field corrected by the offset CCR.

Blow down

The mass flow for the blow down water can be specified externally now.

Identification Mode

It is possible to specify the cold water temperature T4 from outside (FMODE=-1). The cooling range will be calculated in this case.

Adaptation Polynomial ADAPT / Adaptation function EADAPT

Either the cooling range or the cold water temperature can be calculated or corrected with an adaptation polynomial or adaptation function.

Notes:

For this component, the physical behaviour is determined by a characteristic field. Outside this field, results may occur which are physically not reasonable. In Release 8.00, a mechanism was implemented that restricts the temperature to the range of the characteristic field. This action may yield to different results compared. If this change has to be avoided, you can deactivate the restriction by deactivating the flag FSPECL in the property sheet of the cooling tower.

There may be situations where it is not possible to close the energy balances based on the settings. Often, this was caused by a high setting for the spray losses. Therefore, in Ebsilon Release 9, the spray losses were reduced automatically (with a warning). On customer’s wish, this automatic reduction was removed (as of Release 10). Instead, the air mass flow was restricted to a plausible range between 0.2 and 5 of the cooling water mass flow (with a warning as well).
This may cause small changes in the results (mass flow, water fraction) on the air side.

User Input Values

T4CN	Cold water temperature (nominal)
DP34N	Pressure loss line 34 (nominal) Optional input, in case left empty, both pressures must be specified outside.
MSM3	Drift loss fraction (MS/M3)
M6M3	Blow Down fraction (M6/M3) Optional input, in case left empty, the blow down mass flow must be specified outside.
FMODE	Flag for calculation mode Like in Parent Profile (Sub Profile option only) Expression =0: GLOBAL =1: local off-design with normalized performance nomograph [ T4 = T3- (CCR+CR) ] =2: Use performance nomograph [ T4 = T3 - CR ] = -1: T4-specification from outside (identification mode, characteristic diagram is not used)
FSPEC	Flag for make-up mode Like in Parent Profile (Sub Profile option only) Expression =0: Circulation mode i.e. M3=M4 and M5=M2-M1+M6 =1: Discharge mode i.e. M4=M3-(M2-M1+M6) and M5=0
FCHRX	X value of characteristic Like in Parent Profile (Sub Profile option only) Expression = 0: Ordinary air temperature = 1: Wet bulb temperature
FSPECL	Temperature limitations Like in Parent Profile (Sub Profile option only) Expression =0: No limitation =1: Temperature restricted according to characteristic field
FADAPT	Flag for using the adaptation polynomial ADAPT / adaptation function EADPT Like in Parent Profile (Sub Profile option only) Expression =0: not used and not evaluated =1: CR = Polynomial * CR from performance nomograph =2: CR = Polynomial =3: T4 = T4CNPolynomial =1000: Not used, but ADAPT evaluated as RADAPT (Reduction of the computing time) = -1: CR = adaptation function CR from performance nomograph = -2: CR = adaptation function = -3: T4 = T4CN * adaptation function = -1000: Not used, but EADAPT evaluated as RADAPT (Reduction of the computing time)
EADAPT	Adaptation function
QRFAN_1	Characteristic curve parameter relative fan power 1
QRFAN_2	Characteristic curve parameter relative fan power 2
LOAD_1	Characteristic curve parameter M3/M3N_1
LOAD_2	Characteristic curve parameter M3/M3N_2
CR_1	Characteristic curve parameter cooling range 1
CR_2	Characteristic curve parameter cooling range 2
CR_3	Characteristic curve parameter cooling range 3
CR_4	Characteristic curve parameter cooling range 4
CR_5	Characteristic curve parameter cooling range 5
CR_6	Characteristic curve parameter cooling range 6
QFANN	Fan power (nominal)
T1N	Temperature at air inlet (nominal)
T3N	Warm water temperature (nominal)
CCR	Correction of the cooling range
M1N	Air mass flow (nominal)
M3N	Cooling water mass flow (nominal)

The identification value marked in blue is a reference value for off-design calculations. The actual off-design values refer to the values used in the equations.

