Chapter 9 HVAC for Smaller buildings Flashcards
Thermostat :
were simple on off devices; when they dropped below a set point, the heat was turned on
Refrigerant:
during which processes it gives off and takes on heat, respectively.
compressor (rotary, scroll, screw):The piston-type compressor in Fig. 9.1 can instead be one of several other types: rotary, scroll, or screw compressors, each with characteristics suitable to particular applications.
CFC (chlorofluorcarbon)
chlorofluorocarbon (CFC) gases. The threats are stratospheric ozone depletion and global warming.
HCFC (hydrochlorofluorcarbon)
hydrochlorofluorocarbon (HCFC) refrigerants—still a threat to our atmosphere, but better than CFC. HCFC still contains chlorine, a major influence on ozone depletion. Thus, HCFCs themselves are due to be phased out in he first decades after 2000
lithium bromide:
This process is illustrated in Fig. 9.2. No CFCs or
HCFCs are used here; the process uses distilled
water as the refrigerant and lithium bromide (salt
solution) as the absorber.
Fans
Before the advent of mechanical air conditioning, cooling was commonly achieved with simple air motion provided by fans.
unit air conditioner
Perched in windows in full view of passersby, these window-box air conditioners
noisily remind us that many of our buildings still are not centrally mechanically cooled.
absorptance (solar)
Roof color is the first consideration; white or near-white roofs are the first step toward energy savings through control of sol-air temperature.
albedo
overall solar resistance
emissivity
the higher the emissivity, the faster a roof surface reradiates its heat to the sky.
solar reflectance index (SRI)
was developed to allow quick comparisons between roofing products. The SRI scale ranges from 0 (approximately the combination of 5% albedo and 90% emittance; roughly equal to black asphalt shingles) to 100 at 80% albedo and 90% emittance
evaporative cooler
These are also affectionately termed swamp coolers and desert coolers and are familiar devices in hot, arid climates. They require a small amount of electricity to run a fan and some water to increase the RH of the air they supply to the building.
fireplace
may be lovely to look at, but the amount of air exhausted up the chimney can quickly cause more heat losses than heat gained from the fire. The colder the outside air, the greater the net heat loss.
wood stove
more of the fuel’s heat was captured for the room rather than wasted to the outdoors.
Wood stoves are frequently used as the sole mechanical heat source for an entire building, such as a residence or a small commercial building that is passively solar heated.
pellet stove
The form and content of this fuel produce a highly efficient burn with less pollution emitted. The fuel is cleaner and takes less storage space than cordwood; an electric auger automatically feeds fuel into the burnplace to maintain a fire.
flue
leading from a wood stove carries very hot gases that are a potential source of heat (and pollution). The flue can be exposed to a space, making its radiant heat available, or simple heat exchangers can be constructed
catalytic converter
these devices impose limits on the fuel: plastic, colored newsprint, metallic substances, and sulfur are ruinous to combustors, which means that the stove must be used as a wood burner, not a trash incinerator.
masonary chimeny
overcome many of the metal wood stoves’ disadvantages. Their footprint is rather small compared to their height; typically, they are used to heat the entire building (such as a residence). An inner vertical firebox supports a hot, clean burn, resulting in efficient combustion; combustion gases then flow downward in outer chambers, transferring heat to exterior masonry surfaces.
contraflow
Finnish masonry heaters
combustion air
This supply duct should be arranged to remain open at all times. Combustion air should not be drawn from the general building space. It is a waste of energy, and contemporary “tight” construction inhibits such airflow.
heating capacity
Heating capacity is the rate of useful heat output with the boiler operating under steady-state conditions, often expressed in MBh (1000 Btu/h). This “useful heat” assumes that the boiler is within the heated envelope of the building; thus, the heat that escapes from the boiler walls is available to help heat the building.
furnace
manage to remove so much heat from the exhaust gases that smaller flues at much lower temperatures result.
Baseboard
heating device; also fintube
convector
Heating device
series loop system
usually run at the building’s perimeter.
two-pipe reverse return
provides the same supply water temperature to each baseboard or radiator, because it is not cooled either by passing through a previous baseboard or accepting the cooler return water. Equal friction, resulting in equal flow, is achieved
EXAMPLE 9.1 What length of baseboard convector (Table 9.3) is necessary along a 20-ft [6-m]-long living room wall? From heat loss calculations, the living room requires 9000 Btu/h (2635 W). The average water temperature is 180ºF, and water flow is about 1 gpm (0.6 L/s).
SOLUTION
With 180ºF supply water and 1 gpm flow rate, Table 9.3 shows that 580 Btu/h will be delivered for each lineal foot of baseboard convector; 9000 Btu/h ÷ 580 Btu/h ft = 15.5 ft. Choose a baseboard combination of 16-ft overall length, consisting of two 8-ft (finished length) pieces. The total active finned length will be about 15.5 ft.
zone
Each can be heated to different temperatures as called for by thermostats in each separate apartment.
Duct (Ductwork)
Ducts are constructed of sheet metal or glass fiber
and are either round or rectangular. Ductwork will conduct noise unless these suggestions are followed:
- Do not place the blower too close to a return grille.
- Select quiet motors and cushioned mountings.
- Do not permit connection or contact of conduits or
- water piping with the blower housing.
- Use a flexible connection between furnace bonnet and ductwork.
- Ducts also can be lined with sound-absorbing material to further discourage noise transfer, but beware of materials that encourage mold and mildew growth.
Duct sizes may be selected on the basis of
permissible air velocity in the duct (Table 9.4).
Damper
are necessary to balance the system and adjust it to the desires of the occupants
supply diffusers/registers and return grilles
allow a well-planned stream of conditioned air to thoroughly permeate all work areas.
Direct Expansion (DX Coil)
the process is usually known as direct expansion. The cooling coils therefore are often
referred to as DX coils. A popular arrangement in which the airflow airflow through the furnace and coils is horizontal.
In a single-package (also called unitary) system
only one piece of equipment is involved. A single-package air-air heat pump moves heat between an outdoor air stream and an indoor air stream; although kept separate, both streams pass through a single outdoor unit.
Split System
air heat pump moves heat via a refrigerant loop between the outdoor unit (which also contains the compressor), through which outdoor air passes, and the indoor unit
geothermal
Ground-air heat pumps, also called geothermal heat pumps or geoexchange systems, are found in several configurations throughout North America. They often provide domestic hot water in addition to heating and cooling. An environmentally safe refrigerant is circulated through a loop installed underground (or in a pond or lake), taking heat from the soil in winter and dischargingheat to the soil in summer.
ton
(12,000 Btuh of cooling)
SRI
(solar reflectance index; dimensionless)
feet of head
(feet of water column)
AFUE
(annual fuel utilization efficiency; dimensionless)
COP
(coefficient of performance; dimensionless)
EER
(energy efficiency ratio; Btuh/Watt)
IPLV
(integrated part load value; dimensionless)
SEER
(seasonal energy efficiency ratio; Btuh/Watt)
HSPF
(heating seasonal performance factor; Btuh/Watt
SHF
(sensible heat factor; dimensionless)
cfm
(air flow rate; cubic feet per minute)
