大多数英文空调系统中英文翻译

2021年1月19日发(作者：ovann)

英文文献

Air Conditioning Systems

Air conditioning has rapidly grown over the past 50 years, from a luxury
to a standard system included in most residential and commercial buildings. In
1970,
36%
of
residences
in
the
U.S.
were
either
fully
air
conditioned
or
utilized
a room air conditioner for cooling (Blue, et al., 1979). By 1997, this number
had
more
than
doubled
to
77%,
and
that
year
also
marked
the
first
time
that
over
half (50.9%) of residences in the U.S. had central air conditioners (Census
Bureau, 1999). An estimated 83% of all new
homes constructed in 1998 had central air conditioners (Census Bureau, 1999).
Air conditioning has also grown rapidly in commercial buildings. From 1970 to
1995, the percentage of commercial buildings with air conditioning increased
from 54 to 73% (Jackson and Johnson, 1978, and DOE, 1998).
Air
conditioning
in
buildings
is
usually
accomplished
with
the
use
of
mechanical
or
heat-activated
equipment.
In
most
applications,
the
air
conditioner
must
provide
both
cooling
and
dehumidification
to
maintain
comfort
in the building. Air conditioning systems are also used in other applications,
such
as
automobiles,
trucks,
aircraft,
ships,
and
industrial
facilities.
However,
the description of equipment in this chapter is limited to those commonly used
in commercial and residential buildings.
Commercial buildings range from large high-rise office buildings to the
corner convenience store. Because of the range in size and types of buildings
in
the
commercial
sector,
there
is
a
wide
variety
of
equipment
applied
in
these
buildings. For larger buildings, the air conditioning equipment is part of a
total
system
design
that
includes
items
such
as
a
piping
system,
air
distribution
system, and cooling tower. Proper design of these systems requires a qualified
engineer. The residential building sector is dominated
by
single
family
homes
and
low-rise
apartments/condominiums.
The
cooling
equipment applied in these
buildings comes in standard “packages” that are
often both sized and installed by the air conditioning contractor.
The
chapter
starts
with
a
general
discussion
of
the
vapor
compression
refrigeration cycle then moves to refrigerants and their selection, followed
by packaged Chilled Water Systems
。

1.1 Vapor Compression Cycle
Even
though
there
is
a
large
range
in
sizes
and
variety
of
air
conditioning
systems used in buildings, most systems utilize the vapor compression cycle to
produce the desired cooling and dehumidification. This cycle is also used for
refrigerating
and
freezing
foods
and
for
automotive
air
conditioning.
The
first
patent
on
a
mechanically
driven
refrigeration
system
was
issued
to
Jacob
Perkins
in 1834 in London, and the first viable commercial system was produced in 1857
by James Harrison and D.E. s vapor compression, there are two less
common methods used to produce cooling in buildings: the absorption cycle and
evaporative cooling. These are described later in the chapter. With the vapor
compression
cycle,
a
working
fluid,
which
is
called
the
refrigerant,
evaporates
and condenses at suitable pressures for practical equipment designs.
The four basic components in every vapor compression refrigeration system
are
the
compressor,
condenser,
expansion
device,
and
evaporator.
The
compressor
raises
the
pressure
of
the
refrigerant
vapor
so
that
the
refrigerant
saturation
temperature
is
slightly
above
the
temperature
of
the
cooling
medium
used
in
the
condenser.
The
type
of
compressor
used
depends
on
the
application
of
the
system.
Large electric chillers typically use a centrifugal compressor while small
residential equipment uses a reciprocating or scroll compressor.
The condenser is a heat exchanger used to reject heat from the refrigerant
to a cooling medium. The refrigerant enters the condenser and usually leaves
as a subcooled liquid. Typical cooling mediums used in condensers are air and
water. Most residential-sized equipment uses air as the cooling medium in the
condenser, while many larger chillers use water. After leaving the condenser,
the liquid refrigerant expands to a lower pressure in the expansion valve.

