PRELIMINARY ANALYSIS OF EMPLOYING A CHILLER INSTEAD OF THE RECIPROCATING COMPRESSOR ON BOARD LPG GAS CARRIERS

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1 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, are assumed for comparison investigated cycle. Table. Propane parameters Temperature Pressure (abs) Manifold + C.5 MPa Cargo tank - C.6 MPa Assumed sea water temperature is +5 C and related with it condensing temperature is t c = + C.

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.

2 7 ZESZYTY NAUKOWE AKADEMII MORSKIEJ W GDYNI, nr 76, grudzień. RECIPROCATING COMPRESSOR CYCLE Linde`s cycle performed with using reciprocating compressor is shown in Fig.., R9 Ref :W.C.Reynolds: Thermodynamic Properties in SI s in [kj/(kg K)]. v in [m^/kg]. T in [şc] M.J. Skovrup & H.J.H Knudsen ,,4, 9, 8, 7, 6, 5, 4,,, v=,6 v=,8 v=, v=,5 v=, v=, v=,4 v=,6 v=,8 v=, v=,5 v=, s =,4 s =,5 s =,6 s =,7 s =,8 s =,9 s =, s =, s =, s =,,5,6,7,8,9,,5,,,4,5 v=,, x =,,,,4,5,6,7,8, s =,8,,,4,6,8,,, ,6,7 Fig.. Propane cycle On this Mollier diagram [,, ] specific refrigeration capacity of the cycle (specific evaporation heat) is the difference between enthalpy points and 4. Below in Table are shown quantities calculated for assumed in Table parameters of cargo and sea water. Table. Propane cycle quantities Specific evaporation heat q e = 6.65 kj/kg Specific condensing heat q c = kj/kg Specific compression work w = 87.9 kj/kg Coefficient of performance COP =. Point in Fig. denotes parameters of propane on suction line of the compressor, while point 4 after expansion condensate of propane.

-4-5 5,,4, 9, 8, 7, 6, 5, 4,,, - - 4 4 v=,6 v=,8 v=, v=,5 v=, v=, v=,4 v=,6 v=,8 v=, v=,5 v=, - - 4 s =,4 s =,5 s =,6 s =,7 s =,8 s =,9 s =, s =, s =, s =,,5,6,7,8,9,,5,,,4,5 v=,, x =,,,,4,5,6,7,8,9

3 D. Nanowski, Preliminary analysis of employing a chiller instead of the reciprocating compressor 7. CHILLER CYCLE Calculation the chiller with propylene as a refrigerant, with assumption condensing temperature t c = + C and evaporating temperature t e = C, gives quantities of the cycle shown in Table. Linde`s cycle is shown in Fig.. Table. Propylene cycle quantities Specific evaporation heat Specific condensing heat Specific compression work q e = 7.96 kj/kg q c = 6.5 kj/kg w = 9.9 kj/kg Coefficient of performance COP =.,, 9, 8, 7, 6, 5, 4,,,, R7 Ref :W.C.Reynolds: Thermodynamic properties in SI s in [kj/(kg K)]. v in [m^/kg]. T in [şc] M.J. Skovrup & H.J.H Knudsen v=,6 v=,8 v=, v=,5 v=, v=, v=,4 v=,6 v=,8 v=, v=,5 v=, v=, x =,,,,4,5,6,7,8,9 s =,8,,,4,6,8,,, s =,4 s =,5 s =,6 s =,7 4 s =,8, s =,9 s =, s =, s =, 6 8,4,5,6,7,8,9,,5,,,4,5,6,7 Fig.. Chiller propylene cycle In this case specific refrigeration capacity (h h 4 ) of propylene is used to subcool the liquid cargo (propane) in the manifold, for example from ambient temperature + C (Table ) to C as shown (process ) in Fig.. Such subcooling of liquid cargo causes decreasing the final dryness fraction X after expansion - (X =.6). It means that it is lower in the cargo tank than after process 4 (X =.6 in Fig. ) and that almost whole cargo after passing through the chiller and expansion process to the cargo tank pressure.6 MPa (abs) stay in liquid phase.

Table. Linde`s cycle is shown in Fig.. Table. Propylene cycle quantities Specific evaporation heat Specific condensing heat Specific compression work q e = 7.96 kj/kg q c = 6.5 kj/kg w = 9.

4 7 ZESZYTY NAUKOWE AKADEMII MORSKIEJ W GDYNI, nr 76, grudzień 6, 5, 4, - R9 Ref :W.C.Reynolds: Thermodynamic Properties in SI 8, s in [kj/(kg K)]. v in [m^/kg]. T in [şc] M.J. Skovrup & H.J.H Knudsen , v=,6 v=,8 v=, v=,5 v=, v=, v=,4 v=,6, v=,8 v=, s =,4,, - v=,5 v=, -, v=, s =,5,4, x =,,,,4,5,6,7,8,9 s =,8,,,4,6,8,,, Fig.. 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. As shown figures in Table and Table, COP for both cycles is the same. Of course there is some temperature (assumed K) difference required during heat exchange in the chiller between propylene C) and lowest temperature of cargo ( C), but this irreversibility [5, 8, 9] is not critical.. 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.. Separation the gas plant from the cargo allows: to replace reciprocating compressors with screw one, more flexible to control (possible capacity adjusting from %) 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.

. 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

5 D. Nanowski, Preliminary analysis of employing a chiller instead of the reciprocating compressor 7 REFERENCES. Bohdal T., Charun H., Czapp M., Urządzenia chłodnicze sprężarkowe parowe, Wydawnictwa Naukowo-Techniczne, Warszawa.. CoolPack.49, Software, IPU Technology Development, Denmark.. Królicki Z., Termodynamiczne podstawy obniżania temperatury, Oficyna Wydawnicza Politechniki Wrocławskiej, Wrocław McGuire G., White B., Liquefied gas handling principles on ships and in terminals, Witherby &Co, London. 5. Mieczyński M., Istota symetrii termodynamiki klasycznej i współczesnej, Oficyna Wydawnicza Politechniki Wrocławskiej, Wrocław. 6. Nanowski D., Regulacja wydajności chłodniczej systemu etylenowego z kaskadą dla mieszaniny propan-etan, cz., Technika Chłodnicza I Klimatyzacyjna,, nr 9, p Nanowski D., Regulacja wydajności chłodniczej systemu etylenowego z kaskadą dla mieszaniny propan-etan, cz., Technika Chłodnicza I Klimatyzacyjna,, nr, p Szargut J., Egzergia, Wydawnictwa Politechniki Śląskiej, Gliwice Winterbone D., Advanced thermodynamics for engineers, Arnold, London 997. 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. 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.

, Termodynamiczne podstawy obniżania temperatury, Oficyna Wydawnicza Politechniki Wrocławskiej, Wrocław 6. 4. McGuire G., White B.

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