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Description  |
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This invention relates to heat pumps of the mechanical vapour recompression
type and more particularly to working fluids for use therein.
Heat pumps which transfer heat from a low temperature zone to a higher
temperature zone are well known. In the mechanical vapour recompression
type, a fluid of suitable boiling point evaporates at low pressure taking
heat from the surrounding lower temperature zone. The resulting vapour is
then compressed and passed to a condenser where it condenses and gives off
heat to the higher temperature zone. The condensate is then returned
through an expansion valve to the evaporator, so completing the cycle. The
mechanical energy required for compressing the vapour and pumping the
fluid is provided by, for example, an electric motor or an internal
combustion engine.
The suitability of a material as a heat pump working fluid depends upon a
number of factors. Thus, in addition to having a suitable boiling point,
it must be generally acceptable in respect of toxicity, flammability and
corrosivity.
Materials that have been proposed as heat pump fluids include, in
particular, fluorinated hydrocarbons such as trichlorofluoromethane and
1,2,2-trichloro-1,1,2-trifluoroethane. Whilst these fluids have been
generally satisfactory for the purpose for which they were intended, they
are less suitable for heat pumps operating at high output temperatures
because of their instability and corrosivity, especially in the presence
of lubricating oils.
The present invention provides a mechanical vapour recompression heat pump
system wherein the working fluid is a saturated fluorohydrocarbon or
fluorohydrocarbon ether having from 3 to 5 carbon atoms or a fluorinated
amine containing up to 6 carbon atoms and having the formula:
##STR3##
wherein each of X and X.sup.1, independently, represents hydrogen or
fluorine,
R represents hydrogen or a lower alkyl radical,
n represents an integer from 1 to 4,
each of m and q represents an integer from 0 to 2,
p represents an integer from 0 to 4 provided that
p is not zero when X.sup.1 is fluorine, and
r represents an integer from 1 to 3;
or the formula:
##STR4##
wherein W represents hydrogen or fluorine, a represents an integer from 3
to 5, b represents an integer from 1 to 3, and c represents an integer
from 0 to 2 provided that c is not zero when W is fluorine.
The term "fluorohydrocarbon" as used herein means a compound containing
atoms of carbon, hydrogen and fluorine only, whilst "fluorohydrocarbon
ether" means an ether containing atoms of carbon, hydrogen, fluorine and
oxygen only, both hydrogen and fluorine being present in each case.
For increased stability, the fluorohydrocarbons and fluorohydrocarbon
ethers should not contain groups from which hydrogen fluoride is likely to
be eliminated during use. Examples of such groups include --CH.sub.2
CH.sub.2 F and --CH.sub.2 --CHF--CH.sub.2 --. Where low flammability is an
important criterion, the hydrogen/fluorine atomic ratio should not exceed
unity.
The fluorohydrocarbons and ethers may have acyclic (Linear or branched) or
cyclic structures.
Examples of acyclic fluorohydrocarbons which may be used include those
having the following structures:
H(CF.sub.2).sub.x H (3)
wherein x is 3, 4 or 5,
H(CF.sub.2).sub.x CH.sub.2 F (4)
wherein x is 2 or 3,
CF.sub.3 (CF.sub.2).sub.x CH.sub.2 F (5)
wherein x is 1, 2 or 3,
CF.sub.3 CHF(CF.sub.2).sub.x CH.sub.2 F (6)
wherein x is 0, 1 or 2,
CF.sub.3 CHF(CF.sub.2).sub.x CHF.sub.2 ( 7)
wherein x is 0, 1 or 2.
The acyclic fluorohydrocarbons may be obtained by methods that have been
fully described in the prior art.
Acyclic fluorohydrocarbon ethers which may be used include fluorinated
dialkyl ethers having from 3 to 5 carbon atoms of which an alpha-carbon
atom, relative to the oxygen atom, carries at least one fluorine
substituent and the adjacent beta-carbon atom carries at least one
hydrogen substituent. Ethers of this class may be prepared by the ionic
addition of unsaturated fluorinated species to an alcohol. As examples of
this class of ether, there may be mentioned
1-methoxy-1,1,2,2-tetrafluoroethane, and
1-(2,2,2-trifluoroethoxy)-1,1,2,2-tetrafluoroethane, and
1-methoxy-1,1,2-trifluoroethane.
Other acyclic hydrocarbon ethers which may be used include compounds of the
formula:
ROCF.sub.2 H (8)
wherein R represents a fluorinated C.sub.2-4 alkyl group. Ethers of this
class may be prepared by the addition of difluorocarbene to a fluorinated
alcohol. Examples include 1-difluoro-methoxy-2,2,2-trifluoroethane.
