preliminary analysis of employing a chiller instead of the

Dariusz Nanowski
Gdynia Maritime University
PRELIMINARY ANALYSIS OF EMPLOYING A CHILLER INSTEAD
OF THE RECIPROCATING COMPRESSOR
ON BOARD LPG GAS CARRIERS
This paper presents an investigation of thermodynamic calculations of refrigeration cycles where two
ways of cooling down propane as a LPG carrier cargo are considered. On the assumption of some
cargo loading conditions, the thermodynamic comparison of universally used reciprocating
compressors and on the other hand chiller on board LPG carrier has been made. One of the main
criteria to assess efficiency of used gas plant is coefficient of performance (COP). It is related with
power of electric motors used for driving the compressors and cost of required for this purpose fuel.
Additional issues considered are these, which are related to practical building and using the gas
plant. A short analysis of two different ways of taking away heat from the cargo enables to make some
interesting conclusions.
INTRODUCTION
When propane is loaded to the cargo tanks of the gas carrier some heat has to
be removed from cargo in order to keep the pressure in this tanks on required level
[6, 7]. The common way to do it on board LPG carriers is using the reciprocating
compressor [4]. In this way cargo vapour from the cargo tanks is compressed and
by its condensing in the condenser the heat is removed to the sea water.
Another possible way to remove heat from the cargo is cooling down a liquid
cargo during loading at the manifold with using independent refrigerant cycle – the
chiller. In this paper a preliminary thermodynamic analysis of both cycles is carried
out. The following parameters, shown in Table 1, are assumed for comparison
investigated cycle.
Table 1. Propane parameters
Manifold
Cargo tank
Temperature
+2°C
-30°C
Pressure (abs)
0.5 MPa
0.16 MPa
Assumed sea water temperature is +25°C and related with it condensing
temperature is tc = +30°C.
70
ZESZYTY NAUKOWE AKADEMII MORSKIEJ W GDYNI, nr 76, grudzień 2012
1. RECIPROCATING COMPRESSOR CYCLE
Linde`s cycle performed with using reciprocating compressor is shown in
Fig. 1.
0,030
50
R290 Ref :W.C.Reynolds: Thermodynamic Properties in SI
DTU, Department of Energy Engineering
s in [kJ/(kg K)]. v in [m^3/kg]. T in [şC]
M.J. Skovrup & H.J.H Knudsen. 12-04-23
20
0,0
v=
100
150
200
0,30
1,20
250
0,40
1,40
2,90
2,80
3,1
0
0,50
0,60
0,70
0,80
0,90
1,60
1,80
2,00
2,20
300
350
400
0,20
0
0,30
3,2
0,40
3,3
0
-30
0
0,2
v=
,30
v= 0
0,20
1,00
s=
-20
5
0,1
v=
4
x = 0,10
s = 0,80
2,70
3,0
0
0
0,1
v=
-40
1,00
s=
-10
0
,08
=0
s=
2,00
-30
-20
v
0,15
1
s=
3,00
0
60
0,0
s=
v=
0,10
s=
-10
4,00
10
40
0,0
v=
0
5,00
0
s=
6,00
3
0,0
0,080
0,090
s=2
,60
v=
0,070
20
,40
15
s=2
7,00
Pressure [Bar]
v=
0,0
10
8,00
0,
0,060
2
30
0
01
s=2
,50
30
v=
v=
20
10,00
9,00
0,050
40
v
0, = 0
00 ,0
80 06
0
3
0,040
50
40
20,00
0,50
0,60
-40
-40
2,40
450
500
Enthalpy [kJ/kg]
0,70
-20
0
550
600
20
40
650
60
700
80
100
750
120
800
850
Fig. 1. Propane cycle
On this Mollier diagram [1, 2, 3] specific refrigeration capacity of the cycle
(specific evaporation heat) is the difference between enthalpy points 1 and 4.
Below in Table 2 are shown quantities calculated for assumed in Table 1
parameters of cargo and sea water.
Table 2. Propane cycle quantities
Specific evaporation heat
qe = 262.65 kJ/kg
Specific condensing heat
qc = 349.94 kJ/kg
Specific compression work
w = 87.29 kJ/kg
Coefficient of performance
COP = 3.01
Point 1 in Fig. 1 denotes parameters of propane on suction line of the
compressor, while point 4 after expansion condensate of propane.
71
D. Nanowski, Preliminary analysis of employing a chiller instead of the reciprocating compressor…
2. CHILLER CYCLE
Calculation the chiller with propylene as a refrigerant, with assumption
condensing temperature tc = +30°C and evaporating temperature te = –30°C, gives
quantities of the cycle shown in Table 3. Linde`s cycle is shown in Fig. 2.
