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.