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Description  |
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This invention relates to heat pumps of the absorption 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 absorption type of heat pump, a
fluid of suitable boiling point evaporates at low pressure taking heat
from the surrounding lower temperature zone. The resulting vapour then
passes to an absorber where it is absorbed in a solvent having a higher
boiling point than that of the fluid. The solution so formed is then
passed to a boiler or generator where it is heated to drive off the fluid
as vapour, the hot solvent being returned by way of a heat exchanger to
the absorber. As the fluid evaporates, the pressure developed is
sufficient to cause it to condense in a condenser and thereby release heat
to the higher temperature zone. The condensate is then returned through an
expansion valve to the evaporator to complete the cycle.
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. In an absorption type heat pump system, the solvent must also
be satisfactory in these respects. Ideally, the boiling point of the
solvent is as high as possible to minimise vaporisation and loss of
solvent from the generator along with the working fluid. It is also
important, in order to minimise the mass flow of solvent between absorber
and generator and thereby limit the size of the solution heat exchanger
required, that the working fluid and solvent should interact in such a way
that the solution of the one in the other exhibits a negative deviation
from Raoult's law, both components of the solution exerting a lower vapour
pressure than would be expected having regard to the vapour pressures of
the pure components. This situation arises when an affinity or attraction
exists between the molecules of the working fluid and the solvent
molecules, an affinity which can often be interpreted in terms of hydrogen
bonding.
Materials that have been used as working fluids in absorption type heat
pumps include ammonia, the solvent then being water. Other fluids proposed
include fluorinated hydrocarbons such as monochlorodifluoromethane,
1-chloro-2,2,2-trifluoroethane and 1,1,1,2-tetrafluoroethane. Solvents for
use with these fluids include materials of a slightly basic nature capable
of interacting with acidic hydrogen atoms present in the fluoro compounds.
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.
The present invention provides an absorption heat pump system wherein the
working fluid is a saturated fluorohydrocarbon or fluorohydrocarbon ether
having from 3 to 5 carbon atoms.
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:
1. H(CF.sub.2).sub.n H wherein n is 3, 4 or 5,
2. H(CF.sub.2).sub.n CH.sub.2 F wherein n is 2 or 3,
3. CF.sub.3 (CF.sub.2).sub.n CH.sub.2 F wherein n is 1, 2 or 3,
4. CF.sub.3 CHF(CF.sub.2).sub.n CH.sub.2 F wherein n is 0, 1 or 2,
5. CF.sub.3 CHF(CF.sub.2).sub.n CHF.sub.2 wherein n 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,
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:
6. ROCF.sub.2 H
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-difluoromethoxy-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:
##STR1##
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:
##STR2##
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 fluorohydrocarbons and ethers may be obtained by known methods.
The fluoro compounds described herein have boiling points at atmospheric
pressure in the temperature range 15.degree.-75.degree. C. and are
especially suitable as working fluids in heat pumps of which the primary
purpose is heating of the high temperature zone rather then refrigeration
of the low temperature zone, for example pumps having output temperatures
in the range 45.degree.-80.degree. C. and maximum working pressures in the
range 1.5-10 bar.
The solvents to be used with the working fluids of the invention include
those solvents already used or proposed for use with fluoro compounds in
absorption heat pumps. The fluoro compounds described herein generally
behave as proton donors and are advantageously used in combination with
solvents having a proton acceptor capability. Such solvents include ethers
such as tetraglyme, amides which may be lactams such as the
N-alkylpyrrolidones, for example N-methylpyrrolidone, sulphonamides, for
example tetraethylsulphamide and ureas including cyclic ureas of the
formula:
##STR3##
wherein each of Q.sup.1 and Q.sup.2, independently, represents hydrogen or
a lower alkyl radical. An example of such a cyclic urea is
1,3-dimethyl-2-imidazolidinone.
The heat pump working fluids proposed herein may be used in conjunction
with suitable solvents in absorption heat pumps of conventional design.
The usefulness of a material as a heat pump working fluid is usually
expressed as a coefficient of performance (COP) which is the ratio of
quantity of heat extracted to amount of work expended.
In order to estimate the coefficient of performance for a fluid, the
following data are required:
(i) the vapour pressure curve of the pure fluid;
(ii) the vapour pressure curve of the pure solvent;
(iii) the vapour pressure curves for a number of fluid/solvent solutions
covering the upper and lower concentration limits to be found in the heat
pump;
(iv) the molecular weight of the solvent;
(v) the vapour and liquid specific heats of the fluid over the temperature
range to which it is to be exposed;
(vi) the liquid specific heat of the solvent, and
(vii) the liquid specific heats of the solvent/fluid solutions over the
concentration range found in the pump.
