EX 2: 13- Heat Loss and Heat Gain

spaces with large fresh air requirements

(like swimming pools and woodworking shops) may have ventilation as the largest heat gain

carpet floors

------- has low thermal capacity and conductivity it heats rapidly, resulting in rapid heating of the room little heat has been stored in the floor- ambient temperature is rapidly reached

ΔT

= Ti - To (interior minus outside temp)

Estimation of Heat Gain

A crude, but simple procedure

Heating Design for a Simple Building

A warehouse does not need sophisticated equipment. Gas unit heaters are often used. First heat losses are calculated. In this example, the office has a design heat loss of 1500 Btu per hour. The remaining space has a 170,000 Btu per hour heat loss with a large portion due to infiltration at the loading Dock

Air Exchange

Affected by thermal mass of interior, with latent heat as an important factor,

qa = Ps + Pl (in Btu/h) qa = (Ps + Pl) x 3.413 Btu/W (if P is in watts)

Appliance heat gain equation

cooling load factor

CLF a factor that incorporates the effects of thermal mass

DETD

Design Equivalent temperature Difference

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Example of Warehouse

air exchange rate

Iinf in cfm (ft3/min)

SC

Shading coefficient

Heat Gain

Solar impact and thermal mass make for a more complex problem than heat loss: conduction + thermal mass + solar effects ari exchange + thermal mass of interior + latent heat

Cooling Load

The amount of heat needed to be removed to maintain the interior design temperatur

TETD

Total equivalent temperature difference table 2.14 for sunlit walls *opaque doors are considered to be a group A wall

appliance heat gain

______ that heat water or burn gas ass sensible and latent Heat usually latent heat gains are added separately from sensible heat gains.

in place of ΔT

a CLTD TETD DETD values were developed fro varying construction conditions and their performance over a time period.

unit heater

a heating unit consisting of a fan, filter, and heat source, that is not connected to a duct system.

people heat gain

add sensible and latent heat

equipment heat gain

adds sensible heat (ex. copiers)

cooling load factors

an essential consideration for detailed heat gain analysis visible, near and far infrared radiation arrive at floor the energy portion absorbed is converted to heat heat is added to the room by far infrared radiation, conduction, and convection the heat transfer continues for a time after the lights are turned off

other heat sources

anything that affects heat gain in a building or space - windows -

the most sensible gains

apply a CLF (cooling load factor) that expresses the portion of heat added to air at the hour considered

latent heat gains

are always instantaneous

ballast

are not used on incandescent and LED lights

people

are usually the greatest cooling load in auditoria and churches. High thermal mass will reduce peak sensible cooling load.

qsol= A x SHGF x SC

basic formula for estimating solar heat gain A= area (ft2) SHGF= solar heat gain factor (Btu/h/ft3) SC= Shading coefficient

radiant energy

cannot be transmitted directly to a transparent medium (such as air) - radiant energy is transferred to a radiation-absorbent medium - radiation is converted to heat - heat is transmitted through conduction and convection to air

CLTD

cooling load temperature difference

small roof skylight

could be the greatest heat gain in a room in summer, but have a net heat loss in winter.

For heat loss

effects of thermal mass are usually ignored

qe= P x 3.431 Btu/W

equipment heat gain equation

interior rooms

example of spaces that never have heat losses, but always have heat gains... even in the middle of winter

basic formulas for heat loss

for conduction: qc = u × A × ΔT for air exchange: qi = Icfm × 1.08 × ΔT where q is in Btu/h

D column Table 2.12

for dark roof

CLTD

for sunlit walls

sensible heat and latent heat gain

from people and appliance

north facing portions of a building

generally see the smallest impact of solar cooling load

air exchange heat gain

has two components: 1. sensible heat (as considered in heat loss calculations) 2. latent heat which accounts for the latent heat of vaporization of the water in the air

