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
Temperature Pressure (abs)
Manifold +2°C 0.5 MPa
Cargo tank -30°C 0.16 MPa
Assumed sea water temperature is +25°C and related with it condensing
1. RECIPROCATING COMPRESSOR CYCLE
Linde`s cycle performed with using reciprocating compressor is shown in Fig. 1. Enthalpy [kJ/kg] 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 Pr es su re [B ar ] 1,00 2,00 3,00 4,00 5,00 6,00 7,00 8,00 9,00 10,00 20,00 s = 2 ,40 s = 2 ,50 s = 2 ,60 s = 2 ,70 s = 2 ,80 s = 2 ,90 s = 3 ,00 s = 3 ,10 s = 3 ,20 s = 3 ,30 -40 -40 -30 -20 -20 -10 0 0 10 20 20 30 40 40 50 60 80 100 120 0,030 0,040 0,050 0,060 0,070 0,080 0,090 0,10 0,15 0,20 0,30 0,40 0,50 0,60 0,70 -40 -3 0 -20 -1 0 0 10 20 30 40 50 x = 0,10 0,20 0,30 0,40 0,50 0,60 0,70 0,80 0,90 s = 0,80 1,00 1,20 1,40 1,60 1,80 2,00 2,20 2,40 v= 0, 0060 v= 0,00 80 v= 0, 010 v= 0,0 15 v= 0,0 20 v= 0,0 30 v= 0,040 v= 0,060 v= 0,08 0 v= 0,10 v= 0,15 v= 0,20 v= 0,30 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 R290Ref :W.C.Reynolds: Thermodynamic Properties in SI
1
2 3
4
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.
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
Specific evaporation heat 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
Enthalpy [kJ/kg] 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 Pr es su re [B ar ] 1,00 2,00 3,00 4,00 5,00 6,00 7,00 8,00 9,00 10,00 20,00 s = 2 ,40 s = 2 ,50 s = 2, 60 s = 2 ,70 s = 2 ,80 s = 2 ,90 s = 3 ,00 s = 3 ,10 s = 3 ,20 -40 -40 -30 -20 -20 -10 0 0 10 20 20 30 40 40 60 80 100 120 0,030 0,040 0,050 0,060 0,070 0,080 0,090 0,10 0,15 0,20 0,30 0,40 0,50 0,60 0,70 -40 -3 0 -2 0 -1 0 0 10 20 30 40 x = 0,10 0,20 0,30 0,40 0,50 0,60 0,70 0,80 0,90 s = 0,80 1,00 1,20 1,40 1,60 1,80 2,00 2,20 2,40 v= 0, 0060 v= 0, 0080 v= 0, 010 v= 0, 015 v= 0,0 20 v= 0,0 30 v= 0,0 40 v= 0,060 v= 0,08 0 v= 0,10 v= 0,15 v= 0,20 v= 0,30 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 R1270Ref :W.C.Reynolds: Thermodynamic properties in SI
1
2 3
4
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.
Enthalpy [kJ/kg] 100 150 200 250 300 350 400 450 500 550 Pre ssu re [Ba r] 1,00 2,00 3,00 4,00 5,00 6,00 7,00 8,00 s = 2, 40 s = 2, 50 -40 -40 -30 -20 0,20 0,30 0,40 -4 0 -3 0 -20 -1 0 0 10 x = 0,10 0,20 0,30 0,40 0,50 0,60 0,70 0,80 0,90 s = 0,80 1,00 1,20 1,40 1,60 1,80 2,00 2,20 2,40 v= 0, 0060 v= 0, 0080 v= 0, 010 v= 0, 015 v= 0, 020 v= 0, 030 v= 0,040 v= 0,060 v= 0,08 0 v= 0,10 v= 0,15 v= 0,20 v= 0,30
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
R290Ref :W.C.Reynolds: Thermodynamic Properties in SI
1 2
3
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
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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, wyko-rzystywanych 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.
