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
GO BACK
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