Buildings with large WEST and EAST window areas

have solar gains as the largest load

west facing parts of the building

have the greatest cooling problems due to low solar angles, high time heat and accumulation of internal heat from building use during the day

summer

heat flows inside

winter

heat flows outside

internal thermal mass

heat gain in an office, school or other building used for part of a day

cooling load factor

heat gain in room is proportional to change in Temperature when change in temperature rises slowly, heat gain rises slowly (vice-versa)

smaller air conditioning equipment

heat is stored in structure to be removed later, during off peak power periods cost savings allows purchase of other energy saving equipment

cooling load factor

if lights are on 24 hours a day, the floor temperature reaches a steady state, with no diurnal variation of temperature

conduction through glazing

is also affected by these factors, occurs at different times of the day from different assemblies based on conductance based on capacitance, based on varying solar absorption based on varying outside air temperature **but direct solar gain is treated separately

ratio of latent to sensible gains

is important in the design of cooling equipment

lighting heat gain

is is assumes that all the light and heat energy will be absorbed by room surfaces and then heat the air

smaller HVAC equipment

is less expensive both initially and throughout the lifecycle

HVAC equipment most closely matches demand

is more efficient (HVAC)

HVAC running continuously

is more efficient than those that run intermittently at high levels.

heat transfer

is proportional to temperature

temperature difference

is replaced with different terms in different methods of analysis to account for the complications of heat gain:

electric lighting

is the single greatest source of heat in office buildings

Effects of low mass , little insulation

large amplitude, little time lag

ql = P x 3.413 Btu/W x CLF

lighting cooling load equation where P= total lighting power in watts, including ballast power CLF= cooling load factor *** CLF for building with internally exposed thermal mass ---apply CLF to the sensible portion only

qL= P X 3.413 Btu/h/W

lighting heat gain equation P= total lighting pawer in watts, including ballast power

sensible heat gain from

lighting, equipment + soar gain through fenestration + thermal mass of interior

latent heat gains

never use a cooling load factor (CLF) they are always instantaneous

maximum conduction

occurs at different times of the day from different assemblies based on conductance based on capacitance, based on varying solar absorption based on varying outside air temperature

maximum heat loss

occurs very late at night after the structure has dissipated store d heat, people are asleep or absent, and lights, equipment and appliances are off. Heat loss is from heat conduction through assemblies, infiltration/exfiltration and ventilation.

newer cooling systems

offer a graduated response to cooling demand most of the demand is still during peak electrical power demand

oversize cooling equipment

operates less efficiently and may fail to remove humidity

Effects of mass and insulation

outdoor temperature varies greatly throughout the day: temperatures have a large amplitude.

qp = Ps + Pl

people heat gain equation, where P is Btu/h

L column Table 2.12

permanently light roof

practical application

quantifying the effects of solar energy and thermal mass has been performed by experimentation and application of empirical knowledge different combinations of insulation and thermal mass have led to the development of tables of various constructions, and how they respond to variations of solar heat gain throughout the day

room air temperature

rate-of-change is therefore inversely proportional to the thermal capacity of materials in the space this effect is more pronounced with radiant-energy sources

Thermal mass effect

series heat flow: R= R1 + R2 + R3 u=1/R

Effects of high mass, little insulation

small amplitude, large time lag

Effects of low mass, well insulated

small amplitude, little time lag

Solar Effect

solar orientation and shading is different for different parts of a building, and varies throughout the day

heating load

the amount of heat needed to be added to maintain the interior design temperature

throw

the distance air travels before its velocity is reduces to an ineffective speed.

for heat gain

the effects of thermal mass are incorporated into calculations with differential equation analysis by computers or empirical data derived by experimentation

heat gain concept

the greater the internally exposed thermal mass, the slower the room air will heat

heat gain

the sum of the accumulation of heat from all sources such as solar, lighting, people, etc. The opposite of heat loss.

sensible heat

thermal mass can store only -----

thermal conductance and area of assemblies:

u x A

qc=

u x A x TETD where ____________ = heat gain by conduction at a particular time of day

offices and classrooms

usually have lighting as their greatest heat gain. They are often over illuminated

windows

when exterior temperature is greater than interior temperature, heat flows from outside to the inside by conduction solar energy that arrives at the _____ may be partially absorbed or reflected

u values

will be somewhat different due to air film and air space R value difference

material with high thermal capacity

will change in temperature slowly

system with an economizer

will help because it uses outside air for cooling (the outside air will need to be preheated to 55 degrees F before it can be used)

conduction

with solar and thermal mass influences infiltration

and temperature difference

ΔT °F