Cyclic fluorohydrocarbons which may be used include three- to five-membered
ring compounds. In particular, there may be mentioned compounds of the
formula:
##STR5##
wherein at least four of the R.sup.1 -R.sup.7 substituents represent
fluorine, the remainder representing hydrogen. Examples of suitable cyclic
fluorohydrocarbons include the following:
##STR6##
Cyclic fluorohydrocarbon ethers which may be used include four- and
five-membered ring compounds. In particular there may be mentioned
fluorinated oxetanes having at least three fluorine substituents, for
example 2,2,3,3-tetrafluoro-oxetane.
The cyclic fluorohydrocarbon and ethers, which may be obtained by known
methods, may be used singly or in the form of mixtures. Furthermore, if
desired, one or more of the fluorohydrocarbons or fluorohydrocarbon ethers
may be used in admixture with one or more of the fluorinated amines of
Formula I or Formula 2 or with known working fluids.
When the amines of Formula I contain two R substituents on the nitrogen
atom, they may be the same or different. Thus, they may both be hydrogen
or they may be identical or different lower alkyl radicals or one may be
hydrogen and the other lower alkyl. Similarly, when r is 2 or 3, the
radicals to which it relates may be identical or different.
One useful class of amines falling within the scope of Formula I is the
class represented by the formula:
(X CF.sub.2 CHQ).sub.r NH.sub.3-r
wherein x and r have the meanings given above and Q is hydrogen, lower
alkyl, difluoromethyl or trifluoromethyl, the amine having a maximum of 6
carbon atoms.
Specific examples of fluorinated amines of Formula I or Formula 2 which may
be used include 2,2,2-trifluoroethylamine and 2,2-difluoroethylamine and
their N-methyl and N,N-dimethyl derivatives,
bis(2,2,2-trifluoroethyl)amine and bis(2,2-difluoroethyl)amine and their
N-methyl derivatives, tris(2,2,2-trifluoroethyl)amine,
tris(2,2-difluoroethyl)amine, 2,2,2,2',2'-pentafluorodiethylamine,
3,3-difluoro-n-propylamine, 2,2,3,3-tetrafluoro-n-propylamine,
1-difluoromethyl-2,2-difluoroethylamine,
1-trifluoromethyl-2,2,2-trifluoroethylamine,
1-methyl-2,2-difluoroethylamine, N-methyloctafluoropyrrolidine and
N-methyl-3,3,4,4-tetrafluoropyrrolidine.
The amines may be used singly or in the form of azeotropic or
non-azeotropic mixtures. Such mixtures may be deliberately prepared in
order to provide a working fluid having particular properties or they may
be the total product of a manufacturing process, for example a process in
which a primary amine is formed together with the corresponding secondary
and tertiary amines. Suitable mixtures include mixtures of
2,2-difluoroethylamine with bis(2,2-difluoroethyl)amine and
tris(2,2-difluoroethyl)amine, mixtures of 2,2,2-trifluoroethylamine with
bis(2,2,2-trifluoroethyl)amine and tris(2,2,2-trifluoroethyl)amine, and
mixtures of 2,2-difluoroethylamine and 2,2,2-trifluoroethylamine. The
fluorinated amines may also be used in admixture with other heat pump
fluids.
In general, the fluorinated amines described herein are known compounds
which may be prepared by methods fully described in the prior art.
The fluorocompounds described herein have boiling points at atmospheric
pressure in the temperature range 15.degree.-120.degree. C. and are
especially, though not exclusively, suitable as working fluids in
mechanical vapour recompression heat pumps of which the primary purpose is
heating of the high temperature zone rather than refrigeration of the low
temperature zone, for example pumps having output temperatures in the
range 100.degree.-150.degree. C. and maximum working pressures in the
range 10-30 bar. The pumps may be of conventional design, for example
those operating on a reverse Rankine Cycle.
A valuable indication of the usefulness of the above mentioned compounds as
heat pump working fluids may be obtained from pressure/enthalpy diagrams
which may be constructed in known manner for any of the compounds given
the vapour pressure at various temperatures (determined experimentally),
the critical temperature (also determined experimentally) and the ideal
gas specific heat (estimated by a group contribution method). From these
diagrams it is possible to estimate the coefficient of performance (COP)
of a fluid when used in a mechanical vapour recompression heat pump for
pumping heat from a source at a specified temperature to a load at a
higher temperature, the coefficient of performance being the ratio of
quantity of heat extracted to amount of work expanded.
The invention is illustrated but not limited by the following Examples.
EXAMPLES 1
The vapour pressure at various temperatures and the critical temperature
were determined for 1-methoxy-1,1,2,2-tetrafluoroethane and the ideal gas
specific heat was estimated by a group contribution method. These figures
were used for the construction of a pressure-enthalpy diagram from which
design parameters were calculated for three typical heat pumps. The
results of the calculations are summarised in Table 1.
TABLE 1
______________________________________
Temper-
ature of
vapor
Condensing Condensing
leaving Evaporating
Temperature
Pressure Evaporator
Pressure
(.degree.C.)
(bar) (.degree.C.)
