04-11 Ohms law and HVAC

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Ohm’s Law & HVAC
Ohm’s Law is used by HVAC technicians, electricians, electronic technicians, engineers, and others who
deal with electrical circuits everyday. It is a necessary fundamental that anyone dealing with electricity
or electronics must learn. Ohm’s Law is used in many different applications to troubleshoot electrical
problems, for sizing wire and designing circuits, and for designing printed circuit boards (PCB’s). Ohm’s
Law is a basic fundamental formula that helps electrical specialists solve problems and is necessary to
master if you are going to be a master technician in HVAC.
Named after physicist Georg Ohm, Ohm’s Law came about by experimentation. Ohm was fascinated
by an invention by Italian scientist Alessandro Volta, the electrochemical cell. Ohm invented his own
instruments to experiment with the new invention which he used to measure volts and current through
different lengths of wire. His experimentation led to the discovery of resistance
in electrical circuits. Thus, the measurement of resistance is measured in Ohms P = Power
using an Ohm meter or by calculating it using Ohm’s Law. The ohm is defined
as the resistance value through which one volt will maintain a current of one I = Current
ampere. Resistance is measured in ohms (Ω).
V = Voltage
V2
I2R R
V
R
VI
P
V
P/R
P
I
LT
S
IR
P
I2
V2
P
S
V/I
P1 / R
The Ohm’s Law wheel defines the
relationships between (P) power, (E)
voltage, (I) current, and (R) resistance.
AM
P
M
W
A
S
T
T
VO
Similarly, Watt’s Law describes a
relationship between voltage,
current and power (Watts) in an
electric circuit. Recognized by
English physicist James Prescott
Joule, understanding Watt’s Law
is useful for calculating circuit
characteristics. Just as Ohm’s
Law defines the ohm, Watt’s Law
defines the watt which is the
amount of power consumed by
a device which, when supplied
with 1 volt of difference across its
terminals will produce 1 ampere of
current.
R = Resistance
ES
ER
To use Ohm’s Law, one only needs to know two of
the variables to be able to calculate the third.
Resistance is equal to the voltage divided by
the current (R=V÷I). Current is equal to the
voltage divided by the resistance (I=V÷R)
and voltage is equal to the current
multiplied by the resistance (V=IxR).
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Basic HVAC Formulas
Derivations of Air Constants
COOLING –
HEATING
HS = CFMT x 1.08 x (T1 - T2)
HL = CFMT x .68 x (W1 - W2)
HT = CFMT x 4.5 x (h1 - h2)
HS = CFMT x 1.08 x (T2 - T1)
HL = CFMT x .68 x (W2 - W1)
HT = CFMT x 4.5 x (h2 - h1)
1
T1 = t1 + CFMO x (t2 - t1)
CFMT
T1 = t1 - CFMO x (t1 - t2)
CFMT
1 If duct heat gain is a factor,
add to T1 : Duct Heat Gain (Btuh)
CFMT x 1.08
T2 = T1 -
HS
CFMT x 1.08
h2 = h1 -
HT
CFMT X 4.5
1
HUMIDIFYING
2
subtract : Duct Heat Loss (Btuh)
from T1
CFMT x 1.08
CFMT =
or 60 x .075
where 13.33 is the specific volume of std.
air (cu.ft./lb.) and .075 = density (lbs./cu.ft.)
or .24 x 4.5
.24 Btu = specific heat of std. air (Btu/lb./°F)
3 Sensible load of outside
air not included.
HT
CFMT X 4.5
4.5 = 60 Min./hr.
13.33
1.08 = .24 x 60
13.33
HS
CFMT x 1.08
HS
1.08 x (T2 - T1 )
h2 = h1 +
Air constants below apply to Std. Dry Air at 70°F and
14.7 P.S.I.A (29.92 in. mercury column). They may be
used in most cooling calculations unless extremely
precise results are desired.
4.5 (to convert CFM to lbs. / hr.)
2 If duct heat loss is a factor,
T2 = T1 +
CFMT = HS (total)
1.08 x (T1 - T2 )
OR
CFMT = HS (internal) 3
1.08 x (t1 - T2 )
and
or
2
60
1060
x
7000
13.33
.68 =
or 4.5 x
1060
7000
Where:
1060 = Avg. Latent Heat of water vapor (Btu./lb.)
7000 = Grains per lb.
Leaving Air W.B. Temperature: Refer to Enthalpy Table and
read W.B. temperature corresponding to enthalpy of leaving air (h2).
1
For Cooling see
For Heating see
2
FOLLOWING FORMULA APPLY FOR EITHER COOLING
OR HEATING and/or HUMIDIFYING
CFMT =
NT V
60 min/hr
NT =
CFMT (60 min/hr)
V
CFMO =
NOV
60 min/hr
NO =
CFMO (60 min/hr)
V
Basic Fan Laws
a = NEW
b = OLD
CFMa = CFMb x RPMa
RPMb
(
(
2
SPa = SPb x RPMa
RPMb
Friction Loss
per 100 ft.
=
RPMa = RPMb x CFMa
CFMb
(
(
3
HPa = HPb x RPMa
RPMb
System design pressure x 100
Total equiv. length of duct
LEGEND FOR FORMULAS
CFMT
CFMO
NT
NO
V
HT
HS
HL
*h1
*h2
T1
T2
t1
t2
W1
W2
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
{
{
Total airflow cu.ft./min.
Outdoor air cu.ft./min.
Total air changes /hr.
Outdoor air changes /hr.
Volume of space cubic feet
Total heat Btuh
Sensible heat Btuh
Latent heat Btuh
Enthalpy or - entering air Btu/lb.
total heat of - lvg. air Btu/lb. dry air
Temperature of entering air °F.D.B.
Temperature of leaving air °F.D.B.
Indoor design temperature °F.D.B.
Outdoor design temperature °F.D.B.
Grains of water
entering cond.
@
per lb. of dry air
leaving cond.
{
{
*See Enthalpy of air (Total Heat Content of Air)
Table for exact values.
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