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there are so many online psychrometric calculators (hvac-calculators)!

great site

Originally Posted by agross

I am trying to enable a piece of equipment through a dry bulb temp and %rh... is there an equation that can take the db and rh and convert it to a wet bulb?? thanks, adam.

call up "aggreko, cooling tower sizing tool and wet bulb calculator" just plug in d.b.
and r.h. you will get wet bulb and enthalpy. keep this on your p.c.

Originally posted by agross
Yes, I'm trying to... we have an old plate and frame that was used for free cooling... apparently when they switched over control systems (Barber-Coleman to Siemens) the new actuators/programming were not included for it, only the chillers and cooling tower. This was well before I started working there so I really don't know all the specifics. Anyway, we had to replace an isolation valve on one of the chillers so we manually switched over to the plate and frame and it worked beautifully, chilled water temps held better than expected. My new goal is to convince my boss that we should use it when OAT & RH are favorable. In order to do so, he wants a sequence of operations and a program. This particular chilled water system supports some critical equipment so there is no room for error. I did some research on plate and frame free cooling I found out that you can only use it when WB is 10F less than desired chilled water temp; because we only have OAT & RH sensors, I need to find a way to calculate the WB. Thanks for all the help, Adam.

I'll give it a try to post the text version of the Basic code I mentioned before. It's written in RapidQ Basic, but that version is generic enough so that the code should be easily converted to most any programming language. I know it took me very little time to convert it from the original VBA to Java Script, and equally little time to convert it to RapidQ. Originally I'd written it in old DOS QBasic, but have lost the original source code somewhere among a pile of old, archived code on CDs I've got piled up. Which I have intended to catalog, for years, but have never seemed to get around to it. :-)

Watch out for line wrap ....




'Just a little routine to calculate the Psychrometric properties
'of air given 3 known conditions.
'
'Given Dry Bulb Temperature, Relative Humidity, and Altitude of a
'location. The routine calculates 8 other properties of the air
'sample. Results can be saved. Mostly useful to HVAC types (Heating
'Ventilation and Air Condicitoning), engineers, science students, etc.
'
'I'd originally written this in another programming language but a
'friend, who is learning to program in RapidQ asked me for an
'an example program in RapidQ that used some simple functions he was
'trying to grasp how to do. ie How to implement math functions, simple
'input boxes, buttons, etc. And how to save a simple text file.
'
'I'm no real programmer, nor expert by any means with RapidQ, so nothing
'here for any experienced programmer to find useful.
'
'However, I thought perhaps a beginner might find some use in my simple
'style and techniques. Or perhaps someone will find the math functions
'for deriving the various other properties of air from the given 3 to
'be of use.
'
'Free for anyone to use as he or she sees fit.
'Bob G - Osiyo53@yahoo.com

$Include "Rapidq.inc"

DECLARE sub calculate_drha
DECLARE Sub Closemain
DECLARE Sub Dummyproc
DECLARE Sub reset_form
DECLARE Sub save_file

DIM inout_label_Font AS Qfont
inout_label_Font.Name = "MS Sans Serif"
inout_label_Font.Size = 8
inout_label_Font.Color = clWindowText
inout_label_Font.AddStyles(fsBold,fsUnderLine)
DIM Label1_Font AS QFont
Label1_Font.Name = "MS Sans Serif"
Label1_Font.Size = 8
Label1_Font.Color = clWindowText
Label1_Font.AddStyles(fsBold)

DIM SaveDialog AS QSAVEDIALOG
SaveDialog.Filter = "Text Files(*.TXT)|*.TXT|All Files(*.*)|*.*"
SaveDialog.FilterIndex = 1 '' Use "Text Files" as default

