4Th Year System Servicing Exam Holly

12,000

Dry air at 70°F, a barometric pressure of of 29.92" Hg (mercury) at sea level. Weighs 0.075lbs per cubic foot.

Is the relative amount of moisture in a specific sample of air.

Has a wet cloth wick around the thermometer bulb. When air passes over the wet wick the water in the wick evaporates and the thermometer bulb cools off. The amount of water that can be evaporated depends upon the relative humidity of the air.

With an ordinary thermometer

By measuring the dry bulb and wet bulb temperatures and using them to find relative humidity on a chart

Is used to measure wet and dry bulb temperatures

Is the condition at which a specific sample of air is at 100%RH. This is also called The Saturation Point

The air can hold no more moisture. If the air cools the moisture will condense out of the air.

Study of the properties of air

21%

A pressure greater than the atmospheric pressure. (Usually the outside air) positive pressure exhausts air to the outside

Less than atmospheric pressure. Pulls outside air into a space.

Attic vents, combustion air vents, fresh air vents, exhaust air vent. Warm air rises and they naturally ventilate.

Whatever amount of air is exhausted needs to be brought back in, a large hood fan is an example where make up air is needed to balance the amount of air exhausted by the hood

Carries heat currents of warm air

Heat carried through some material

Delta T ∆T

Is a measure of how well a material conducts heat

Measure of how well a material resists the flow of heat, insulation has an R value

Combustion chamber

A heat exchanger or coil

Hydronic heating system

Freeze at 32° boils at 212°

Freezes at 0° boils at 100°

The amount of heat required to raise one pound of any substance 1°F

The heat released by a burning fuel

Not a form of energy, cold is the absence of heat, an object cannot be made colder by adding cold to it, heat can just be removed.

Heat you can feel. Temperature measured by a dry bulb thermometer

The heat released or absorbed by a change of state

A liquid chosen for refrigeration because it changes state at a relatively low temp so that latent heat can be released or absorbed easily. Like water.

Gives off heat as it changes from vapor to a liquid

Absorbs heat as it changes from a liquid to a vapor

From the compressor, through the condenser, and to the expansion valve

The the expansion valve through the evaporator, to the compressor

The refrigerant is delivered to the compressor as a low pressure vapor. When the compressor puts it under pressure the vapor heats up. The hot, high pressure vapor is pumped to the condenser.

In the condenser the hot high pressure vapor refrigerant condenses to a liquid because it is under pressure. It is collected in the liquid receiver storage tank. This change of state releases the heat from the refrigerant. Heat is given off to the air as the high pressure liquid is pumped through the condenser coil.

The metering device allows the refrigerant to change from high pressure to low pressure. The liquid on the high side stays under pressure. The pressure drops on the other side of the metering device. The device can be a thermal expansion valve or capillary tubes.

In the evaporator the lower pressure allows the liquid refrigerant to boil and change to vapor. This change of state absorbs heat into the refrigerant. The heat is absorbed from the air or liquid that surrounds the evaporator coil.

The force exerted in all directions by air on the inside surface. It is called static because it is not moving in any one direction.

Measures pressures in duct

Vp+Sp=Tp

Pressure loss due to turbulence in the airflow

The result of changes in duct size, shape, direction, or velocity that prevents air from moving in a smooth straight pattern

Axial and centrifugal

Propeller, tubeaxial, vaneaxial

Straight Blade fans, forward curved Blade fans (squirrel cage) backwards inclined fan blades.

Straight Blade fan, solid particles that might clog other types of fans are less likely to cause a problem.

Where dirt and residue will build up on the blade surfaces

Large HVAC systems where separate housings are constructed on the job. Should not be used where materials could cling to the blades. They may have straight blades or airfoil blades. Airfoil blades improve the static efficiency and reduce noise.

The HP is the rating of the motor. Bhp is the load it is actually operating under. The motor must be large enough to not only overcome the Bhp load, but also overcome the inertia of the fan and drive on startup.

Positive

Variable air volume is a single path type. Each room has a VAV unit on the duct run. It automatically varies the air volume to each room according to needs. The air volume varies but the air temperature remains the same.

Negative

Positive

That the duct system is sized to maintain the same friction loss per hundred feet through the entire system

A friction loss chart, gives duct sizes in round pipe.

Round duct equivalents chart

Measures air velocity at outlets

Measures the air volume at a duct outlet or inlet. Flowhood is placed over an outlet

Pitot tube attached to a U-Tube Manometer

Pitot tube

Opposed blade

Measures pressure drop. Measure before and after large filter banks to tell you if you need to change the filters.

The ratio of one side of the duct to the other. It is the longer dimension divided by the shorter dimension. The aspect ratio should not be more than 4:1

Blast gate

15°