Table 3. Propylene cycle quantities
qe = 271.96 kJ/kg
Specific condensing heat
qc = 362.25 kJ/kg
Specific compression work
w = 90.29 kJ/kg
Coefficient of performance
COP = 3.01
R1270 Ref :W.C.Reynolds: Thermodynamic properties in SI
0,03
40
0
s=
3,0
-20
-30
5
0,1
0
v=
0,30
3,1
0
1
0,40
3,2
0
v=
-40
0,20
,20
s=
-30
2,00
0,15
s=
80
0,0
v=
,10
0
v=
4
2,80
-10
s=
-10
-20
Pressure [Bar]
0
0
60
0,0
v=
3,00
0,10
0
,03
4
0,0
v=
4,00
0,080
0,090
10
2,9
0
0
0
v=
6,00
5,00
20
2,70
7,00
0,0
s=
v=
2,60
15
0,060
0,070
s=
0,0
0,050
s=2
,40
20
,50
v=
8,00
0,040
2
30
s=2
10
10,00
9,00
v= v= 0 v= 0
0, ,00 ,00
01 8 60
0 0
20
3
0
40
s=
DTU, Department of Energy Engineering
s in [kJ/(kg K)]. v in [m^3/kg]. T in [şC]
M.J. Skovrup & H.J.H Knudsen. 12-04-23
30
20,00
Specific evaporation heat
0,50
-40
0,60
,30
v= 0
1,00
x = 0,10
0,20
s = 0,80
1,00
50
100
150
200
0,30
1,20
0,40
1,40
250
0,50
1,60
300
0,70
0,60
0,70
0,80
0,90
1,80
2,00
2,20
2,40
350
400
450
Enthalpy [kJ/kg]
500
-40
550
-20
0
600
20
40
650
60
700
80
100
750
120
800
Fig. 2. Chiller propylene cycle
In this case specific refrigeration capacity (h1–h4) of propylene is used to
subcool the liquid cargo (propane) in the manifold, for example from ambient
temperature +2°C (Table 1) to –20°C as shown (process 1–2) in Fig. 3.
Such subcooling of liquid cargo causes decreasing the final dryness fraction X
after expansion 2-3 (X = 0.06). It means that it is lower in the cargo tank than after
process 3–4 (X = 0.36 in Fig. 1) and that almost whole cargo after passing through
the chiller and expansion process to the cargo tank pressure 0.16 MPa (abs) stay in
liquid phase.
72
ZESZYTY NAUKOWE AKADEMII MORSKIEJ W GDYNI, nr 76, grudzień 2012
R290 Ref :W.C.Reynolds: Thermodynamic Properties in SI
DTU, Department of Energy Engineering
s in [kJ/(kg K)]. v in [m^3/kg]. T in [şC]
M.J. Skovrup & H.J.H Knudsen. 12-04-24
0
8,00
0
08
0,0
1
v=
v=
0
03
0,
v=
40
0,0
v=
0,0
3,00
60
0
v=
-20
0
,08
v=
0
0,1
-20
s=
2,4
Pressure [Bar]
-10
v=
0,
02
0
4,00
0
v=
0 ,0
15
0
5,00
0,0
10
2
v=
6,00
v=
10
0,0
06
7,00
2,00
v=
-30
3
0,1
5
0
v=
-30
,20
0
0,3
100
-40
x = 0,10
s = 0,80
150
0,20
1,00
0,30
200
0,40
1,20
250
0,50
1,40
1,60
300
0,60
1,80
350
0,70
2,00
400
0,80
2,20
450
0,90
2,40
500
s=
2 ,5
-40
0
,30
v= 0
1,00
0
0,2
0,40
-40
550
Enthalpy [kJ/kg]
Fig. 3. Propane processes in chiller
It should be emphasized that subcooling of liquid cargo on the manifold gives
better conditions for increasing total refrigeration capacity of the gas plant and to
limit cargo loading time.
CONCLUSIONS
1. As shown figures in Table 2 and Table 3, COP for both cycles is the same.
Of course there is some temperature (assumed 10 K) difference required during
heat exchange in the chiller between propylene –30°C) and lowest temperature
of cargo (–20°C), but this irreversibility [5, 8, 9] is not critical.
2. The chiller enables to release refrigeration cycle of the gas plant from cargo and
select or find more efficient refrigerant for required range of temperatures –
propylene is only an example.
3. Separation the gas plant from the cargo allows:
• to replace reciprocating compressors with screw one, more flexible to
control (possible capacity adjusting from 10–100%) and with longer time
between required overhauls;
• with safe refrigerant, chiller compressor may be situated with additional
equipment as well as control devices in the same room as electric motors;
• the heat exchanger besides performing subcooling processes, also may warm
the cargo up during its discharging.
D. Nanowski, Preliminary analysis of employing a chiller instead of the reciprocating compressor…
73
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WSTĘPNA ANALIZA ZASTOSOWANIA CHILLERA W ZASTĘPSTWIE
SPRĘŻARKI TŁOKOWEJ W INSTALACJI ŁADUNKOWEJ GAZOWCÓW LPG
Streszczenie
Publikacja prezentuje analizę obliczeń termodynamicznych obiegów chłodniczych, wykorzystywanych w dwóch różnych metodach schładzania propanu jako ładunku na statku LPG. Przy
założeniu określonych warunków załadunku przeprowadzono porównanie wykorzystania sprężarki
tłokowej i chillera ze sprężarką śrubową, gdzie odpowiednio ładunek jest schładzany przy
wykorzystaniu par ładunku lub niezależnego czynnika chłodniczego (chiller). Jednym z głównych
kryteriów oceny doskonałości rozpatrywanych obiegów jest współczynnik wydajności chłodniczej
(COP). Jest on ściśle związany z mocą silników elektrycznych wykorzystywanych do napędu
sprężarek (o mocy ok. 300 kW każdy) i ponoszonymi kosztami paliwa. Dodatkowe kwestie rozważane
w pracy związane są z praktyczną stroną budowy i eksploatacji okrętowych ładunkowych instalacji
gazowych. Krótka analiza usuwania ciepła z ładunku umożliwia przedstawienie interesujących
wniosków.