DIM file1 AS QFILESTREAM

DIM a1 AS DOUBLE
DIM a2 AS DOUBLE
DIM a3 AS DOUBLE
DIM a4 AS DOUBLE
DIM a5 AS DOUBLE
DIM a6 AS DOUBLE
DIM acc AS DOUBLE
DIM atm AS DOUBLE
DIM b1 AS DOUBLE
DIM b2 AS DOUBLE
DIM b3 AS DOUBLE
DIM b4 AS DOUBLE
DIM db AS DOUBLE
DIM dp AS DOUBLE
DIM dx AS DOUBLE
DIM elev AS DOUBLE
DIM fmid AS DOUBLE
DIM h AS DOUBLE
DIM j AS DOUBLE
DIM nn AS DOUBLE
DIM p1 AS DOUBLE
DIM p2 AS DOUBLE
DIM p3 AS DOUBLE
DIM p4 AS DOUBLE
DIM psat AS DOUBLE
DIM pvp AS DOUBLE
DIM pvs AS DOUBLE
DIM rh AS DOUBLE
DIM rhpercent AS DOUBLE
DIM rtb AS DOUBLE
DIM savename AS STRING
DIM ta AS DOUBLE
DIM v AS DOUBLE
DIM vp AS DOUBLE
DIM w AS DOUBLE
DIM wb AS DOUBLE
DIM wh AS DOUBLE
DIM ws AS DOUBLE
DIM wsat AS DOUBLE
DIM wstar AS DOUBLE
DIM wtemp AS DOUBLE
DIM xmid AS DOUBLE
DIM y AS DOUBLE
DIM z AS DOUBLE


'---------------------------------------------------
' Calculate the Logarithm of a number to the base 10
'
FUNCTION log10(number AS DOUBLE)AS DOUBLE
log10 = Log(number) / Log(10#)
END FUNCTION

'---------------------------------------------------
' Calculate Atmospheric pressure given elevation
'
FUNCTION psychro_atm(elev AS DOUBLE)AS DOUBLE
Dim el(21)AS DOUBLE, press(21)AS DOUBLE
'Altitude Press.
el(1) = -1000: press(1) = 31.02
el(2) = -500: press(2) = 30.47
el(3) = 0: press(3) = 29.921
el(4) = 500: press(4) = 29.38
el(5) = 1000: press(5) = 28.86
el(6) = 2000: press(6) = 27.82
el(7) = 3000: press(7) = 26.82
el(8) = 4000: press(8) = 25.82
el(9) = 5000: press(9) = 24.9
el(10) = 6000: press(10) = 23.98
el(11) = 7000: press(11) = 23.09
el(12) = 8000: press(12) = 22.22
el(13) = 9000: press(13) = 21.39
el(14) = 10000: press(14) = 20.48
el(15) = 15000: press(15) = 16.89
el(16) = 20000: press(16) = 13.76
el(17) = 30000: press(17) = 8.9
el(18) = 40000: press(18) = 5.56
el(19) = 50000: press(19) = 3.44
el(20) = 60000: press(20) = 2.14
i = 1
WHILE elev > el(i)
i = i + 1
WEND
psychro_atm = press(i)
END FUNCTION

'-------------------------------------------------------------------------------
' Calculate vapor pressure at saturation given dry bulb temp
'
FUNCTION psychro_pvs(temp AS DOUBLE)AS DOUBLE
a1 = -7.90298
a2 = 5.02808
a3 = -0.00000013816
a4 = 11.344
a5 = 0.0081328
a6 = -3.49149
b1 = -9.09718
b2 = -3.56654
b3 = 0.876793
b4 = 0.0060273
ta = (temp + 459.688) / 1.8
IF ta > 273.16 THEN
z = 373.16 / ta
p1 = (z - 1) * a1
p2 = log10(z) * a2
p3 = ((10 ^ ((1 - (1 / z)) * a4)) - 1) * a3
p4 = ((10 ^ (a6 * (z - 1))) - 1) * a5
ELSE
z = 273.16 / ta
p1 = b1 * (z - 1)
p2 = b2 * log10(z)
p3 = b3 * (1 - (1 / z))
p4 = log10(b4)
END IF
psychro_pvs = 29.921 * (10 ^ (p1 + p2 + p3 + p4))
END FUNCTION

'-------------------------------------------------------------------------------
' Calculate Humidity Ratio given dry bulb temp, relative humidity, and elevation
'
FUNCTION psychro_wrh(db AS DOUBLE, rh AS DOUBLE, atm AS DOUBLE)AS DOUBLE
wsat = psychro_pvs(db)
wtemp = 0.62198 * (wsat / (atm - wsat))
psychro_wrh = rh * wtemp
END FUNCTION

'-------------------------------------------------------------------------------
'Calculate Enthalpy given dry bulb temp and Humidity Ratio
'
FUNCTION psychro_h_w(db AS DOUBLE, w AS DOUBLE)AS DOUBLE
psychro_h_w = (db * 0.24) + (1061 + (0.444 * db))* w
END FUNCTION

