HVAC, Module 2, Temperature and Pressure

Illustrate how British Thermal Units can be used to measure sensible heat transfer processes?

1 Pound Water x ( 212 - 32 F ) = 180 BTU 2 Pound Water x ( 212 - 32 F ) = 360 BTU 10 Pound Water X ( 212 - 32 F ) = 1800 BTU

What is the pressure at sea level?

14.7 psi or 30 inches mercury

How much BTU does it take to melt a pound of ice into 32 F water?

144 BTU's of latent heat of fusion. Each pound of ice has the ability to provide 144 BTU's of cooling capacity as it melts into water.

1 Ton = 12,000 BTU/h (Capacity or load) Explain...

144 BTU's to melt 1 pound of Ice 2000 Pounds in a Ton 2000 x 144 = 288,000 BTU's Divided by 24 Hours in a day. 12,000 BTU/h to melt 1 ton of ice in a day.

Compound Gauge

A service gauge that has both pressure and vacuum scales.

Name the 2 systems of pressure measurement used when working with mechanical refrigeration systems:

Absolute pressure Gauge pressure

Pounds Per Square Inch Absolute

Absolute pressures equal to or above atmospheric pressure. (14.7 psi)

Inches of Mercury Absolute

Absolute pressures that are equal to or below standard or atmospheric pressure (14.7 psi)

Saturation Temperature

Boiling Temperature ( Liquid to Gas )

What is used to accurately measure latent heat as well as sensible heat content?

British Thermal Units

What is used to measure the rate of heat transfer?

Btuh (British Thermal Unit's per hour)

The process of moving from left to right on the latent section of the T-H plot is called___________, from right to left is called _______________.

Evaporating, Condensing

What is the formula for latent heat of fusion?

Example: 1 Pound of Water 1 Pound x 144 BTU/Pound = 144 BTU

What is the formula for latent heat of vaporization?

Example: 1 Pound of Water 1 Pound x 970 BTU/Pound = 970 BTU

How do you calculate pressure? And give an example.

Force (pounds) divided by area. Example: 1Gl water = 8lbs. 1 foot cube of water = 8 gallons or 64 pounds. 64 divided by area of bottom of cube (12 in. x 12 in. = 144 square in.) 64 divided by 144 = 0.44 psi

Pressure

Force per unit of area. Pounds per square inch (PSI) is one way to express pressure.

Pounds per Square Inch Gauge

Gauge pressures equal or above atmospheric pressure.

inches mercury, vacuum

Gauge pressures that are less than, or equal to, atmospheric pressure.

What does British Thermal Units measure?

Heat Content Heat Transfer Heating and Cooling Capacity Heating and Cooling Load Heat Content of Refrigerant (Enthalpy)

Superheating

Heat above its temperature of saturation.

Perfect Vacuum

In gauge pressure terms, a perfect vacuum is 30 inches mercury vacuum, which equals no atmospheric pressure. The pressure cannot be any lower, therefore, it is called a perfect vacuum.

Absolute Pressure

Pressure measurements which are compared to absolutely no pressure at all - not even atmospheric pressure.

What is the formula for calculating heat quantity?

Q = Sp x W ( Delta T ) Example: Heat 1 Pound of Water from 32 to 212 F Q = 1.00 x 1.0 x ( 212 - 32 F ) Q is the sensible heat transfer quantity which we are trying to find. 1.00 is the specific heat of water. 1.0 is the weight of water. 212 - 32 is the change in temperature. ( Delta T ) Answer: 180 BTU

What is the amount of BTU's needed to turn 1 pound of water at 32 F to complete steam. Hint: Sensible heat + Latent Heat = Total Heat Content

Sensible Heat ( 32 - 212 = 180 BTU ) Latent Heat ( 212 to complete steam = 970 BTU ) Total Heat = 1150 BTU

Latent Heat of Vaporization

The amount of heat needed to change the state of a substance from a liquid to a gas.

Latent Heat of Fusion

The heat required to change the state of a substance from a solid to a liquid.

Gauge Pressure

The pressure measured on a gauge, expressed as PSIG or In. Hg. Vac. Pressure measurements which are compared to atmospheric pressure.

Evacuation

The process of removing air, moisture, and other gases from the inside of a refrigeration system.

Enthalpy

The total heat ( in BTU ) which a pound of any substance contains.

Enthalpy

Total heat content expressed in BTU per pound of refrigerant.

T or F - The bigger the job, the greater the number of pounds of refrigerant that must be circulated by the refrigeration system to move the needed heat to or from the air.

True

True or False: Fluids (liquid and gas) exert pressure in all directions, unlike solids that exert pressure straight down.

True

Ton

Unit of measurement that is used to express the approximate load for a space, the capacity of a system, or the capacity of a piece of refrigeration equipment.

Condensing

Vapor to a liquid state

When using gauges for service, install, check out, or troubleshooting, how do you determine absolute pressure?

gauge pressure + atmospheric pressure = absolute pressure

barometric pressure

same as atmospheric pressure. The absolute pressure read on a barometer in inches of mercury.

atmospheric pressure

the pressure caused by the weight of the atmosphere

A system using R-22 as a refrigerant has a PSIG of 63. What is its absolute pressure(PSIA)?

77.7 PSIA (14.7 atmospheric pressure)

Saturated Liquid

A liquid that is about to vaporize

Why can we say that even though refrigeration pressure can be used to find saturation temperature, it does not guarantee that the refrigerant is at saturated conditions?

At any single pressure, the refrigerant can exist as a sub cooled liquid, a saturated liquid, a saturated liquid-vapor mixture, a saturated vapor, or a superheated gas. If the liquid and gas states of the refrigerant are both present in one place, the refrigerant is at its saturation temperature. If liquid is present, it may be at the saturation temperature (saturated liquid) or it may be below the saturated temperature (sub cooled liquid). A temperature reading will be needed in addition to the pressure reading to determine its condition.

British Thermal Unit

The amount of heat required to raise the temperature of 1 pound of water 1 degree. A quantity of heat. It is used to express a machines heating capacity.

Specific Heat

The amount of heat, measured in BTU's, required to raise one pound of a substance 1 degree fahrenheit.

What does temperature measure?

The average speed of motion(velocity) of the molecules that make up a substance. It measures sensible heat.