Air Conditioning Theory

Calculating BTU's of Infiltration for Cooling/Heating

1.1 x CFM x Temperature Difference

Calculating CFM

Air changes per hour x Cu. FT of house x .0167

Four Factors For Human Comfort

Air temperature, air humidity, air cleanliness, air movement, most common comfort condition is 75 degrees F and 50% relative humidity

Finding Air Volume in Duct Systems

Air velocity (ft/min) x area of duct (lxw)/ 144 in^2/ft^2, answer is expressed in cfm

Packaged Air Conditioner

All of the components are built into one cabinet like a window unit, usually found on the side of a building or on top, have no piping that needs to be field installed, and they come charged from the factory

Condensate in A/C Units

An A/C unit in a climate with average humidity will collect 3 pints of condensate per hour of operation for each ton of refrigeration, most units have a drain that goes to the ground and the condensate is absorbed into the ground.

Partition and Exterior Walls

An exterior wall is exposed directly to the outside while a partition wall is exposed to an unconditioned space such as a garage.

High Efficiency Condensers

Are highly efficient because they have more surface area which reduces the compressor head pressure even in the hottest weather, they also have two speed fans one speed for mild weather and one for hot

Cooling the Compressor and Motor

Are suction gas cooled, if they are not cooled the motor will burn out, discharge gas is used in scroll compressors newer compressors are able to run at hotter temperatures, the discharge line should not exceed 225 degrees F

Propeller Fan

Are used for exhaust or condenser fans, they can move large volumes of air at low pressures

Squirrel Cage Blower

Are used in high pressurized duct systems to move air (1 in of water column or higher)

Heat Gain/Heat Loss Calculations

Are used to determine the amount of heat lost and gained by the means of the structure

Compressor Sensor

Are used to sense the temperature inside the compressor, only found in newer compressors

Formulas For Heating and Cooling TD

Cooling Design TD = Outdoor Design Temperature - Indoor Design Temperature Heating Design TD = Indoor Design Temperature - Outdoor Design Temperature

Insulating Ductwork

Ductwork should only be insulated when a 15 degree F temperature difference exists from the inside to the outside of the duct, can be insulated on the outside or inside, 2 in thickness is most common

Air Conditioning, Heating, and Refrigeration Institute (AHRI)

Establishes the ratings for which air-conditioning equipment is evaluated at, A/C units are rated for 80 degrees F with 50% relative humidity, and they rate condensers on a basis of 95 degree F air passing over them

Moisture in Window A/C Units

Evaporators operate at 35 degrees F to prevent frost buildup, the moisture that collects on them falls into a pan and drained to the condenser where it is evaporated, most commonly have rotary compressors

Ductless Systems

Have a outdoor condensing unit connected to one or more indoor fan coils units, and each room has its own unit

Evaporator Symptoms

If a slightly higher than normal superheat is normal if the outside ambient temperatures are high, if a fixed bore metering device is being used

Weight of Dry Air

Is .075 lb/ft^3 at atmospheric pressure

Acoustical Lining

Is applied to the interior surfaces of the ductwork to prevent air escaping

Ventilation Air

Is fresh air intake that is induced before the air is heated or cooled and it is brought into the air conditioner to keep air from being oxygen-starved, once every 4 hours there should be 25% of the indoor air pushed out, it is planned

Enthalpy of Air

Is the air's total heat content including latent and sensible heat, expressed in Btu per pound of air

Static Pressure

Is the pressure in a duct system that is pushing against the walls of the duct, static pressure + velocity pressure= total pressure , a manometer is used to measure it

Dew Point Temperature

Is the temperature at which moisture begins to condense out of the air, air can be dehumidified by passing it over a surface that is below the dew point temperature and moisture will collect on the cold surface, ex evaporator coil

Infiltration Air

Is the term used for random air that leaks into a struture

Velocity Pressure

Is the weight of the air moving through the duct system, it is measured with a manometer

Disconnect Switch

Is used to cut off the power to the equipment for servicing, needs to be within 25 ft of the unit and within sight

Net Wall Area

Is used to describe the area of a wall after the doors and windows are cut into it, Net wall area= gross wall area- door area- window area- area of other components.