2
The expansion valve can be a passive device, such as a capillary tube or
short tube orifice, or an active device, such as a thermal expansion valve or
electronic expansion valve. The purpose of the valve is toregulate the flow of
refrigerant to the evaporator so that the refrigerant is superheated when it
reaches the suction of the compressor.
At
the
exit
of
the
expansion
valve,
the
refrigerant
is
at
a
temperature
below
that
of
the
medium
(air
or
water)
to
be
cooled.
The
refrigerant
travels
through
a
heat
exchanger
called
the
evaporator.
It
absorbs
energy
from
the
air
or
water
circulated
through
the
evaporator.
If
air
is
circulated
through
the
evaporator,
the system is called a
direct expansion system
. If water is circulated through
the evaporator, it is called a
chiller
. In either case, the refrigerant does
not make direct contact with the air or water in the evaporator.
The
refrigerant
is
converted
from
a
low
quality,
two-phase
fluid
to
a
superheated
vapor
under
normal
operating
conditions
in
the
evaporator.
The
vapor
formed
must
be removed by the compressor at a sufficient rate to maintain the low pressure
in the evaporator and keep the cycle operating.
All mechanical cooling results in the production of heat energy that must
be rejected through the
condenser. In many instances, this heat energy is
rejected to the environment directly to the air in the condenser or indirectly
to water where it is rejected in a cooling tower. With some applications, it
is possible to utilize this waste heat energy to provide simultaneous heating
to
the
building.
Recovery
of
this
waste
heat
at
temperatures
up
to
65°C
(150°F)
can be used to reduce costs for space heating.
Capacities
of
air
conditioning
are
often
expressed
in
either
tons
or
kilowatts (kW) of cooling. The ton is a unit of measure related to the ability
of an ice plant to freeze one short ton (907 kg) of ice in 24 hr. Its value is
3.51
kW
(12,000
Btu/hr).
The
kW
of
thermal
cooling
capacity
produced
by
the
air
conditioner must not be confused with the amount of electrical power (also
expressed in kW) required to produce the cooling effect.
2.1 Refrigerants Use and Selection

3
Up until the mid-1980s, refrigerant selection was not an issue in most
building
air
conditioning
applications
because
there
were
no
regulations
on
the
use of refrigerants. Many of the refrigerants historically used for building
air
conditioning
applications
have
been
chlorofluorocarbons
(CFCs)
and
hydrochlorofluorocarbons (HCFCs). Most of these refrigerants are nontoxic and
nonflammable. However, recent U.S. federal regulations (EPA 1993a; EPA 1993b)
and
international
agreements
(UNEP,
1987)
have
placed
restrictions
on
the
production and use of CFCs and HCFCs. Hydrofluorocarbons (HFCs) are now being
used
in
some
applications
where
CFCs
and
HCFCs
were
used.
Having
an
understanding
of refrigerants can help a building owner or engineer make a more informed
decision about the
best
choice
of
refrigerants for specific
applications.
This
section discusses the different refrigerants used in or proposed for building
air conditioning applications and the regulations affecting their use.
The
American
Society
of
Heating,
Refrigerating
and
Air
Conditioning
Engineers
(ASHRAE)
has
a
standard
numbering
system,for
identifying
refrigerants
(ASHRAE,
1992).
Many
popular
CFC,
HCFC,
and
HFC
refrigerants
are
in
the
methane
and ethane series
of
refrigerants. They are
called
halocarbons, or halogenated
hydrocarbons, because of the presence of halogen elements such as fluorine or
chlorine (King, 1986).
Zeotropes
and
azeotropes
are
mixtures
of
two
or
more
different
refrigerants.
A
zeotropic
mixture
changes
saturation
temperatures
as
it
evaporates
(or
condenses) at constant
pressure.
The phenomena is called
temperature glide.
At
atmospheric pressure,
R-407C has
a
boiling (bubble) point
of
–44°C (–47°F)
and
a
condensation
(dew)
point
of
–37°C
(–35°F),
which
gives
it
a
temperature
glide of 7°C (12°F). An azeotropic
mixture behaves like a single component
refrigerant in that the saturation temperature does not change appreciably as
it evaporates or condenses at constant pressure. R-410A has a small enough
temperature
glide
(less
than
5.5°C,
10°F)
that
it
is
considered
a
near-azeotropic refrigerant mixture.
ASHRAE groups refrigerants by their toxicity and flammability (ASHRAE,