'-------------------------------------------------------------------------------
'Calculate dew point temp. given Vapor Pressure
'
FUNCTION psychro_dp(pvp AS DOUBLE) AS DOUBLE
nn=pvp*0.491154
y = LOG(nn)
IF pvp < 0.18036 THEN
psychro_dp = 90.12 + (26.142 * y) + (0.8927 * y * y)
ELSE
psychro_dp = 100.45 + (33.193 * y) + (2.319 * y * y) + 0.17074*(y^3)+1.2063*((pvp*0.491154)^0.1984)
END IF
END FUNCTION

'-------------------------------------------------------------------------------
'Calculate Humidity Ratio given dry bulb temp and pressure
'
FUNCTION psychro_w_pvp(pair AS DOUBLE, pvp AS DOUBLE) AS DOUBLE
psychro_w_pvp = 0.622 * pvp / pair
END FUNCTION

'-------------------------------------------------------------------------------
' Calculate Vapor Pressure given dry bulb temp, wet bulb temp, and pressure
'
FUNCTION psychro_pv1(db AS DOUBLE, wb AS DOUBLE, atm AS DOUBLE) AS DOUBLE
pvp = psychro_pvs(wb)
ws = (pvp / (atm - pvp)) * 0.62198
IF wb <= 32 THEN
wh=((1219.98+0.44*db-0.49*wb)*ws-0.24*(db-wb))/(1219.98+0.44*db-0.49*wb)
psychro_pv1 = atm * (wh / (0.62198 + wh))
ELSE
wh=((1093-0.556*wb)*ws-0.24*(db-wb))/(1093+0.444*db-wb)
psychro_pv1 = atm * (wh / (0.62198 + wh))
END IF
END FUNCTION

'-------------------------------------------------------------------------------
'Calculate Humidity Ratio given dry bulb temp, wet bulb temp, and pressure
'
FUNCTION psychro_w(db AS DOUBLE, wb AS DOUBLE, atm AS DOUBLE) AS DOUBLE
vp = psychro_pv1(db, wb, atm)
psychro_w = 0.622 * vp / (atm - vp)
END FUNCTION

'-------------------------------------------------------------------------------
'Computes wet-bulb temperature iteratively, dry bulb, dew point
'enthalpy and atm pressure using Bisection method.
'
FUNCTION wet_bulb(db AS DOUBLE,dp AS DOUBLE,h AS DOUBLE,atm AS DOUBLE)AS DOUBLE
j=0
acc=0.0001

rtb=dp
dx=db-dp
WHILE j<40
j=j+1
dx=dx*0.5
xmid=rtb+dx
psat=psychro_pvs(xmid)
wstar=psychro_w_pvp(atm-psat, psat)
fmid=psychro_h_w(xmid, wstar)
IF ((h-fmid)>0.0)THEN
rtb=xmid
END IF
IF ABS(dx)<acc THEN
EXIT WHILE
END IF
WEND
wet_bulb=rtb
wb=rtb
END FUNCTION

'-------------------------------------------------------------------------------
'Calculate Specific Volume given dry bulb temp, wet bulb temp, and pressure
'
FUNCTION psychro_v(db AS DOUBLE, wb AS DOUBLE, atm AS DOUBLE)AS DOUBLE
psychro_v = ((0.754 * (db + 459.7)* (1 + (7000 * psychro_w(db, wb, atm) / 4360))) / atm)
END FUNCTION

