The
industrial refrigeration field, like any other field, is becoming more and more
competitive. We therefore need to evaluate and carefully check alternative
systems before deciding on a particular refrigeration system for a specific
application.
For
food processing, fisheries, meat packing or any other similar industry, where
process temperature requirement is around (–)40°C, generally a two-stage
ammonia refrigeration system has been used.
But
we need to rethink the economic viability of such two-stage ammonia systems.
One good alternative to consider is a cascade refrigeration system of two
refrigerants with carbon dioxide on the low temperature side and ammonia on the
high temperature side. This article makes a detailed techno-economic analysis
of a CO2 / NH3 cascade system to check for its advantages and disadvantages
in comparison with a conventional two-stage ammonia system.
Before
going into the detailed analysis of such a system, let us first understand the
basic features of these two types of refrigeration systems. For a two-stage
ammonia system, there is a low-stage or booster compressor and a high-stage
compressor, both operating on a common refrigerant – ammonia. The vapour
compression is carried out in two stages. The booster or low-stage compressor
discharge is introduced to the suction of the high-stage compressor via an
inter-stage cooler (which cools the booster discharge gas by evaporation of
high pressure liquid). A typical simple system with a flash type inter-cooler p
- h (pressure - enthalpy) diagram is shown in Figure 1.
In
a cascade refrigeration system there can be more than one refrigerant depending
on the application or requirement of the plant. In such a cascade system, each
refrigerant circuit is separate. For the present application CO2 will be used as a refrigerant for the low temperature
circuit and ammonia will be used for the high temperature circuit.
The
condenser of the CO2 circuit will act
as the evaporator of the NH3 circuit (generally
known as cascade cooler or cascade condenser). Thus there will be no
inter-stage cooler. For better understanding please refer Figure 2 which shows
a schematic arrangement for a CO2 / NH3 cascade system.
Now
for a detailed techno-economic analysis and comparison, let us consider a
typical refrigeration system for food processing or similar application - for
which the basic requirements are :
·
Process temperature :
(–)40 °C
·
Evaporating temperature
: (–)45 °C (for low-stage or low temperature cycle)
·
Condensing temperature :
(+)40 °C
·
Capacity of plant
required : 100 TR (351.63 kW)
Please
note that these parameters form a basis for comparison so that we can make all
calculations and evaluate both the systems based on these common system
parameters. Similarly for the compressor performance, the same pressure drop or
temperature penalty for the suction / discharge line and the same suction gas
super heating has been considered for both the systems. Also a specific make of
screw or reciprocating compressor has been considered for analysis of operating
parameters.
For
the two-stage ammonia system we will consider operation of the low-stage
compressor at a saturated evaporating temperature (–)45°C and saturated
condensing temperature (–)10°C. Whereas, for the highstage compressor,
operation has been considered at a saturated evaporating temperature of (–)10°C
and saturated condensing temperature of (+)40 °C.
For
the CO2 / NH3 cascade system we will consider operation of the low
temperature circuit CO2 compressor at a
saturated evaporating temperature of (–)45°C and saturated condensing
temperature of (–)5°C. Whereas for the high temperature circuit, NH3 compressor operation will be considered at a saturated
evaporating temperature of (–)10°C and saturated condensing temperature of
(+)40 °C. This overlap of refrigerant temperatures in the cascade condenser is
a “must” for such systems.
A
typical simple CO2 / NH3 cascade system p - h (pressure - enthalpy) diagram is shown
in Figure 3.
With
these system parameters, important plant and relevant operating performance
parameters are calculated and analysed . The evaluation of these parameters is
made for both the options of screw as well as reciprocating compressors. A
detailed analysis and comparison of all these basic operating and performance
parameters is given in Table 1 (for Screw Compressors) and in Table 2 (for Reciprocating
Compressors).
Also,
for better understanding, the comparison of these performance-related
parameters is shown in graphical form in Figure
4, Figure 5, Figure 6, Figure 7, Figure 8 and Figure 9.
Let
us now critically examine these operating performance parameters for both the
refrigeration systems, for comparison and analysis, keeping Table 1 & Table 2 as a reference.