(bar) COP
______________________________________
1 100 6.2 46 1.3 3.47
2 125 10.7 75 2 3.64
3 150 17.2 103 6 3.80
______________________________________
The results demonstrate that 1-methoxy-1,1,2,2-tetrafluoroethane can be
used as a working fluid in heat pumps operating at the stated condensing
and evaporating temperatures. The minimum evaporating temperature is
37.degree. C. (the boiling point of the compound at atmospheric pressure).
The preferred condensing temperature is 167.degree. C. (corresponding to a
pressure of 23 bar). Above this value, the latent heat decreases rapidly
as the temperature approaches the critical temperature (186.degree. C.)
making the fluid less suitable for use above 167.degree. C.
EXAMPLE 2
Using the process outlined in Example 1, a pressure/enthalpy diagram was
constructed for 1-(2,2,2-trifluoroethoxy)-1,1,2,2-tetrafluoroethane. This
was used for the calculation of design parameters for two typical heat
pumps. The results of the calculations are summarised in Table 2.
TABLE 2
______________________________________
Temper-
ature of
vapor
Condensing Condensing
leaving Evaporating
Temperature
Pressure Evaporator
Pressure
(.degree.C.)
(bar) (.degree.C.)
(bar) COP
______________________________________
1 150.degree.
10 123.degree. C.
3.7 6.2
2 175.degree.
16 147.degree. C.
6.5 5.9
______________________________________
EXAMPLE 3
The vapour pressure at various temperatures and the critical temperature
were determined for 2,2,2-trifluoroethylamine and the ideal gas specific
heats were estimated by a group contribution method. These figures were
used for the construction of a pressure enthalpy diagram and also for
calculating design parameters for three typical heat pumps. The results
are summarised in Table 3.
TABLE 3
______________________________________
Temper-
ature of
vapor
Condensing Condensing
leaving Evaporating
Temperature
Pressure evaporator
Pressure
(.degree.C.)
(bar) (.degree.C.)
(bar) COP
______________________________________
1 101 6.97 50 1.68 5.82
2 123 11.12 75 3.56 6.47
3 148 18.16 100 6.71 6.40
______________________________________
The results demonstrate that 2,2,2-trifluoroethylamine can be used as a
working fluid in heat pumps operating at the stated condensing and
evaporating temperatures. The preferred minimum evaporating temperature is
37.degree. C. (the boiling point of the compound at atmospheric pressure).
Above 180.degree. C., the latent heat decreases rapidly as the temperature
approaches the critical temperature (198.degree. C.) making the fluid less
suitable for use above 180.degree. C.
EXAMPLE 4
The techniques described above were applied to 1,1,2,2,3-pentafluoropropane
with the following results.
______________________________________
Temper-
ature of
vapor
Condensing Condensing
leaving Evaporating
Temperature
Pressure Evaporator
Pressure
(.degree.C.)
(bar) (.degree.C.)
(bar) COP
______________________________________
1 140 19 80 3 2.94
2 120 13 60 1.7 3.05
3 100 8.5 60 2 5.10
______________________________________
This fluid is therefore useful as a heat pump working fluid at temperatures
up to about 140.degree. C. Above this temperature, its latent heat is
rapidly decreasing.
EXAMPLE 5
The techniques described above were applied to 2,2,3,3-tetrafluoro-oxetane
with the following results.
______________________________________
Temper-
ature of
vapor
Condensing Condensing
leaving Evaporating
Temperature
Pressure Evaporator
Pressure
(.degree.C.)
(bar) (.degree.C.)
(bar) COP
______________________________________
1 140 19 80 4.0 3.67
2 140 19 100 7.2 5.89
3 120 11.5 80 4.0 6.00
______________________________________
This fluid is therefore useful for output temperatures up to about
145.degree. C. Above that temperature, its latent heat is becoming too
small.
EXAMPLE 6
The techniques described above were applied to
1,1,2,2-tetrafluorocyclobutane with the following results.
______________________________________
Temper-
ature of
vapor
Condensing Condensing
leaving Evaporating
Temperature
Pressure evaporator
Pressure
(.degree.C.)
(bar) (.degree.C.)
(bar) COP
______________________________________
1 153 13 90 1.5 2.96
2 120 6.8 80 1.5 5.37
3 153 13 120 4.7 6.15
______________________________________
This fluid is useful for condensing temperatures up to about 180.degree. C.
EXAMPLE 7
The techniques described above were applied to
1,1,1,2,3,3,4-heptafluoro-n-butane with the following results.
______________________________________
Temper-
ature of
vapor
Condensing Condensing
leaving Evaporating
Temperature
Pressure evaporator
Pressure
(.degree.C.)
(bar) (.degree.C.)
(bar) COP
______________________________________
1 134 10.2 100 2.5 4.7
2 112 6.9 89 2.5 7.27
3 134 10.2 80 1.4 2.5
______________________________________
This fluid is suitable for temperatures up to about 140.degree. C.
* * * * *
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Description |
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