CREATE MainForm AS QForm
Caption = "Bob G's (Osiyo) Free Software"
Top = 129
Left = 203
Width = 460
Height = 343
Center
Wndproc=Dummyproc
Onclose=Closemain
END CREATE
CREATE main_label AS QLabel
Parent = MainForm
Top = 3
Left = 93
Width = 32
Height = 13
Caption = "Calculate Psychrometric Properties of Air"
Color = &hC0C0C0
Font = Label1_Font
AutoSize = False
END CREATE
CREATE db_label AS QLabel
Parent = MainForm
Top = 51
Left = 22
Width = 32
Height = 13
Caption = "Dry Bulb Temp (F) : "
Color = &hC0C0C0
Font = Label1_Font
AutoSize = False
END CREATE
CREATE rh_label AS QLabel
Parent = MainForm
Top = 75
Left = 5
Width = 109
Height = 13
Caption = "Relative Humidity (%) : "
Color = &hC0C0C0
Font = Label1_Font
AutoSize = False
END CREATE
CREATE elev_label AS QLabel
Parent = MainForm
Top = 98
Left = 7
Width = 32
Height = 13
Caption = "Elev Above Sea (FT) : "
Color = &hC0C0C0
Font = Label1_Font
AutoSize = False
END CREATE
CREATE input_label AS QLabel
Parent = MainForm
Top = 30
Left = 51
Width = 32
Height = 13
Caption = "INPUTS"
Color = &hC0C0C0
Font = inout_label_Font
AutoSize = False
END CREATE
CREATE calc_btn AS QButton
Parent = MainForm
Top = 72
Left = 238
Width = 75
Height = 25
Caption = "CALCULATE"
OnClick = calculate_drha
END CREATE
CREATE reset_btn AS QButton
Parent = MainForm
Top = 72
Left = 358
Width = 75
Height = 25
Caption = "RESET"
OnClick = reset_form
END CREATE
CREATE db_edit AS QEdit
Parent = MainForm
Top = 48
Left = 136
Width = 49
Height = 21
Text = "70.0"
Color = &hFFFFFF
MaxLength = 5
AutoSize = False
END CREATE
CREATE rh_edit AS QEdit
Parent = MainForm
Top = 72
Left = 136
Width = 49
Height = 21
Text = "50.0"
Color = &hFFFFFF
MaxLength = 5
END CREATE
CREATE elev_edit AS QEdit
Parent = MainForm
Top = 96
Left = 135
Width = 49
Height = 21
Text = "1000"
Color = &hFFFFFF
MaxLength = 6
END CREATE
CREATE output_label AS QLabel
Parent = MainForm
Top = 129
Left = 182
Width = 32
Height = 13
Caption = "OUTPUTS"
Color = &hC0C0C0
Font = inout_label_Font
AutoSize = False
END CREATE
CREATE atm_label AS QLabel
Parent = MainForm
Top = 154
Left = 18
Width = 32
Height = 13
Caption = "Atmospheric Press (in.Hg) : "
Color = &hC0C0C0
Font = Label1_Font
AutoSize = False
END CREATE
CREATE svp_label AS QLabel
Parent = MainForm
Top = 179
Left = 6
Width = 32
Height = 13
Caption = "Saturated Vap Press (in.Hg) : "
Color = &hC0C0C0
Font = Label1_Font
AutoSize = False
END CREATE
CREATE pvp_label AS QLabel
Parent = MainForm
Top = 202
Left = 25
Width = 32
Height = 13
Caption = "Partial Vap Press (in.Hg) : "
Color = &hC0C0C0
Font = Label1_Font
AutoSize = False
END CREATE
CREATE hr_label AS QLabel
Parent = MainForm
Top = 226
Left = 44
Width = 32
Height = 13
Caption = "Humidity Ratio (lb/lb) : "
Color = &hC0C0C0
Font = Label1_Font
AutoSize = False
END CREATE
CREATE atm_edit AS QEdit
Parent = MainForm
Top = 150
Left = 179
Width = 49
Height = 21
Text = "0.0"
Color = &hFFFFFF
MaxLength = 7
AutoSize = False
ReadOnly = True
END CREATE
CREATE svp_edit AS QEdit
Parent = MainForm
Top = 174
Left = 179
Width = 49
Height = 21
Text = "0.0"
Color = &hFFFFFF
MaxLength = 7
ReadOnly = True
AutoSize = False
END CREATE
CREATE pvp_edit AS QEdit
Parent = MainForm
Top = 199
Left = 179
Width = 49
Height = 21
Text = "0.0"
Color = &hFFFFFF
MaxLength = 7
ReadOnly = True
AutoSize = False
END CREATE
CREATE hr_edit AS QEdit
Parent = MainForm
Top = 223
Left = 178
Width = 49
Height = 21
Text = "0.0"
Color = &hFFFFFF
MaxLength = 7
AutoSize = False
ReadOnly = True
END CREATE
CREATE svol_label AS QLabel
Parent = MainForm
Top = 154
Left = 250
Width = 32
Height = 13
Caption = "Specific Vol (cuft/lb) : "
Color = &hC0C0C0
Font = Label1_Font
AutoSize = False
END CREATE
CREATE dp_label AS QLabel
Parent = MainForm
Top = 179
Left = 291
Width = 32
Height = 13
Caption = "Dew Point (F) : "
Color = &hC0C0C0
Font = Label1_Font
AutoSize = False
END CREATE
CREATE wb_label AS QLabel
Parent = MainForm
Top = 202
Left = 297
Width = 32
Height = 13
Caption = "Wet Bulb (F) : "
Color = &hC0C0C0
Font = Label1_Font
AutoSize = False
END CREATE
CREATE ent_label AS QLabel
Parent = MainForm
Top = 227
Left = 266
Width = 32
Height = 13
Caption = "Enthalpy (BTU/lb) : "
Color = &hC0C0C0
Font = Label1_Font
END CREATE
CREATE svol_edit AS QEdit
Parent = MainForm
Top = 150
Left = 383
Width = 49
Height = 21
Text = "0.0"
Color = &hFFFFFF
MaxLength = 7
AutoSize = False
ReadOnly = True
END CREATE
CREATE dp_edit AS QEdit
Parent = MainForm
Top = 175
Left = 382
Width = 49
Height = 21
Text = "0.0"
Color = &hFFFFFF
MaxLength = 7
AutoSize = False
ReadOnly = True
END CREATE
CREATE wb_edit AS QEdit
Parent = MainForm
Top = 200
Left = 382
Width = 49
Height = 21
Text = "0.0"
Color = &hFFFFFF
MaxLength = 7
AutoSize = False
ReadOnly = True
END CREATE
CREATE ent_edit AS QEdit
Parent = MainForm
Top = 225
Left = 381
Width = 49
Height = 21
Text = "0.0"
Color = &hFFFFFF
MaxLength = 7
AutoSize = False
ReadOnly = True
END CREATE
CREATE file_btn AS QButton
Parent = MainForm
Top = 264
Left = 176
Width = 75
Height = 25
Caption = "Save to File"
OnClick = save_file
END CREATE
MainForm.ShowModal