Columns 3 & 4 show the value of parameters for the low-stage ammonia of the
two-stage system and low temperature CO2 circuit of the
cascade system respectively. Similarly columns 6 & 7 show the value of
parameters for the high-stage ammonia of the two-stage system and high
temperature NH3 circuit of the
cascade system respectively.
For
analysis / comparison of these two systems, the following important operating /
performance parameters are considered :
·
Capacity of compressor
required
·
Coefficient of
performance (COP)
·
Compressor shaft power
·
Oil cooler load &
oil flow (for screw compressor)
·
Volumetric efficiency of
compressor
·
Adiabatic compression
efficiency of compressor
·
Condenser capacity &
water flow rate required
·
Discharge vapour
temperature
·
Compression ratio
·
Suction mass &
volume flow rate required
·
Compressor swept volume
required
·
Number of compressors
required
·
Total number of
compressors required (for low + high stage)
·
Compressor head / side
cover cooling medium required (for recip compressor)
From
a study of Table 1 and Table 2 we can conclude the following advantages / benefits of the
CO2 / NH3 cascade system over a two-stage ammonia system:
1. Compressor size (or compressor swept volume) required for
the CO2 low-stage side is
appreciably smaller as compared to the low-stage ammonia.
CO2 has a much lower vapour specific volume at low temperatures
compared to NH3 . This is
approximately 97% less at a saturated vapour of (–)45°C. Basically, compressors
are selected based on volume flow rate requirement of a particular plant.
Greater the vapour volume flow rate requirement - larger the compressor that is
required. Hence with this advantage the compressor size for the low-stage is
drastically reduced. This effect will have an added advantage when considering
superheated suction vapour, as the difference of suction vapour specific volume
(for CO2 & NH3 ) will be more for superheated vapour than saturated vapour.
As
per Table 1,
we require only one CO2 low-stage screw
compressor against nine similar capacity NH3 lowstage screw
compressors. Similarly, from Table 2 we require only one CO2 low-stage
reciprocating compressor against eleven similar capacity NH3 low-stage reciprocating compressors.
The
major contributing factor for the initial cost of a refrigeration plant is the
compressor (approximately 15 to 25%) and thus the CO2 low-stage system will appreciably reduce the initial plant
cost . Each screw compressor is to be taken as a module consisting of
compressor and its independent items such as motor, oil cooler, oil separator,
suction / discharge valves, suction strainer, coupling, capacity control
arrangement, interconnected piping, instruments & controls, cabling, base
frame and structural items. Thus when the number of screw compressors is
reduced from nine to one, there will be a huge reduction in cost of all such
related items.
A
related saving is the smaller plant room required and the lower cost of
installation.
With
the CO2 low-stage system
requirement of only 302.79 m3/h compressor swept
volume we can easily consider the option of adopting a reciprocating
compressor; which in case of ammonia may not be a viable option-as the swept
volume required is very high (3420.32 cu.m/hr.). A refrigeration plant with
reciprocating compressors will be much cheaper (approximately 10 to 15%) as
compared to a plant with screw compressors. Thus we can clearly conclude that
with the option of reciprocating compressor, instead of screw compressor, there
will be an appreciable saving in initial plant cost.
2. The compression ratio required for the low-stage is much
lower for CO2 . It is
approximately 44 to 49% less compared to the ammonia booster stage.
The
advantages of a lower compression ratio are better volumetric efficiency, lower
discharge gas temperature and higher adiabatic compression efficiency. All
these advantages of a lower compression ratio will have a greater effect on a
reciprocating compressor compared to a screw compressor and this is clear from Table 1and Table 2.
As
the discharge gas temperature is much lower for the low side CO2 compressor, the chances of oil decomposition and its related
operating problems are absent. Because of this lower discharge gas temperature
and appreciably lower compression ratio, we have the option to adopt a
reciprocating compressor for the lowstage CO2compressor. In the case
of ammonia, it is almost impossible to consider a reciprocating compressor for
such a high compression ratio and discharge gas temperature.