U-Values

Is used to describe the thermal conductivity of a material, the higher the U value the easier it is for heat to pass through the material, it is the inverse of R value, ex R-value=30, U-value= 1/30

Manuel J

Is used to determine the heat gain and heat loss of a residentialstructure

Distribtuor

Is used to distribute the correct amount of refrigerant to individual circuits

Velometer

Is used to measure the air speed, and how fast it is moving past a particular point

R-Value

Is used to to rate the thermal resistance of insulation materials, the higher the R value the harder it is for heat to pass through the material

Placing Condensing Units in Split-Type Systems

Make sure the roof drainage doesn't drain on top of the unit, and that there is proper air circulation, try and place units in shady areas

Airflow in A/C units

More airflow will cause higher coil temperatures, less airflow will cause lower coil temperatures , airflow can be adjusted to achieve proper evaporator temperature for proper humidity, closer fin spacing will also take more moisture out of the air

Sizing Ductwork

Oversized ductwork will result in the slowing of the air as it flows through and as well as more expensive to fabritcate, and undersized duct will increase noise levels and excessive air velocity

Formula For Heat Lost/Gained Through a Wall

Q(Quantity of heat in btu/h)=Area of the wall x HTM (heat transfer medium)(cooling design temp or heating design temp)

Sensible Heat Formula

Qs (BTU'S of Sensible Heat) = 1.08 x cfm x TD

Total Heat Formula

Qt (Total Heat)= 4.5 x cfm x total heat difference, is used to select equipment for the correct sensible and latent heat capacities

Absolute Humidity

The amount of water vapor that is contained in a air sample, expressed in grains or pounds 7,000 grains = 1 lb, is not commonly used for field calculations

Split System Air Conditioner

The condenser is located outside, and evaporator inside and line sets are used to connect them

Wet Bulb Depression

The difference of a dry bulb temperature and a wet bulb temperature measurement., is found by subtracting the wet bulb temp from the dry bulb temp, a larger web bulb depression is caused by a higher humidity level and vis versa, a psychrometer is used to measure it

Using Gauge to Determine Duct Weight

The gauge of the sheet metal can be used to find the weight of the sheet metal by using a constant of 41.82 and multiplying it by the gauge of metal, ex: 24 gauge-.024 x 41.82= 1lb per square foot of metal

Wet Bulb Temperature

The moisture of air taken with a psychometer, the closer the wet bulb temp is to the dry bulb temp the higher the RH

Relative Humidity

The relationship between the weight of water vapor in a pound of air compared to the weight of water vapor that a pound of air can hold at 100% saturation condition, high relative humidity is good in the winter, and a low relative humidity is good in the summer

Dry Bulb Temperature

The sensible heat level of the air taken with a psychometer , the dry bulb temp is always higher than the wet bulb temp

Psychrometrics

The study of the properties of air, when you move horizontal on the chart there is a sensible heat change, and when you move vertical there is a latent heat change

Condenser Operating Conditions

They normally operate at 30 degree F temperature difference, on high efficiency equipment they operate at a 20 degree F temperature difference

Evaporator Operating Conditions

They normally operate at 40 degree F boiling temperature with a 35 degree F temperature difference, with normal comfort conditions

Controlling Humidity in A/C Units

Units dehumidify and cool the air, if the humidity is very high, the coil temperature should be lowerd remove more moisture, and if humidity is too low the coil temperature can be raised

Temperature Split

Used to describe the difference in temperature between heat exchange mediums which are the refrigerant and the air, usually 20 degrees F for evaporators

Psychrometric Chart

Used to plot and compare comfort conditions in an occupied space, during the summer the most common comfort level is 75 degrees F 50% RH

Voltage in Window A/C Units

Usually operate at 115v

Equation For Finding Weight of Air in a Room

Volume of room (lwh) x .075lb/ft^3

Cubic Ft Per Minute (CFM) Equation

Volume of the room x .25, divided by 60 (minutes), is used to find how much cubic ft of air per minute must be introduced to change 25% of the air per hour, most systems operate at 400cfm/ton, the number of windows in a room is also taken into consideration

Humidity

Water vapor suspended in the air

Condenser Symptoms

When a high discharge-line temp is discovered, a technician can prove a leak by building up the head pressure to 300 psig for R-22 and then shut the unit off, if there is a leak it will begin to whistle through the compressor

Oversized A/C Equipment

Will cause excessive starting and stopping which will lead to more wear and tear and less dehumidification, also more expensive operating and maintenance cost