Tech Corner Ohm’S Law & HVAC

Tech Corner 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 To use Ohm’s Law, one only needs to know two of R = Resistance the variables to be able to calculate the third. Resistance is equal to the voltage divided by A the current (R=V÷I). Current is equal to the S M voltage divided by the resistance (I=V÷R) T P and voltage is equal to the current T 2 E multiplied by the resistance (V=IxR). A V V R E Similarly, Watt’s Law describes a W 2 R R P S relationship between voltage, I R

current and power (Watts) in an

electric circuit. Recognized by

English physicist James Prescott VI

P / R

Joule, understanding Watt’s Law

is useful for calculating circuit

characteristics. Just as Ohm’s

V / I Law defines the ohm, Watt’s Law P / R

defines the watt which is the

amount of power consumed by

2 a device which, when supplied P V with 1 volt of difference across its V terminals will produce 1 ampere of O I P P S current. IR 2 LT I M The Ohm’s Law wheel defines the S H relationships between (P) power, (E) O voltage, (I) current, and (R) resistance.

OhmsLaw_HVAC_TechTip_201608 Basic HVAC Formulas Derivations of Air Constants

and COOLING – HEATING or HUMIDIFYING 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 H = CFM x 1.08 x (T - T ) H = CFM x 1.08 x (T - T ) used in most cooling calculations unless extremely S T 1 2 S T 2 1 precise results are desired. HL = CFMT x .68 x (W1 - W2) HL = CFMT x .68 x (W2 - W1) HT = CFMT x 4.5 x (h1 - h2) HT = CFMT x 4.5 x (h2 - h1) 4.5 (to convert CFM to lbs. / hr.) CFM 1 CFM 2 T1 = t1 + O x (t2 - t1) T1 = t1 - O x (t1 - t2) CFM CFM 4.5 = 60 Min./hr. or 60 x .075 T T 13.33

2 1 If duct heat gain is a factor, If duct heat loss is a factor, where 13.33 is the specific volume of std. add to T1 : Duct Heat Gain (Btuh) subtract : Duct Heat Loss (Btuh) air (cu.ft./lb.) and .075 = density (lbs./cu.ft.) CFMT x 1.08 from T1 CFMT x 1.08 .24 x 60 H H 1.08 = or .24 x 4.5 T2 = T1 - S T2 = T1 + S 13.33 CFMT x 1.08 CFMT x 1.08 .24 Btu = specific heat of std. air (Btu/lb./°F) H H CFMT = S (total) CFMT = S 1.08 x (T - T ) 1.08 x (T - T ) 1 2 2 1 60 1060 1060 OR .68 = x or 4.5 x H 3 Sensible load of outside 13.33 7000 7000 CFMT = S (internal) 3 1.08 x (t1 - T2 ) air not included. Where: HT HT 1060 = Avg. Latent Heat of water vapor (Btu./lb.) h2 = h1 - 1 h2 = h1 + 2 CFMT X 4.5 CFMT X 4.5 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). LEGEND FOR FORMULAS

For Cooling see 1 For Heating see 2 CFMT = Total airflow cu.ft./min. FOLLOWING FORMULA APPLY FOR EITHER COOLING CFMO = Outdoor air cu.ft./min. OR HEATING and/or HUMIDIFYING NT = Total air changes /hr. NO = Outdoor air changes /hr. N V CFM (60 min/hr) CFMT = T NT = T V = Volume of space cubic feet 60 min/hr V HT = Total heat Btuh N V CFM (60 min/hr) HS = Sensible heat Btuh CFMO = O NO = O 60 min/hr V HL = Latent heat Btuh Basic Fan Laws *h1 = Enthalpy or - entering air Btu/lb. *h2 = { total heat of { - lvg. air Btu/lb. dry air a = NEW b = OLD T1 = Temperature of entering air °F.D.B. T2 = Temperature of leaving air °F.D.B. CFM = CFMb x RPMa RPM = RPM x CFMa a a b t1 = Indoor design temperature °F.D.B. RPMb CFMb t2 = Outdoor design temperature °F.D.B. W1 = Grains of water entering cond. RPM 2 RPM 3 @ SPa = SPb x a HPa = HPb x a W2 = { per lb. of dry air { leaving cond. ( RPMb ( ( RPM b ( *See Enthalpy of air (Total Heat Content of Air) Table for exact values. System design pressure x 100 Friction Loss = per 100 ft. Total equiv. length of duct AMERICAN REFRIGERATION SUPPLIES, INC. Your ONE STOP HVACR Wholesaler Serving... Arizona n California n Nevada n New Mexico n Texas n Virginia Phone (602) 243-2792 www.arsnet.com Fax (602) 243-2703