'-------------------------------------------------------------------------------
'Compute other properties given Dry Bulb Temp.,Relative Humidity and Altitude.
'
SUB calculate_drha
db=VAL(db_edit.Text)
elev=VAL(elev_edit.Text)
rhpercent=VAL(rh_edit.Text)
rh=rhpercent/100

'calc atmospheric pressure
atm=psychro_atm(elev)
atm_edit.Text = LEFT$((STR$(atm)),7)

'calc vapor pressure at saturation
pvs=psychro_pvs(db)
svp_edit.Text = LEFT$((STR$(pvs)),7)

'Calculate Partial Vapor Pressure.
pvp=pvs*rh
pvp_edit.TEXT = LEFT$((STR$(pvp)),7)

'calc humidity ratio
w=psychro_wrh(db, rh, atm)
hr_edit.Text = LEFT$((STR$(w)),7)

'calculate Enthalpy
h=psychro_h_w(db, w)
ent_edit.TEXT = LEFT$((STR$(h)),7)

'calculate Dew Point.
dp=psychro_dp(pvp)
dp_edit.Text = LEFT$((STR$(dp)),7)

'Calculate wet bulb temperature using an iterative method.
IF rh <1.000 THEN
wb=wet_bulb(db,dp,h,atm)
ELSE
wb=db
END IF
wb_edit.Text = LEFT$((STR$(wb)),7)

'calculate Specific Volume
v=psychro_v(db, wb, atm)
svol_edit.Text = LEFT$((STR$(v)),7)
END SUB

SUB Dummyproc ' To Minimize Properly
END SUB

SUB Closemain
Application.Terminate
END SUB

SUB reset_form
db_edit.Text = "70.0"
elev_edit.Text = "1000"
rh_edit.Text = "50.0"
atm_edit.Text = "0.0"
svp_edit.Text = "0.0"
pvp_edit.TEXT = "0.0"
hr_edit.Text = "0.0"
ent_edit.TEXT = "0.0"
dp_edit.Text = "0.0"
wb_edit.Text = "0.0"
svol_edit.Text = "0.0"
END SUB