The data required may be obtained experimentally and/or from published
information.
The invention is illustrated but not limited by the following Examples:
EXAMPLE 1
An absorption heat pump can be constructed based on
cis-1,2,3,3,4,4-hexafluorocyclobutane as the working fluid and
1,3-dimethyl-2-imidazolidinone (N N'-dimethylethylene urea) as the
absorbing solvent.
The coefficient of performance of such a device will be 1.34 if the
temperatures in the various components have the following values.
Evaporator: 0.degree. C.;
Absorber: 40.degree. C.;
Generator: 170.degree. C.;
Condenser: 50.degree. C.
This device will supply heat to circulating warm air central heating
systems at 40.degree.-45.degree. C. For an output of 10 KW the mass flow
rates of the solvent and fluid are:
Solvent 0.0713 Kg/sec around generator/solution heat exchanger/absorber
system
Fluid 0.0287 Kg/sec around the generator/condenser
evaporator/absorber/solution heat exchanger system.
EXAMPLE 2
An absorption heat pump can be constructed based on
cis-1,2,3,3,4,4-hexafluorocyclobutane as the working fluid and tetraglyme
as the absorbing solvent.
The coefficient of performance of such a device will be 1.31 if the
temperatures in the various components have the following values:
Evaporator: 0.degree. C.;
Absorber: 40.degree. C.;
Generator: 170.degree. C.;
Condenser: 50.degree. C.
This device will be suitable for supplying heat to circulating warm air
central heating systems at 40.degree.-45.degree. C. For an output of 10 KW
the mass flow rates of the fluid and solvent are:
______________________________________
Solvent 0.0936 Kg/sec
basis as for Example 1
Fluid 0.0257 Kg/sec
______________________________________
EXAMPLE 3
An absorption heat pump can be constructed based on
1-(2,2,2-trifluoroethoxy)-1,1,2,2-tetrafluoroethane as the working fluid
and tetraglyme as the absorbing solvent.
The coefficient of performance will be 1.22 if the temperatures in the
various components have the following values:
Evaporator: 0.degree. C.;
Absorber: 40.degree. C.;
Generator: 170.degree. C.;
Condenser: 50.degree. C.
This device will be suitable for supplying heat to circulating warm air
central heating systems at 40.degree.-45.degree. C. For an output of 10 KW
the mass flow rates of the fluid and solvent are:
Solvent: 0.140 Kg/sec;
Fluid: 0.0339 Kg/sec.
EXAMPLE 4
An absorption heat pump can be constructed based on
1,1,2,2,3-pentafluoropropane as the working fluid and
1,3-dimethyl-2-imidazolidinone as the absorbing solvent.
The coefficient of performance of such a device will be 1.46 if the
temperatures in the various components have the following values.
Evaporator: 10.degree. C.;
Absorber: 55.degree. C.;
Generator: 200.degree. C.;
Condenser: 65.degree. C.
This device will supply heat to circulating warm water central heating
systems at 55.degree.-60.degree. C. For an output of 10 KW the mass flow
rates of the solvent and fluid are:
Solvent 0.031 Kg/sec around generator/solution heat exchanger/absorber
system.
Fluid 0.063 Kg/sec around the generator/condenser
evaporator/absorber/solution heat exchanger system.
EXAMPLE 5
An absorption heat pump can be constructed based on
1,1,2,2,3-pentafluoropropane as the working fluid and
1,3-dimethyl-2-imidazolidinone as the absorbing solvent.
The coefficient of performance of such a device will be 1.51 if the
temperatures in the various components have the following values.
Evaporator: 0.degree. C.;
Absorber: 40.degree. C.;
Generator: 180.degree. C.;
Condenser: 50.degree. C.
This device will supply heat to circulating warm air central heating
systems at 40.degree.-45.degree. C. For an output of 10 KW the mass flow
rates of the solvent and fluid are:
Solvent 0.057 Kg/sec around generator/solution heat exchanger/absorber
system.
Fluid 0.030 Kg/sec around the generator/condenser
evaporator/absorber/solution heat exchanger system.
EXAMPLE 6
An absorption heat pump can be constructed based on
1,1,2,2,3-pentafluoropropane as the working fluid and
1,3-dimethyl-2-imidazolidinone as the absorbing solvent.
The coefficient of performance of such a device will be 1.40 if the
temperatures in the various components have the following values.
