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HVACR416 - Design
Heat Loss / Heat GainPart 1
Why?
• The primary function of Air Conditioning is to maintain conditions that are…
o Conductive to human comfort
o Required by a product or process within a space.
How?
• To perform this function the equipment installed must be of proper capacity and controlled throughout the year.
• Proper capacity is determined by actual PEAK load.
• Partial load conditions are handled by other controls during the year.
Loads
• It is impossible to measure the actual loads in a peak condition.
• Loads must be estimated based on heat gains and heat losses.
• The heat gain and heat loss will not ever equal the exact equipment size as there are other considerations.
System Design
• Proper system design takes into account:
o Building Heat Load
o Building Requirements
o People Requirements and Comfort
o Air Flow
o Humidity
o Energy Costs
A few items you need to know
• The amount of heat instantaneously coming into a space.
• A heat loss is the amount of heat instantaneously leaving a space.
ASHRAE
• American Society of Heating, Refrigeration and Air Conditioning Engineers.
• ASHRAE is the body that sets the standards for all heating, ventilation and air conditioning formulas and calculations.
BTU
• The BTU is the British Thermal Unit.
• 1 BTU = the amount of heat it takes to raise 1 pound of water one degree.
• All heating and air conditioning calculations are based on the BTU.
• 12,000 BTU’s is equal to 1 Ton.
CFM
• Cubic Feet Per Minute
• The CFM is the measurement used for air flow.
• 400 CFM of air is the standard for 1 Ton of Air Conditioning.
Square Foot
• Used to measure floor and wall space.
• This is Length x Width
• If a space is 12’ long and 12’ wide it is 144 sq. ft.
• A space 12 inches by 12 inches = 1 sq. ft.
Cubic Foot
• A cubic foot is a measurement of length, width and height.
• The cubic foot is used for a measurement of VOLUME.
• For example a room that has a ceiling height of 10ft and a length of 10ft and a width of 10ft has a volume of 1000 cu. ft. (or ft3)
Infiltration
• Heat that moves into and out of a structure through doors, cracks, and holes in a building.
• Infiltration is uncontrolled air/heat movement.
• Infiltration gets worse the larger the temperature variance from inside to outside.
Ventilation
• A planned and controlled movement of air and heat into or out of the building envelope.
• Ventilation occurs in rest-rooms, through air exchangers, or over grills in kitchens.
• Some ventilation is mechanical, some is natural.
Natural Ventilation
• Remember Hot Air Rises
• Cool Air Falls
• Heat moves from hot to cold.
• Air moves from high pressure to low pressure environments.
• All of these points are used in natural ventilation.
Sensible Heat & Latent Heat• Sensible Heat is…..• The heat that is measurable.
• Latent Heat is……• Heat that is not measurable and causes a change
in state.
o For example water to steamo Ice to watero Water to iceo Vapor to liquido Liquid to vapor
Types of Heat
• Conduction:
o Heat transfer from one molecule to another within a substance or from one substance to another.
• Convection:o Heat transfer from one place to another using a fluid.
• Radiation:o Heat transfer via rays, or infra-red light. The sun is an
example of Radiation.
Load Estimating
• The first step in load estimating is to organize the sources of heat as internal or external loads.
• This is done through a building survey.
External Loads
• External loads are loads that have conducted heat, solar heat and outside air load from ventilation or infiltration.
Internal Loads
• Internal heat may come from people, lights, electric motors, office machines, computers, appliances, kitchen equipment, and processes.
• Internal heat may also come from the equipment you are using to cool the space.
How external heat affects a space?
• The heat that flows through a wall or other structure depends on the temperature difference on the two sides.
• The larger the temperature difference the greater the heat flow.
• The lower the temperature difference the lower the heat flow.
Heat Flow
• The amount of heat flow also depends on the area, and the type of construction.
• The type of construction is known as the “U Factor”.
• The total heat flow is known as Q and is a measure of BTU/hr or BTU’s per hour.
Heat Flow
INSIDE – 70 degreesOUTSIDE – 95 degrees
Wall 12ft long and 10ft high
Tc = Temperature CoolerArea
Tw = Temperature WarmerArea
Q = BTU/H of Heat Transfer
U = Heat Transfer Factor of the Wall
Heat Transfer
• The formula for heat transfer is:
o Q = U x Area x (Temp W – Temp C)
• In English this translates to total conducted heat (Q) is found by multiplying the U factor by the Area by the temperature difference across the wall.
Heat Transfer Example
• For example – a 30 ft long by 10 ft high partition wall has a temperature of 90 degrees on the unconditioned (outside) side and 80 degrees on the conditioned (inside) side.
• From a set of “U” factor tables, the value of a typical 8 inch masonry partition is .40 BTU / sq. ft. / degree temp difference
Heat Transfer Example
• The wall has an area of 300 sq. ft.
• The temperature difference is 10 degrees.
• The conducted heat is calculated out to .40 x 300 x 10 which equals 1200 btu/hr.
Heat Transfer
• In addition to heat transfer through walls and partitions heat can enter a building through:
o The roof
o The walls
o The windows
• Glass is a great conductor of heat.
Heat Transfer Example
• If the same area of wall in the last example:
o 30’ x 10’ = 300 sq. ft. Was single paned glass with a U factor of 1.13 then:
Q = 1.13 x 300 x 10 or 3390 BTU/hr.
Heat Transfer
• One square foot of ordinary window passes as much heat as about 4.5 sq. ft of residential wall, or 4 sq. ft. of residential ceiling, or 3.5 sq. ft. of commercial wall, or 3 sq. ft. of commercial ceiling.