The equations are designed for quick and easy application in the field. They are intended for the technician who needs to determine if a system or a component is performing in the desired range of operation. Some equations are approximations that provide a practical degree of accuracy. Some norms are also provided that indicate what can be expected from a typical installation.
Topics covered include:
ESTIMATING INPUT OF A GAS-FIRED BOILERTo estimate the input in Btuh of a gas-fired boiler:
CFH = (cu. ft./rev x 3,600)/Seconds per revolution
Btuh = CFH x 1,000 Btu/cu. ft.
MBH = (cu. ft./rev x 3.6 x 1,000 Btu/cu. ft.)/Seconds per revolution
ESTIMATING INPUT OF A NO. 2 OIL-FIRED BOILERTo estimate the input of a No. 2 oil-fired boiler:
gph = gal. x 3,600/Seconds timed
Example: If 2 gallons requires 30 seconds:
gph = 2 gal. x 3,600/30
gph = 240
Btuh = gph x 140,000 Btu/gal.
ESTIMATING OUTPUT OF A GAS-FIRED OR OIL-FIRED BOILERTo estimate the output in Btuh of a gas-fired or oil-fired boiler, assume an efficiency of 80 percent, or determine efficiency from flue gas analysis. Estimate the boiler output with this equation:
Output (Btuh) = Input (Btuh) x Efficiency
CALCULATING HEAT EXCHANGE AT ANY POINT IN HYDRONIC SYSTEMDetermine heat exchange in Btuh at any point in a hydronic system with this equation:
Btuh = gpm x âˆ†T x 500
The âˆ†T is the difference between the entering and leaving water temperature.
CALCULATING TONS OF REFRIGERATION FOR CHILLER OR COOLING COILTo calculate the tons of refrigeration used by a chiller or cooling coil, first calculate the Btuh:
Btuh = gpm x âˆ†T x 500
Since one ton of refrigeration equals 12,000 Btuh, use this equation to calculate the tons of refrigeration used:
Tons of refrigeration = Btuh/12,000
COUNTERFLOW PIPING FOR COOLING COILFluid (chilled water or refrigerant) must enter the cooling coil on the side of the coil that is downstream of the airflow and leave on the upstream side. This pattern is called counterflow. If a coil is piped in the other direction (parallel pattern), the desired leaving air temperature will not be obtained.
TYPICAL AIR PRESSURE DROP ACROSS A COOLING COILTypical air pressure drop across a clean dry coil is 0.5 inches to 0.7 inches wg.
TYPICAL âˆ†T IN COILS AT MAXIMUM LOADTypical chilled water âˆ†T in cooling coils:
Typical hot water âˆ†T in heating coils:
ESTIMATING SENSIBLE HEAT FLOW THROUGH HEATING OR COOLING COILFor standard air, estimate the sensible heat flow through a coil with this equation:
Btuh = 1.1 x cfm x âˆ†T
Note: 1.08 is more exact for this equation, but 1.1 is often used for estimating.
The âˆ†T is the difference between the entering water temperature and the leaving water temperature.
COEFFICIENT OF PERFORMANCE (COP)COP (coefficient of performance) is a term used to indicate the efficiency of refrigeration equipment. The higher the COP, the more efficient the equipment is. Effective cooling depends on the temperature as well as the moisture content of air.
COP = Effective cooling out/Work in
Excerpted and reprinted from HVAC Technician's Handbook by Leo A. Meyer, one of the books in the Indoor Environment Technician's Library series published by LAMA Books. For over 30 years, Meyer has been writing and publishing training materials for the HVAC industry. His books cover a wide range of topics, including heating and cooling, indoor air quality, sheet metal work, electricity basics, safety, and others. For more information, visit www.lamabooks.com.
Publication date: 02/06/2006