4
1994).Group
A1
is
nonflammable
and
least
toxic,
while
Group
B3
is
flammable
and
most toxic. Toxicity is based on the upper safety limit for airborne exposure
to the refrigerant. If the refrigerant is nontoxic in quantities less than 400
parts per million, it is a Class A refrigerant. If exposure to less than 400
parts per million is toxic, then the substance is given the B designation. The
numerical designations refer to the flammability of the refrigerant. The last
column of Table 4.2.1 shows the toxicity and flammability rating of common
refrigerants.
Refrigerant 22 is an HCFC, is used in many of the same applications, and
is still the refrigerant of choice in many reciprocating and screw chillers as
well as small commercial and residential packaged equipment. It operates at a
much higher pressure than either R-11 or R-12. Restrictions on the production
of
HCFCs
will
start
in
2004.
In
2010,
R-22
cannot
be
used
in
new
air
conditioning
equipment. R-22 cannot be produced after 2020 (EPA, 1993b).
R-407C
and
R-410A
are
both
mixtures
of
HFCs.
Both
are
considered
replacements
for R-22. R-407C is expected to be a drop-in replacement refrigerant for R-22.
Its
evaporating
and
condensing
pressures
for
air
conditioning
applications
are
close to those of R-22 (Table 4.2.3). However, replacement of R-22 with R-407C
should be done only after consulting with the equipment manufacturer. At a
minimum,
the
lubricant
and
expansion
device
will
need
to
be
replaced.
The
first
residential-sized
air
conditioning
equipment
using
R-410A
was
introduced
in
the
U.S.
in
1998.
Systems
using
R-410A
operate
at
approximately
50%
higher
pressure
than R-22 (Table 4.2.3); thus, R-410A cannot be used as a drop-in refrigerant
for
R-22.
R-410A
systems
utilize
compressors,
expansion
valves,
and
heat
exchangers designed specifically for use with that refrigerant.
Ammonia is widely used in industrial refrigeration applications and in
ammonia water absorption chillers. It is moderately flammable and has a class
B
toxicity
rating
but
has
had
limited
applications
in
commercial
buildings
unless
the chiller plant can be isolated from the building being cooled (Toth, 1994,
Stoecker,
1994).
As
a
refrigerant,
ammonia
has
many
desirable
qualities.
It
has

5
a
high
specific
heat
and
high
thermal
conductivity.
Its
enthalpy
of
vaporization
is typically 6 to 8 times higher than that of the commonly used halocarbons,
and it provides higher heat transfer compared to halocarbons. It can be used
in both reciprocating and centrifugal compressors.
Research is underway to investigate the use of natural refrigerants, such
as
carbon
dioxide
(R-744)
and
hydrocarbons
in
air
conditioning
and
refrigeration
systems
(Bullock,
1997,
and
Kramer,
1991).
Carbon
dioxide
operates
at
much
higher
pressures than conventional HCFCs or HFCs and requires operation above the
critical
point
in
typical
air
conditioning
applications.
Hydrocarbon
refrigerants, often thought of as too hazardous because of flammability, can
be
used
in
conventional
compressors
and
have
been
used
in
industrial
applications.
R-290, propane, has operating pressures close to R-22 and has been proposed as
a
replacement
for
R-22
(Kramer,
1991).
Currently,
there
are
no
commercial
systems
sold in the U.S. for building operations that use either carbon dioxide or
flammable refrigerants.
3.1 Chilled Water Systems
Chilled water systems were used in less than 4% of commercial buildings in
the U.S. in 1995. However, because chillers are usually installed in larger
buildings,
chillers
cooled
over
28%
of
the
U.S.
commercial
building
floor
space
that same year (DOE, 1998). Five types of chillers are commonly applied to
commercial
buildings:
reciprocating,
screw,
scroll,
centrifugal,
and
absorption.
The first four utilize the vapor compression cycle to produce chilled water.
They
differ
primarily
in
the
type
of
compressor
used.
Absorption
chillers
utilize
thermal energy (typically steam or combustion source) in an absorption cycle
with
either
an
ammonia-water
or
water-lithium
bromide
solution
to
produce
chilled water.
3.2 Overall System
An estimated
86% of
chillers
are applied in multiple
chiller arrangements
like
that
shown
in
the
figure
(Bitondo
and
Tozzi,
1999).
In
chilled
water
systems,