As
the compressor discharge gas temperature is appreciably lower for the CO2 low-stage screw compressor (59°C as compared to 71.50°C for
ammonia) the oil cooler load will also be less for CO2 low-stage screw compressor (can be seen in Table 1). Thus
the size of the oil cooler will be smaller, oil circulation rate will be lower,
total oil required for the compressor will be less and the oil pump capacity
and its power consumption will also be reduced. All these will result in
reduction of initial plant cost and operating cost for the CO2 lowstage side of a cascade system.
Similarly,
because of low discharge gas temperature (62.40°C as compared to 112.90°C for
ammonia) no external water head and side cover cooling is required for a CO2 low-stage reciprocating compressor. In the case of ammonia
low-stage reciprocating compressor, a water head / side cover cooling is a
"must" under these operating conditions. Thus no water piping with
valves, fittings and supports is required for the CO2 low stage reciprocating compressor and this will reduce the
cooling water pump power consumption. All of which would result in a reduction
of initial as well as operating cost.
With
a low discharge gas temperature there is less chance of oil decomposition with
its related problems and failure of the discharge valve plate of the
compressor. Thus, less service and maintenance is required for such plants with
CO2 .
Table 1 : Comparison of operating
parameters between two-stage NH3 and
CO2 /
NH3 cascade
system for Screw compressors
|
||||||
Sr. No.
|
Parameter
|
Low stage of
|
Low temp.
|
Remarks
|
High stage of
|
High temp.
|
2-stage system
|
circuit of cascade
|
2-stage system
|
circuit of cascade
|
|||
1
|
Refrigerant
|
Ammonia
|
Carbon
dioxide
|
|
Ammonia
|
Ammonia
|
2
|
Refrigerant
designation
|
R
717
|
R
744
|
|
R
717
|
R
717
|
3
|
Sat.
evap. / cond. temp. in °C.
|
(-)45
/ (-) 10
|
(-)45
/ (-) 5
|
|
(-)10
/ (+) 40
|
(-)10
/ (+) 40
|
4
|
Type
of compressor
|
Screw
|
Screw
|
|
Screw
|
Screw
|
5
|
Suction
pressure in bar abs.
|
0.5
|
8.3
|
For
R717 less than atm. pressure
|
2.9
|
2.9
|
6
|
Discharge
pressure in bar abs.
|
3.6
|
30.5
|
87.05%
less for R717
|
15.7
|
15.7
|
7
|
Capacity
of comp. reqd. in TR
|
100
|
100
|
|
133.45
|
131.55
|
8
|
Capacity
of comp. reqd. in kW
|
351.63
|
351.63
|
|
469.23
|
462.55
|
9
|
COP
(comp. cap. / power)
|
2.99
|
3.17
|
6.02%
more for R744
|
3.31
|
3.31
|
10
|
Comp.
shaft power in kW
|
117.6
|
110.92
|
5.68%
less for R744
|
141.76
|
139.74
|
11
|
Oil
cooler load in kW
|
53.32
|
5.13
|
90.38%
less for R744
|
74.62
|
73.56
|
12
|
Oil
flow reqd. in LPM
|
104.15
|
36.35
|
65.09%
less for R744
|
88.29
|
87.04
|
13
|
Volumetric
effy. of comp. in %
|
88.8
|
88.5
|
Nearly
same since screw comp.
|
91.4
|
91.4
|
14
|
Adiabatic
(compression) effy. in %
|
65.8
|
78.5
|
19.30
% more for R744
|
80.3
|
80.3
|
15
|
Condenser
capacity reqd. in kW
|
N.A.
|
N.A.
|
|
610.99
|
602.3
|
16
|
Condenser
water flow rate in LPM
|
N.A.
|
N.A.
|
|
2189.39
|
2158.24
|
17
|
Discharge
vapour temp. °C
|
71.5
|
59
|
12.50°C
less for R744
|
82.3
|
82.3
|
18
|
Compression
ratio
|
7.2
|
3.67
|
49.03%
less for R744
|
5.41
|
5.41
|
19
|
Suction
mass flow in kg/ hr.
|
1026.49
|
5145.41
|
80.05
% more for R744
|
1588.68
|
1566.07
|
20
|
Suction
vol. flow rate cu.m./ hr.