Evaporator: 0.degree. C.;
Absorber: 55.degree. C.;
Generator: 190.degree. C.;
Condenser: 65.degree. C.
This device will supply heat to circulating warm water central heating
systems at 55.degree.-60.degree. C. For an output of 10 KW the mass flow
rates of the solvent and fluid are:
Solvent 0.120 Kg/sec around generator/solution heat exchanger/absorber
system.
Fluid 0.030 Kg/sec around the generator/condenser
evaporator/absorber/solution heat exchanger system.
EXAMPLE 7
An absorption heat pump can be constructed based on
1-(2,2,2-trifluoroethoxy)-1,1,2,2-tetrafluoroethane as the working fluid
and tetraglyme as the absorbing solvent.
The coefficient of performance of such a device will be 1.4 if the
temperatures in the various components have the following values.
Evaporator: 0.degree. C.;
Absorber: 50.degree. C.;
Generator: 180.degree. C.;
Condenser: 60.degree. C.
This device will supply heat to circulating warm air or warm water central
heating systems at 50.degree.-55.degree. C. For an output of 10 KW the
mass flow rates of the solvent and fluid are:
Solvent 0.266 Kg/sec around generator/solution heat exchanger/absorber
system.
Fluid 0.031 Kg/sec around the generator/condenser
evaporator/absorber/solution heat exchanger system.
EXAMPLE 8
An absorption heat pump can be constructed based on
1-(2,2,2-trifluoroethoxy)-1,1,2,2-tetrafluoroethane as the working fluid
and tetraglyme as the absorbing solvent.
The coefficient of performance of such a device will be 1.25 if the
temperatures in the various components have the following values.
Evaporator: 10.degree. C.;
Absorber: 55.degree. C.;
Generator: 190.degree. C.;
Condenser: 65.degree. C.
This device will supply heat to circulating warm water central heating
systems at 55.degree.-60.degree. C. For an output of 10 KW the mass flow
rates of the solvent and fluid are:
Solvent 0.173 Kg/sec around generator/solution heat exchanger/absorber
system.
Fluid 0.033 Kg/sec around the generator/condenser
evaporator/absorber/solution heat exchanger system.
EXAMPLE 9
An absorption heat pump can be constructed based on
cis-1,2,3,3,4,4-hexafluorocyclobutane as the working fluid and
1,3-dimethyl-2-imidazolidinone as the absorbing solvent.
The coefficient of performance of such a device will be 1.26 if the
temperatures in the various components have the following values.
Evaporator: 10.degree. C.;
Absorber: 50.degree. C.;
Generator: 180.degree. C.;
Condenser: 60.degree. C.
This device will supply heat to circulating warm water central heating
systems at 50.degree.-55.degree. C. For an output of 10 KW the mass flow
rates of the solvent and fluid are:
Solvent 0.076 Kg/sec around generator/solution heat exchanger/absorber
system.
Fluid 0.029 Kg/sec around the generator/condenser
evaporator/absorber/solution heat exchanger system.
EXAMPLE 10
An absorption heat pump can be constructed based on
cis-1,2,3,3,4,4-hexafluorocyclobutane as the working fluid and tetraglyme
as the absorbing solvent.
The coefficient of performance of such a device will be 1.36 if the
temperatures in the various components have the following values.
Evaporator: 5.degree. C.;
Absorber: 50.degree. C.;
Generator: 180.degree. C.;
Condenser: 60.degree. C.
This device will supply heat to circulating warm water central heating
systems at 50.degree.-55.degree. C. For an output of 10 KW the mass flow
rates of the solvent and fluid are:
Solvent 0.133 Kg/sec around generator/solution heat exchanger/absorber
system.
Fluid 0.026 Kg/sec around the generator/condenser
evaporator/absorber/solution heat exchanger system.
EXAMPLE 11
An absorption heat pump can be constructed based on
cis-1,2,3,3,4,4-hexafluorocyclobutane as the working fluid and tetraglyme
as the absorbing solvent.
The coefficient of performance of such a device will be 1.36 if the
temperatures in the various components have the following values.
Evaporator: 10.degree. C.;
Absorber: 50.degree. C.;
Generator: 200.degree. C.;
Condenser: 60.degree. C.
This device will supply heat to circulating warm air central heating
systems at 50.degree.-55.degree. C. For an output of 10 KW the mass flow
rates of the solvent and fluid are:
Solvent 0.116 Kg/sec around generator/solution heat exchanger/absorber
system.
Fluid 0.027 Kg/sec around the generator/condenser
evaporator/absorber/solution heat exchanger system.
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Description |
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