6
return water from the building is circulated through each chiller evaporator
where it is cooled to an acceptable temperature (typically 4 to 7°C)
(39 to
45°F). The chilled water is then distributed to water
-to-air heat exchangers
spread throughout the facility. In these heat exchangers, air is cooled and
dehumidified
by
the
cold
water.
During
the
process,
the
chilled
water
increases
in temperature and must be returned to the chiller(s).
The chillers are water-cooled chillers. Water is circulated through the
condenser of each chiller where it absorbs heat energy rejected from the high
pressure
refrigerant.
The
water
is
then
pumped
to
a
cooling
tower
where
the
water
is
cooled
through
an
evaporation
process.
Cooling
towers
are
described
in
a
later
section.
Chillers
can
also
be
air
cooled.
In
this
configuration,
the
condenserwould be a refrigerant-to-air heat exchanger with air absorbing the
heat energy rejected by the high pressure refrigerant.
Chillers
nominally
range
in
capacities
from
30
to
18,000
kW
(8
to
5100
ton).
Most chillers sold in the U.S. are electric and utilize vapor compression
refrigeration
to
produce
chilled
water.
Compressors
for
these
systems
are
either
reciprocating,
screw,
scroll,
or
centrifugal
in
design.
A
small
number
of
centrifugal chillers are sold that use either an internal combustion engine or
steam drive instead of an electric motor to drive the compressor.
The
type
of
chiller
used
in
a
building
depends
on
the
application.
For
large
office buildings or in chiller plants serving multiple buildings, centrifugal
compressors are often used. In applications under 1000 kW (280 tons) cooling
capacities,
reciprocating
or
screw
chillers
may
be
more
appropriate.
In
smaller
applications, below 100 kW (30 tons), reciprocating or scroll chillers are
typically used.
3.3 Vapor Compression Chillers
The
nominal
capacity
ranges
for
the
four
types
of
electrically
driven
vapor
compression
chillers.
Each
chiller
derives
its
name
from
the
type
of
compressor
used in the chiller. The systems range in capacities from the smallest scroll
(30 kW; 8 tons) to the largest centrifugal (18,000 kW; 5000 tons).Chillers can

7
utilize
either
an
HCFC
(R-22
and
R-123)
or
HFC
(R-134a)
refrigerant.
The
steady
state efficiency of chillers is often stated as a ratio of the power input (in
kW) to the chilling capacity (in tons). A capacity rating of one ton is equal
to
3.52
kW
or
12,000
btu/h.
With
this
measure
of
efficiency,
the
smaller
number
is
better.
centrifugal
chillers
are
the
most
efficient;
whereas,
reciprocating
chillers have the worst efficiency of the four types. The efficiency numbers
provided in the table are the steady state full-load efficiency determined in
accordance to ASHRAE Standard 30 (ASHRAE, 1995). These efficiency numbers do
not include the auxiliary equipment, such as pumps and cooling tower fans that
can add from 0.06 to 0.31 kW/ton to the numbers shown
Chillers
run
at
part
load
capacity
most
of
the
time.
Only
during
the
highest
thermal loads in the building will a chiller operate near its rated capacity.
As
a
consequence,
it
is
important
to
know
how
the
efficiency
of
the
chiller
varies
with part load capacity. a representative data for the efficiency (in kW/ton)
as a function of percentage full load capacity for a reciprocating, screw, and
scroll chiller plus a centrifugal chiller with inlet vane control and one with
variable frequency drive (VFD) for the compressor. The reciprocating chiller
increases in efficiency as it operates at a smaller percentage of full load.
In contrast, the efficiency of a centrifugal with inlet vane control is
relatively constant until theload falls to about 60% of its rated capacity and
its kW/ton increases to almost twice its fully loaded value.
In 1998, the Air Conditioning and Refrigeration Institute (ARI) developed
a new standard that incorporates into their ratings part load performance of
chillers
(ARI
1998c).
Part
load
efficiency
is
expressed
by
a
single
number
called
the integrated part load value (IPLV). The IPLV takes data similar to that in
Figure 4.2.3
and weights it at the 25%, 50%, 75%, and 100% loads to produce a
single integrated efficiency number. The weighting factors at these loads are
0.12, 0.45, 0.42, and 0.01, respectively. The equation to determine IPLV is:


8

Most of the IPLV is determined by the efficiency at the 50% and 75% part
load
values.
Manufacturers
will
provide,
on
request,
IPLVs
as
well
as
part
load
efficiencies.
The four compressors used in vapor compression chillers are each briefly
described below. While centrifugal
and
screw compressors are
primarily used
in
chiller applications, reciprocating and scroll compressors are also used in
smaller unitary packaged air conditioners and heat pumps.

3.4 Reciprocating Compressors
The
reciprocating
compressor
is
a
positive
displacement
compressor.
On
the
intake stroke of
the
piston, a
fixed
amount of gas
is pulled into the cylinder.
On
the
compression
stroke,
the
gas
is
compressed
until
the
discharge
valve
opens.
The quantity of gas compressed on each stroke is equal to the displacement of
the cylinder. Compressors used in chillers have multiple cylinders, depending
on the capacity of the compressor. Reciprocating compressors use refrigerants
with low specific volumes and relatively high pressures. Most reciprocating
chillers used in building applications currently employ R-22.
Modern high-speed reciprocating compressors are generally limited to a
pressure
ratio
of
approximately
nine.
The
reciprocating
compressor
is
basically
a constant-volume variable-head machine. It handles various
discharge pressures with
relatively
small changes in inlet-volume
flow rate
as
shown by the heavy line (labeled 16 cylinders).Condenser operation in many
chillers
is
related
to
ambient
conditions,
for
example,
through
cooling
towers,

9
so that on cooler days the condenser pressure can be reduced. When the air
conditioning load is lowered, less refrigerant circulation is required. The
resulting load characteristic is represented by the solid line that runs from
the upper right to lower left.
The
compressor
must
be
capable
of
matching
the
pressure
and
flow
requirements
imposed by the system. The reciprocating compressor matches the imposed
discharge pressure at any level up to its limiting pressure ratio. Varying
capacity requirements can be met by providing devices that unload
individual or multiple cylinders. This unloading is accomplished by blocking
the suction or discharge valves that open either manually or automatically.
Capacity
can
also
be
controlled
through
the
use
of
variable
speed
or
multi-speed
motors. When capacity control is implemented on a compressor, other factors
at part-load conditions need to considered, such as (a) effect on compressor
vibration and sound when unloaders are used, (b) the need for good oil return
because
of
lower
refrigerant
velocities,
and
(c)
proper
functioning
of
expansion
devices at the lower capacities.
With most reciprocating compressors, oil is pumped into the refrigeration
system from the compressor during normal operation. Systems must be designed
carefully to return oil to the compressor crankcase to provide for continuous
lubrication and also to avoid contaminating heat-exchanger surfaces.
Reciprocating compressors usually are arranged to start unloaded so that
normal torque motors are adequate for starting. When gas engines are used for
reciprocating compressor drives, careful matching of the torque requirements
of the compressor and engine must be considered.
3.5 Screw Compressors
Screw
compressors,
first
introduced
in
1958
(Thevenot,
1979),
are
positive
displacement
compressors.
They
are
available
in
the
capacity
ranges
that
overlap
with reciprocating compressors and small centrifugal compressors. Both
twin-screw and single-screw compressors are used in chillers. The twin-screw
compressor is also called the helical rotary compressor.
A
cutaway of a

10