|
2166.58
|
249.09
|
88.50%
less for R744
|
687.25
|
677.47
|
21
|
Suction
line size in mm NB
|
150
|
65
|
Appreciable
lower size for R744
|
100
|
100
|
22
|
Suctn.
line thermal insulation thk. in mm
|
150
|
125
|
Less
insultn. reqd. for R744
|
75
|
75
|
23
|
Wet
return line size reqd. in mm NB
|
200
|
80
|
Appreciable
lower size for R744
|
N.A.
|
N.A.
|
24
|
Wet
return line insulation thk. in mm
|
200
|
125
|
Less
insultn. reqd. for R744
|
N.A.
|
N.A.
|
25
|
Discharge
line size reqd. in mm NB
|
100
|
50
|
Appreciable
lower size for R744
|
65
|
65
|
26
|
Comp.
swept vol. reqd. in cu.m./hr.
|
2439.84
|
281.46
|
88.46%
less for R744
|
751.91
|
741.21
|
27
|
No.
of comp. (@282 cu.m/hr. each) reqd.
|
9
|
1
|
8
nos. additional comp. reqd. for R717
|
3
|
3
|
28
|
Total
power for comps. (low+high) in kW
|
|
|
250.66
kW for cascade system
|
|
|
29
|
Total
no. of comps. reqd. (low+high)
|
|
|
4
nos. for cascade system
|
|
|
3. The COP (coefficient of performance) for the CO2 low stage compressor is much higher compared to the ammonia compressor for the required operating conditions.
Thus,
there is a reduction in compressor power consumption, both for the screw as
well as the reciprocating compressor (5.68% less for screw compressor and
18.18% for reciprocating compressor). Based on the total plant capacity we will
require a lower kW rating motor for CO2 low-stage
compressor (screw or reciprocating). This advantage of lower power consumption
has an effect on the entire plant life operating cost. We are all aware of the
present-day crisis of electric power and its ever-increasing price all over the
world. Therefore, the lower power requirement of a CO2 low-stage compressor for such a cascade system has high
potential in reducing operating cost for end users in future refrigeration
plants.
The
high-side ammonia compressor capacity required will be lower for a cascade
system (as compared to the high stage of a conventional two-stage ammonia
system). This is because of lower low-stage compressor power for CO2 . From Table 1 & Table 2 this is 1.42% less for a screw compressor and 5.03% less for
a reciprocating compressor.
Similarly,
the power consumption of the high-stage compressor will be less (1.42% for a
screw compressor and 5.03% for a reciprocating compressor) for a cascade system
compared to a two-stage ammonia system.
Thus
the total power consumption of compressors (low-stage + high-stage) for cascade
system is less (3.35% for screw and 11.13% for reciprocating) for a CO2 / NH3 cascade system
compared to a two-stage ammonia system.
We
can also conclude that the condenser capacity required is lower (1.42 % for
screw and 5.03% for reciprocating) for a CO2 / NH3 cascade system compared to a two-stage ammonia system. Thus
the condenser size will be smaller, water flow rate across the condenser will be
less, pipeline size and valves etc. shall be smaller, cooling water pump
capacity and power consumption for such a pump will be less, cooling tower
capacity required also will be less, cooling tower fan capacity and power
consumption by the fan motor will also be marginally less. All these will
result in a further reduction in initial as the well as operating cost of
plant.
Table 2 : Comparison of operating parameters between
two-stage cascade system for Reciprocating compressors
|
||||||
Sr. No.
|
Parameter
|
Low stage of
|
Low temp.
|
Remarks
|
High stage of
|
High temp.
|
2-stage system
|
circuit of cascade
|
2-stage system
|
circuit of cascade
|
|||
1
|
Refrigerant
|
Ammonia
|
Carbon
dioxide
|
|
Ammonia
|
Ammonia
|
2
|
Refrigerant
designation
|
R
717
|
R
744
|
|
R
717
|
R
717
|
3
|
Sat.
evap. / cond. temp. in °C.
|
(-)45
/ (-) 10
|
(-)45
/ (-) 5
|
|
(-)10
/ (+) 40
|
(-)10
/ (+) 40
|
4
|
Type
of compressor
|
Reciprocating
|
Reciprocating
|
|
Reciprocating
|
Reciprocating
|
5
|
Suction
pressure in bar abs.