SUB save_file
IF SaveDialog.Execute THEN
savename = SaveDialog.FileName
IF UCASE$(RIGHT$(savename,4))<>".TXT" THEN
savename = savename + ".TXT"
END IF
sl1$ = "Dry Bulb Temp (F) : " + db_edit.Text
sl2$ = "Relative Humidity (%) : " + rh_edit.Text
sl3$ = "Elev Above Sea (FT) : " + elev_edit.Text
sl4$ = "Atmospheric Press (in.Hg) : " + atm_edit.Text
sl5$ = "Saturated Vap Press (in.Hg) : " + svp_edit.Text
sl6$ = "Partial Vap Press (in.Hg) : " + pvp_edit.TEXT
sl7$ = "Humidity Ratio (lb/lb) : " + hr_edit.Text
sl8$ = "Specific Vol (cuft/lb) : " + svol_edit.Text
sl9$ = "Dew Point (F) : " + dp_edit.Text
sl10$ = "Wet Bulb (F) : " + wb_edit.Text
sl11$ = "Enthalpy (BTU/lb) : " + ent_edit.TEXT
file1.Open(savename,fmCreate) ' create a new file
file1.WriteLine("Inputs Used ..............................")
file1.WriteLine("")
file1.WriteLine(sl1$)
file1.WriteLine(sl2$)
file1.WriteLine(sl3$)
file1.WriteLine("")
file1.WriteLine("")
file1.WriteLine("Calculated Results .......................")
file1.WriteLine("")
file1.WriteLine(sl4$)
file1.WriteLine(sl5$)
file1.WriteLine(sl6$)
file1.WriteLine(sl7$)
file1.WriteLine(sl8$)
file1.WriteLine(sl9$)
file1.WriteLine(sl10$)
file1.WriteLine(sl11$)
file1.WriteLine("")
file1.Close
END IF
END SUB

'End Program

Originally posted by schoolguy
Here is a good one:



X = 273.16 / ((DryBulb - 32) / 1.8 + 273.16)
X = -10.7959 * (1 - X) - (2.1836 * ln(X)) + 2.2196
X = 29.92 * (1 / exp(2.3026 * X))
X = RH * X / 100
X = 0.622 * X / (29.92 - X)
Enthalpy = (0.24 + (0.444 * X)) * DryBulb + (1061 * X)

Not bad, the algorithm gives a pretty decent approximation. I tested it against a known mathematical model and yours gives good results in simpler form. Doesn't correct for altitude, etc but the answers are perfectly useable for ordinary HVAC work.

However, you're calculating enthalphy. Original poster requested Wet Bulb.

I see that as one works the math, by line #4 above, you
extract the partial vapor pressure. That's the X=RH*X/100
line, in case line counting gets confusing due to
reformatting of lines on your screen.

Now, there is no such thing as a math formula that will give
one the EXACT, one and only answer, for Wet Bulb
calculations. The only thing math can do is approximate.
And as to how close a math algorithm is to the reality of a
measured WBT, depends on how complicated one wishes to get
with your math.

If one is trying to stay fairly simple, once one has worked
out the first 4 lines above, you've arrived at Partial Vapor
Pressure (close enough for most HVAC purposes)for the given
DBT and RH.

Using the PVP, find approx Dew Point.

nn=pvp*0.491154
y = LOG(nn)
IF pvp < 0.18036 THEN
dp = 90.12 + (26.142 * y) + (0.8927 * y * y)
ELSE
dp = 100.45 + (33.193 * y) + (2.319 * y * y)
dp = dp + 0.17074*(y^3)+1.2063*((pvp*0.491154)^0.1984)
END IF

(Watch out for line wrap)

From here, one can keep it simple and use the "1/3 Rule"
that metrologists often use to approximate WBT. Which gives
a fairly decent result under most operating conditions for
the equipment the original poster mentions.

Under the 1/3 Rule one figures out the Dew Point Depression (DPD), which is the difference between Dry Bulb and Dew Point. Then divide DPD by 3. And subtract that result from Dry Bulb.

To be safe, I'd add 1.0 to the WBT answer before doing the
subtraction to determine if Desired Coil Temp minus WBT >= 10.

Other than that. One just about has to resort to using an
iterative mathematical model to extract a more accurate
answer. Which is what those electronic meters that read DBT
and RH then calculate WBT when yah push a button do. And
what I used in the Basic language programming example I
pointed to in my original response. That example uses an
algorithm published by ASHRAE, that was tested against
actual measured values and shown to be accurate enough for
HVAC design purposes.

Now, if the original poster is looking for a nice, simple X=
Y*Z type formula. I'm afraid he's SOL. There is no such
simple mathematical formula possible that'll give any sort
of meaningful and useful answer.