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 curve: Warm water (T3) characteristic curve: T3=f(T1,QRFAN_1)

X-axis               1          T1                               1^st point
                        2          T1                               2^nd point
                        .
                      N         T1                               last point

Y-axis               1          T3(QRFAN_1)             1^st point
                        2          T3(QRFAN_1)           2^nd point
                        .
                        N         T3(QRFAN_1)            last point

Characteristic curve: Warm water (T3) characteristic curve: T3=f(T1,QRFAN_2)

X-axis               1          T1                               1^st point
                        2          T1                               2^nd point
                        .
                      N         T1                               last point

Y-axis               1          T3(QRFAN_2)            1^st point
                        2          T3(QRFAN_2)            2^nd point
                        .
                        N         T3(QRFAN_2)           last point

Characteristic curve: Warm water (T3) characteristic curve: T3=f(T1,LOAD_1)

X-axis               1          T1                               1^st point
                        2          T1                               2^nd point
                        .
                      N         T1                               last point

Y-axis               1          T3(LOAD_1)               1^st point
                        2          T3(LOAD_1)               2^nd point
                        .
                        N         T3(LOAD_1)               last point

Characteristic curve: Warm water (T3) characteristic curve: T3=f(T1,LOAD_2)

X-axis               1          T1                               1^st point
                        2          T1                               2^nd point
                        .
                      N         T1                               last point

Y-axis              1          T3(LOAD_2)               1^st point
                        2          T3(LOAD_2)               2^nd point
                        .
                        N         T3(LOAD_2)                last point

Characteristic curve: Warm water (T3) characteristic curve: T3=f(T1,CR_1)

X-axis             1          T1                               1^st point
                        2          T1                               2^nd point
                        .
                      N         T1                              last point

Y-axis              1          T3(CR_1)                  1^st point
                        2          T3(CR_1)                 2^nd point
                        .
                        N         T3(CR_1)                    last point

Characteristic curve: Warm water (T3) characteristic curve: T3=f(T1,CR_2)

X-axis               1          T1                               1^st point
                        2          T1                               2^nd point
                        .
                      N         T1                               last point

Y-axis             1          T3(CR_2)                    1^st point
                        2          T3(CR_2)                    2^nd point
                        .
                        N         T3(CR_2)                     last point

Characteristic curve: Warm water (T3) characteristic curve: T3=f(T1,CR_3)

X-axis            1          T1                               1^st point
                        2          T1                               2^nd point
                        .
                      N         T1                                last point

Y-axis             1          T3(CR_3)                 1^st point
                        2          T3(CR_3)                  2^nd point
                        .
                        N         T3(CR_3)                    last point

Characteristic curve: Warm water (T3) characteristic curve: T3=f(T1,CR_4)

X-axis               1          T1                               1^st point
                        2          T1                               2^nd point
                        .
                      N         T1                                last point

Y-axis               1          T3(CR_4)                    1^st point
                        2          T3(CR_4)                    2^nd point
                        .
                        N         T3(CR_4)                   last point

Characteristic curve: Warm water (T3) characteristic curve: T3=f(T1,CR_5)

X-axis            1          T1                               1^st point
                        2          T1                               2^nd point
                        .
                      N         T1                               last point

Y-axis               1          T3(CR_5)                   1^st point
                        2          T3(CR_5)                     2^nd point
                        .
                        N         T3(CR_5)                      last point

Characteristic curve: Warm water (T3) characteristic curve: T3=f(T1,CR_6)

X-axis               1          T1                               1^st point
                        2          T1                               2^nd point
                        .
                      N         T1                                last point

Y-axis               1          T3(CR_6)                     1^st point
                        2          T3(CR_6)                     2^nd point
                        .
                        N         T3(CR_6)                      last point