|
0.53
|
8.16
|
For
R717 less than atm. pressure
|
2.85
|
2.85
|
6
|
Discharge
pressure in bar abs.
|
3.6
|
30.84
|
83.33%
less for R717
|
15.77
|
15.77
|
7
|
Capacity
of comp. reqd. in TR
|
100
|
100
|
|
133.31
|
131.35
|
8
|
Capacity
of comp. reqd. in kW
|
351.63
|
351.63
|
|
486.35
|
461.86
|
9
|
COP
(comp. cap. / power)
|
2.61
|
3.19
|
18.18%
more for R744
|
3.13
|
3.13
|
10
|
Comp.
shaft power in kW
|
134.72
|
110.23
|
18.18%
less for R744
|
155.38
|
147.56
|
11
|
Comp.
head / side cover cooling
|
both
water
|
both
water
|
90.38%
less for R744
|
both
water
|
both
water
|
12
|
Volumetric
effy. of comp. in %
|
63
|
82
|
|
68
|
68
|
13
|
Condenser
capacity reqd. in kW
|
N.A.
|
N.A.
|
|
641.74
|
609.42
|
14
|
Condenser
water flow rate in LPM
|
N.A.
|
N.A.
|
|
2299.56
|
2183.75
|
15
|
Discharge
vapour temp. °C
|
112.9
|
62.4
|
50.50°C
less for R744
|
137.2
|
137.2
|
16
|
Compression
ratio
|
6.79
|
3.78
|
44.30%
less for R744
|
5.53
|
5.53
|
17
|
Suction
mass flow in kg/ hr.
|
1022.57
|
5134.13
|
80.08
% more for R744
|
1651
|
1567.87
|
18
|
Suction
vol. flow rate cu.m./ hr.
|
2154.8
|
248.29
|
88.48%
less for R744
|
719.84
|
683.6
|
19
|
Suction
line size in mm NB
|
150
|
65
|
Appreciable
lower size for R744
|
100
|
100
|
20
|
Suctn.
line thermal insulation thk. in mm
|
150
|
125
|
Less
insultn. reqd. for R744
|
75
|
75
|
21
|
Wet
return line size reqd. in mm NB
|
200
|
80
|
Appreciable
lower size for R744
|
N.A.
|
N.A.
|
22
|
Wet
return line insulation thk. in mm
|
200
|
125
|
Less
insultn. reqd. for R744
|
N.A.
|
N.A.
|
23
|
Discharge
line size reqd. in mm NB
|
100
|
50
|
Appreciable
lower size for R744
|
65
|
65
|
24
|
Comp.
swept vol. reqd. in cu.m./hr.
|
3420.32
|
302.79
|
91.15%
less for R744
|
1058.59
|
1005.29
|
25
|
No.
of comp. (@282 cu.m/hr. each) reqd.
|
11
|
1
|
10
nos. additional comp. reqd. for R717
|
3
|
3
|
26
|
Total
power for comps. (low+high) in kW
|
|
|
257.80
kW for cascade system
|
|
|
27
|
Total
no. of comps. reqd. (low+high)
|
|
|
4
nos. for cascade system
|
|
|
4. The low-stage compressor suction pressure is higher for CO2 , higher than atmospheric pressure; thus there is no chance of entry of air from a low side leakage and its related operating problems. In the case of an ammonia plant this is a common problem in low temperature applications. Hence, for a two-stage ammonia system, a costly automatic air purger with its controls and piping is always used to get rid of this problem of entry of non-condensable air in the system from the low side.
So
a costly automatic air purger with its controls, instruments, valves, piping, and
thermal insulation, can be totally eliminated for the CO2 plant. This will have an appreciable effect on reducing the
initial cost of the plant. Also there is no chance of accumulation of
noncondensable air in the system causing high condensing pressure which
increases compressor power requirement for the compressor.
5. CO2 suction vapour
specific volume is much lower compared to ammonia ; hence for a similar
capacity plant the suction line size will be smaller.