Yes, I'm trying to... we have an old plate and frame that was used for free cooling... apparently when they switched over control systems (Barber-Coleman to Siemens) the new actuators/programming were not included for it, only the chillers and cooling tower. This was well before I started working there so I really don't know all the specifics. Anyway, we had to replace an isolation valve on one of the chillers so we manually switched over to the plate and frame and it worked beautifully, chilled water temps held better than expected. My new goal is to convince my boss that we should use it when OAT & RH are favorable. In order to do so, he wants a sequence of operations and a program. This particular chilled water system supports some critical equipment so there is no room for error. I did some research on plate and frame free cooling I found out that you can only use it when WB is 10F less than desired chilled water temp; because we only have OAT & RH sensors, I need to find a way to calculate the WB. Thanks for all the help, Adam.

Here is a good one:



X = 273.16 / ((DryBulb - 32) / 1.8 + 273.16)
X = -10.7959 * (1 - X) - (2.1836 * ln(X)) + 2.2196
X = 29.92 * (1 / exp(2.3026 * X))
X = RH * X / 100
X = 0.622 * X / (29.92 - X)
Enthalpy = (0.24 + (0.444 * X)) * DryBulb + (1061 * X)

Agross, are you programming, if you are I have just the thing

Originally posted by osiyo

Originally posted by -80guru
get a sling psycrometer. It has WB DB and RH

True. But then, he asked for the equations. So I presumed he might be writing a program.

If he just wants to do the conversion manually, he could also use a standard psychrometric chart.

Yes,I was afraid you control guys were talking about something else...

here ya go:

RH=100e(Tw-Td/28.116)-17.4935(Td-Tw)e-Td/28.116

RH is in percent.Td is dry bulb in F.Tw is wet bulb in F.
The symbol e refers to the exponential function.The exponential function is taken to the power represented by the number above the symbol.(The function of e is the usual one used where 2.71828 is the value of e)

The formula for grains of water per pound of dry air is:
grain/pound=.09065RHeTd/28.116

Now remember that I could only copy that from the book to here in "keyboard style".I think you can decipher it though.

Originally posted by -80guru
get a sling psycrometer. It has WB DB and RH

True. But then, he asked for the equations. So I presumed he might be writing a program.

If he just wants to do the conversion manually, he could also use a standard psychrometric chart.

Originally posted by agross
thanks for the input osiyo, i'm having some trouble w/ the link, can you help at all?

Okay, try this. Go to the below site.

http://groups.yahoo.com/group/rapidq/

You'll have to subscribe to the group to look into the "Files" section. A painless process and costs nothing. They just ask for a valid email address. I've been a subscriber to various Yahoo Groups for years, and near as I can tell they've never passed around my email addy. Nor will they send yah annoying mail unless you allow it.

Anyway, subscribe. Takes 30 seconds. Then look into the "Files" section. And you'll see "airpsychro.bas". It's a simple text file. You don't need any special program to view, download, or edit it.

I'd just paste it here, however I'm not sure this site would like an 18k text file pasted into their edit box. Plus it'd word wrap in this site's edit box, which would make it hard to read.

If that doesn't work, say so. And if you have an available email addy I'll email it direct.

get a sling psycrometer. It has WB DB and RH

thanks for the input osiyo, i'm having some trouble w/ the link, can you help at all?

Originally posted by agross
I am trying to enable a piece of equipment through a dry bulb temp and %rh... is there an equation that can take the db and rh and convert it to a wet bulb?? thanks, adam.

Do you read the Basic programming language, at least a little?

If so, you should find the math formulas you want here:

http://f6.grp.yahoofs.com/v1/UM1FQql...AirPsychro.bas

It's a Basic program I wrote that'll accept DB, RH, and Altitude inputs, then give you 6 other values as a result. Wet Bulb, Enthalpy, etc.

Use it as you wish.

Even if you don't understand Basic, it's not hard to follow the code and find the math routines. If yah know basic math, you can understand the formulas.

If the link doesn't work for you, let me know and I'll try some other way of getting the text of the file to you.

I am sure there must be an"equation"for this but I sure dont know it...anyway you might want to look at this thread:
RH and TEMP conversion chart---- by nicholasa---IAQ section

I am trying to enable a piece of equipment through a dry bulb temp and %rh... is there an equation that can take the db and rh and convert it to a wet bulb?? thanks, adam.