HVAC ContractingTechnical

Heat Transfer

This article is a quick reference on heat transfer. It includes practical information you may need to look up.

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 the input of a gas-fired boiler

Estimating the input of a No. 2 oil-fired boiler

Estimating the output of a gas-fired or oil-fired boiler

Calculating heat exchange at any point in a hydronic system

Calculating tons of refrigeration for chiller or cooling coil

Counterflow piping for cooling coil

Typical air pressure drop across a cooling coil

Typical âˆ†T in coils at maximum load

Estimating sensible heat flow through heating or cooling coil

COP - coefficient of performance of a refrigeration cycle

ESTIMATING INPUT OF A GAS-FIRED BOILER

To estimate the input in Btuh of a gas-fired boiler:

Determine the cubic feet of gas per revolution (cu. ft./rev), indicated by the timing dial on the gas meter. This is called the meter factor.

With the boiler at high fire, use a stopwatch to measure the seconds per revolution of the timing dial.

Determine the CFH (cu. ft./hour) with this equation:

CFH = (cu. ft./rev x 3,600)/Seconds per revolution

The heat content (calorific value) of gas is approximately 1,000 Btu/cu. ft.

Estimate the boiler input in Btuh with this equation:

Btuh = CFH x 1,000 Btu/cu. ft.

The two equations can be combined and used to calculate the MBH (thousands of Btuh) with this equation:

MBH = (cu. ft./rev x 3.6 x 1,000 Btu/cu. ft.)/Seconds per revolution

ESTIMATING INPUT OF A NO. 2 OIL-FIRED BOILER

To estimate the input of a No. 2 oil-fired boiler:

With boiler at high fire, time the oil meter by using a stopwatch to measure the gallons of oil used in a given number of seconds.

Determine oil consumption in gph (gallons per hour) by using this equation (based on the fact that there are 3,600 seconds in an hour):

gph = gal. x 3,600/Seconds timed

Example: If 2 gallons requires 30 seconds:

gph = 2 gal. x 3,600/30

gph = 240

The heat content (calorific value) of No. 2 oil is approximately 140,000 Btu/gal.

Estimate the boiler input in Btuh with this equation:

Btuh = gph x 140,000 Btu/gal.

ESTIMATING OUTPUT OF A GAS-FIRED OR OIL-FIRED BOILER

To 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 SYSTEM

Determine 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 COIL

To 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 COIL

Fluid (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 COIL

Typical air pressure drop across a clean dry coil is 0.5 inches to 0.7 inches wg.

TYPICAL âˆ†T IN COILS AT MAXIMUM LOAD

Typical chilled water âˆ†T in cooling coils:

Before the mid-1970s, cooling coils were sized for a chilled water âˆ†T of approximately 8Â°F.

Recent systems have cooling coils sized at a chilled water âˆ†T of 16Â°F or more.

Typical hot water âˆ†T in heating coils:

Before the mid-1970s, heating coils were sized for a hot water âˆ†T of approximately 20Â°F.

Recent systems have heating coils sized for a âˆ†T of 40Â°F or more.

ESTIMATING SENSIBLE HEAT FLOW THROUGH HEATING OR COOLING COIL

For 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

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