From
Table 1 and Table 2 we find that for 100 TR (351.63 kW) plant with (–)45 °C
evaporation temperature the suction line size will be 65 mm NB for carbon
dioxide as compared to 150 mm NB for ammonia. Since the suction line size is
smaller the thermal insulation requirement will be also less (insulation
thickness with EPS 125 mm for CO2 as compared to 150
mm for NH3 ).
As
the suction line size is smaller for CO2 , hence suction
valves, strainer, fittings etc. will also be of smaller size. All these items
for such low temperature application require low temperature carbon steel
(LTCS) or suitable grade Stainless Steel (S.S.) material, which are extremely
costly compared to general carbon steel materials. Thus with CO2 refrigerant in the low temperature side of a cascade system
there is a significant reduction of initial plant cost and installation cost.
6. The vapour volume flow
rate for CO2 at suction
temperature is appreciably lower compared to NH3 for the same capacity and temperature. Hence the accumulator
/ liquid separator used for separating the suction vapour from the liquid to
avoid liquid carry over to the compressors can be much smaller. This vessel
also calls for LTCS or special grade S.S. material; hence the advantage of a
smaller size accumulator will also result in an appreciable reduction in
initial plant cost.
The
thermal insulation of the accumulator will also be reduced compared to a
similar capacity ammonia accumulator. This will result in further reduction in
initial cost of plant as well as installation cost.
With
a similar logic for pumped re-circulation system the wet return line size and
its thermal insulation requirement will also be less compared to ammonia.
7. The discharge condition
CO2 vapour has lower
specific volume compared to ammonia. Hence the discharge line size for a CO2 plant will be smaller compared to similar capacity ammonia.
This will also result in advantages of lower piping, fittings and smaller size
valves resulting in a further reduction in the overall plant cost.
8. Because of lower suction / wet return lines, lower size
discharge line and a smaller accumulator, the total first charge of refrigerant
for such a CO2 / NH3 cascade system will be smaller than a conventional two-stage
NH3 system.
The
estimated total initial refrigerant charge requirement will be 60 to 70% less
as compared to a two-stage ammonia plant.
CO2 is approximately 37% cheaper than ammonia. Thus there will
be an additional benefit in future cost saving while replenishing the
refrigerant.
9. CO2 gas is non-toxic and non-flammable. Hence carbon dioxide can
be used in direct contact with food items.
CO2 is also odourless and it is a better and a safer refrigerant
for food processing or other industries. Also, it is environment-friendly and
not lethal like ammonia for human inhaling. Hence in food processing
industries, where customers object to ammonia because of possible leakage in
the food processing area, they can safely decide to go for carbon dioxide, by
adopting the CO2 / NH3 cascade system.
10. CO2 compressors
require special synthetic lubricating oil with food grade quality and this is
80% more costly compared to standard lubricating oil required for ammonia
compressor. But the requirement of such compressor oil for CO2 is lower because of less number of compressors or smaller
compressors and smaller oil cooler. In the case of a two-stage ammonia system,
for both the stages (booster & high stage) we need to use a better quality
oil suitable for low temperature ammonia service, as both the stages are
interconnected . Whereas standard oil for ammonia for (–)10°C temperature can
be used for the cascade system ammonia side (since both circuits are separate
and independent).
Thus the overall cost of the first charge of compressor oil (low temperature side CO2 compressor oil and high temperature side ammonia compressor standard oil) will be less as compared to a two-stage ammonia plant.
Table3 : Comparison of important properties
|
|||
Sr.
No.
|
Parameter
|
Ammonia
|
Carbon
dioxide
|
1
|
Chemical
formula
|
NH3
|
CO2
|
2
|
Molecular
weight
|
17
|
44
|
3
|
Refrigerant
designation
|
R
717
|
R
744
|
4
|
Critical
temp. °C
|
133
|
31.06
|
5
|
Critical
pressure Bar abs.
|
113
|
73.84
|
6
|
Type
|
Inorganic
Compound
|
|
7
|
Boiling
point °C at std. atm. pressure( NBP)
|
(-)
33.30
|
(-)78.30
|
8
|
Safety
group
|
B2
(evidence of
|
A1
(toxicity
|
toxicity
identified,
|
not
identified, No
|
||
lower
flammability limit)
|
Flame
propagation)
|
||
9
|
Sp.