Physics Used

Equations

All cases

Moisture

========

MOL:= Molar mass

Y1H2O = X1H2O*MOL1SUM/18.0152

P1S = fsat(T1)

PHI1 = Y1H2O*P1/P1S

LOAD = M3/M3N

if (FMODE=2), then {

off-design with characteristic curve

}

if (GLOBAL = design and FMODE = GLOBAL), then {

Design case

LOAD=1

}

if (GLOBAL = off-design or FMODE=1), then {

off-design with normalized performance nomograph

}

Characteristic diagram interpolation

============================

Interpolation moisture

WW1 = f(T1) from characteristic curve 1 with QRFAN_1

WW2 = f(T1) from characteristic curve 2 with QRFAN_2

ZW = WW1+(H7-QRFAN_1)/( QRFAN _2- QRFAN _1)*(WW2-WW1)

Interpolation load

WW1 = f(ZW) from characteristic curve 3 with LOAD_1

WW2 = f(ZW) from characteristic curve 4 with LOAD_2

ZW = WW1+(LOAD-LOAD_1)/(LOAD_2-LOAD_1)*(WW2-WW1)

Interpolation warm water temperature

WW1 = f(ZW) from characteristic curve 5 with CR_1

WW2 = f(ZW) from characteristic curve 6 with CR_2

WW3 = f(ZW) from characteristic curve 7 with CR_3

WW4 = f(ZW) from characteristic curve 8 with CR_4

WW5 = f(ZW) from characteristic curve 9 with CR_5

WW6 = f(ZW) from characteristic curve 10 with CR_6

if (T3 >= WW(I) UND T3 < WW(I+1)), then {

ZW = (WW(I+1)-WW(I))

CR = CR_(I)+[T3-WW(I)] / ZW * [CR_(I+1)-CR_(I) ]

}

T4-calculation

=================

if (FMODE=2), then {

T4 = T3-CR}

else {

if (GLOBAL = design and FMODE = GLOBAL), then {

T4 = T4CN

ZW = T4N-T3N

CCR = ZW-CR }

else {

ZW = CCR+CR

T4 = T3-ZW }

}

State 4

=======

HX4 = f(P4,T4)

HX6 = HX4

Approximation state 2

=====================

FAC = .8

T2 = FAC*T3+(1-FAC)*T4

P2S = fsat(T2)

Y2H2O = P2S/P2

XH2O = Y2H2O*18.0152/MOL1SUM

DSR = XH2O-X1H2O

MSM1 = M3*MSM3/M1

M2M1 = 1+DSR+MSM1

X2H2OG = XH2O/M2M1

ZW = 1/M2M1

X2H2OL = MSM1*ZW

if ( i != H2O), then {

X2i=X1i*ZW }

HX2 = f(P2,T2)

Calculation of M1

=================

H2_1 = f(P1,T2)

H2_2 = f(P3,T2)

if (FSPEC = 0), then {

ZW = HX2_1-H1+DSR*(H2_2-H4)

M1 = M3*(H3-H4)/ZW }

else {

ZW = H2_1-H1+DSR*(H2_2-H4)

MX1 = M3*(H3-H4)/ZW

}

Pressure equations

=================

F = (M3/M3N) ** 2

DP34 = DP34N * F

P4 = P3 - DP34

P2 = P1

P4 = P6

P4 = P5

Enthalpy equations

===================

H2 = HX2

H4 = HX4

H6 = HX6

Mass flow equations

======================

if FSPEC=0, then {

M3 = M4 }

else {

M3*(M6M3+MSM3)+DSR*M1 = M4

}

M1 = MX1

M2 = M2M1*M1

M6 = M6M3*M3

if FSPEC=0, then {

M5 = (M6M3+ MSM3)*M3+DSR*M1 }

else {

M5 = 0 }

Component Displays

Display Option 1

Example

Click here >> Component 79 Demo << to load an example.

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