Heat at const. press.( Cp) kJ / kg .K
|
2.1269
|
0.8709
|
10
|
Sp.
Heat at const. vol.( Cv) kJ / kg .K
|
1.6705
|
0.6783
|
11
|
Ratio
of Cp / Cv
|
1.2732
|
1.2839
|
12
|
Gas
constant (R) J / kg .K
|
487
|
189
|
13
|
Flammability
|
Flammable
with 16 to
|
Not
flammable in air
|
25%
by vol. In air
|
|||
14
|
Health
hazard
|
Injurious
/ lethal for
|
Not
injurious or lethal
|
0.5
to1% conc. for
|
|||
0.5
Hr. exposure
|
|||
15
|
ODP
factor (ozone depletion potential)
|
Nil
|
Nil
|
16
|
Sat.
press. at (-)45°C sat temp. bar abs.
|
0.545
|
8.336
|
17
|
Sat.
press. at (-)5°C sat temp. bar abs.
|
3.548
|
30.47
|
18
|
Sp.
vol. sat. vap.at (-)45°C temp. cu.m / kg
|
2.00458
|
0.0459
|
19
|
Sp.
vol. sat. liq..at (-)5°C L / kg
|
1.5495
|
1.0447
|
20
|
Consideration
for food contact
|
Direct
contact with
|
Can
have direct
|
food
not permitted
|
contact
with food
|
||
21
|
Odour
|
Pungent
smell
|
Odourless
|
Thus the overall cost of the first charge of compressor oil (low temperature side CO2 compressor oil and high temperature side ammonia compressor standard oil) will be less as compared to a two-stage ammonia plant.
Please
also refer to Table 3 for a comparison between CO2 and NH3 as refrigerants
for various important properties and parameters; this shows that as a
refrigerant, carbon dioxide can be considered a better refrigerant compared to
ammonia. In fact, it was being used long before we became familiar with CFC,
HFC, ammonia or Hydro carbons as refrigerants.
But
CO2 cannot be used on
the high-stage side of the plant, as condensing pressure at 40 °C temperature
will be much higher than ammonia. This calls for a condenser design pressure
which is extremely high and not economically viable. Hence it is used only in
the low temperature side. Whereas ammonia is used on the high temperature side
of the cycle, with its benefit of a lower condensing pressure. Thus by having
these two refrigerants in a cascade refrigeration system we can make the plant
design most cost-effective and optimum, taking advantage of the properties of
both the refrigerants, CO2 & NH3 .
However,
like any other system, the CO2 / NH3 cascade system has some disadvantages as compared to a
two-stage ammonia refrigeration system. The major disadvantages of such a
cascade refrigeration system are:
1.
For carbon dioxide the
saturated pressure is much higher (more than 75 bar) when liquid refrigerant is
warmed to ambient temperature (say 40°C). This condition would require that all
the components in the low temperature circuit be suitable for such high
pressure, which is economically not viable.
To make the plant viable a suitable volume fade-out vessel is provided on the
CO2 circuit (in
between the condenser and the chiller) to permit the liquid refrigerant to be
warmed to room temperature. When the plant is shut down for a long period, such
a situation may occur. A fade-out vessel is simply an empty vessel that is open
to the cascade refrigerant CO2 . This is designed
with suitable volume so that when the system is shut down and temperature rises
the liquid can expand to vapour without exceeding a reasonable limiting
pressure. Lower the limiting pressure, larger is the volume required of the
fade-out vessel. Thus we can have, say 40°C equalising temperature which has
enough room to expand to vapour at a pressure not higher than 32 bar absolute.
This is additional equipment required for cascade systems
2.
In the case of liquid
overfeed refrigeration system, the CO2 liquid pump capacity
required is 2.5 to 3.5 times higher than an NH3 pump for similar operating parameters. Thus, liquid line
sizes for such a pump suction and discharge will be higher compared to ammonia.
3.
The CO2 side vessel and exchanger design pressure is higher compared
to the booster ammonia side vessel and heat exchanger.
In
spite of the above mentioned disadvantages the CO2 / NH3 cascade system can
be considered a better and more cost-effective proposal for a system requiring
less than (–)40 °C evaporation.