Applied Thermal Engineering 29 (2009) 3218–3223
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Miniature heat pipes as compressor cooling devices F.C. Possamai a, I. Setter a, L.L. Vasiliev b,* a b
Embraco – Empresa Brasileira de Compressors S.A., Rua Rui Barbosa, 1020, P.O. Box 91, 89219-901 Joinville, SC, Brazil Luikov Heat and Mass Transfer Institute, P. Brovka 15, 220072 Minsk, Belarus
a r t i c l e
i n f o
Article history: Received 24 June 2008 Accepted 15 April 2009 Available online 3 May 2009 Keywords: Heat pipe Compressor Cooling system
a b s t r a c t The purpose of this paper is an analytical and experimental study on heat transfer and temperature distribution for hermetic refrigeration compressor using miniature heat pipes as two-phase thermal control system. Heat pipe based coolers, such as miniature and micro heat pipe spreaders, loop heat pipes and loop thermosyphons ensure the temperature decrease of the most important parts of the compressor – cylinder head, cylinder, oil and compressor shell down to 10–15 °C. The experimental validation of the analytical study was performed for four different designs of miniature heat pipes. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Compressors are used for several purposes in industrial, commercial and residential applications. For example, gas compressors in industrial applications are used for refrigeration, air conditioning, heating, pipeline conveying, natural gas gathering, catalytic cracking, polymerization and other chemical processes [1]. Among the different components of small refrigerating machinery, the compressor has the most important effect on energy consumption. The knowledge of its performance is critical for improving the overall performance of a system. Currently, the increasing demand of high efficiency, low noise and low cost hermetic compressors are strong reasons for the development of general and accurate prediction methodologies. Hermetic compressors, used for many refrigeration applications, have both the electric motor and the compressor inside a welded steel housing, providing a true hermetic seal. In general, a part of the heat generated in the compressor is dissipated to the environment, across the shell, and the rest of the heat is transported to the condenser by the refrigerant, resulting in an overall thermal balance. Compressor isentropic efficiency is directly affected by this thermal balance. The lower is the internal parts working temperature the better is the isentropic efficiency. The development of devices able to improve the thermal balance wills positively impacts energetic efficiency. A good understanding on heat transfer and temperature distribution of several components inside the compressor helps on establishing the proper geometry and materials selection [2]. It is necessary to ensure not only the thermal parameters of the cooler, but also reliability, high heat transfer rate, modern technology, low price, its universal * Corresponding author. Tel./fax: +375 172 84 21 33. E-mail addresses:
[email protected],
[email protected] (L.L. Vasiliev). 1359-4311/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2009.04.030
application, etc. The schema of a typical compressor is shown on Fig. 1. Some modern compact heat releasing devices usually generate large heat flows over very small areas. The main problem of their successful functioning is related to its cooling problem. To cool compressors one can use air and liquid coolers, as well as coolers designed on the principle of two-phase heat transfer flow (heat pipes) in a closed space [3,4]. 2. Heat transfer inside compressors The main modes of heat transfer considered inside the compressor casing are conduction and convection. Radiation heat transfer effect is not efficient due to the low component temperatures and relatively small temperature differences between the elements inside the compressor. As for the heat transfer between the compressor casing and its surroundings, convection and radiation are considered in [2] such as: The conduction heat transfer:
Q ¼ ki Ai DT=Dx
ð1Þ
The convective heat transfer is given by Newton’s law of cooling
Q ¼ hi Ai ðT i TÞ ¼ hi Ai DT
ð2Þ
As for radiation heat transfer, it is assumed to be that of radiation exchange between small surfaces within a larger enclosure. Thus, the heat transfer equation can be expressed as:
Q ¼ hi Ai ðT i T 1 Þ
ð3Þ T 1 ÞðT 2i
T 21 Þ.
with hr ¼ er1 ðT i þ þ Actually, the oil circulation inside the shell in one of the most important heat transfer mechanism, but it is not efficient enough in order to transfer all heat from hot parts to the shell, for example the cylinder head, due to its small heat transfer area.
F.C. Possamai et al. / Applied Thermal Engineering 29 (2009) 3218–3223
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Nomenclature Q Ki Ai hi hr
e r1 r d0 h
heat input (heat losses) thermal conductivity heat transfer area heat transfer coefficient or enthalpy radiation heat transfer coefficient emissivity of a surface Stefan–Boltzmann constant surface tension coefficient the mean hydraulic pore diameter the angle of wetting of the wick
Heat pipe application for thermal control of different compact heat exchangers, motors and compressors is very efficient to reduce the mass and cost of the product and ensure the passive, compact heat transfer solution. There are several possibilities for the appliance of heat pipes to cool compressors. One of the effective modes of the compressor thermal control is heat pipe application as heat exchanger between heat releasing components inside the compressor and oil, as well as the compressor and its surroundings. The most suitable heat pipes are: miniature heat pipes (mHP), micro heat pipes (MHP), heat pipe spreader, loop heat pipes and loop thermosyphons with capillary assisted evaporator. Actually, micro heat pipes (MHP) and miniature heat pipes (mHP) as small scale heat transfer devices are used to cool different kinds of Micro Electro Mechanical Systems (MEMS) and electronic components. It is also interesting to apply them as compressors cooling technology, as shown in Fig. 2. The modern mHP and MHP can be used to cool heat releasing elements of the compressor by applying the oil as a heat sink. The other types of mHP, loop heat pipes and loop thermosyphons, can be used to cool the oil inside the hermetic shell of the compressor by applying the atmosphere as a heat sink. The typical hydraulic diameter of MHPs is on the order of 10– 500 lm, the hydraulic diameter of mHPs is on the order of 2– 6 mm. The mHP theory is slightly different for conventional twophase systems due to a substantial interaction between the vapor flow and the liquid flow inside, and due to a small diameter of a vapor channel. In the assembling presented in Fig. 2, there are two heat pipes being used to transport the heat. Heat pipe (a) does the heat transportation from the compressor cylinder head that is the hottest compressor region to the oil reservoir in the bottom
K coefficient of permeability of the wick G porosity of the wick D diameter of the particles D coefficient of the tortuosity of the wick channels Dpl and Dpv friction pressure losses in liquid and vapor phases of heat pipe Dp|| and Dp\ pressure losses along and across of the heat pipe due to action of the gravity force Dpd pressure losses due to working fluid trapping by the wick in the MHP condenser
part. The heat pipe (b) transports the heat from the oil to the outside of the compressor. 3. Miniature heat pipes Miniature heat pipes (mHP) as vacant heat transfer devices to cool compressors may be interesting to be used as a flat, or flattened (1–3 mm thick) shape, 4–8 mm in width and 100–300 mm in length. They require an effective wick design, as they often have 90° bends. The effective thermal resistance of the heat pipe is one of the important parameters and is not constant, but the outcome of a large number of variables, such as heat pipe geometry, evaporator length, condenser length, wick structure and working fluid. The total thermal resistance of the heat pipe is the sum of resistances due to thermal conduction through the wall (heat pipe envelope) and the mHP wick, evaporation or boiling, axial vapor flow, condensation and conduction losses back through the condenser section wick and heat pipe wall. The maximum heat transport capability of the mHP is managed by several limiting factors which shall be observed when designing a heat pipe. Two main properties of the mHP wick are the pore size and the wick permeability. The pore size (radius) determines the fluid pumping pressure (capillary head) of the wick. The permeability determines the frictional losses of the fluid as it flows through the wick. Sintered powder wick has relatively high thermal conductivity, and it means a higher heat flux performance capability due to the enhanced fin effect and ‘‘micro heat pipe” effect, which the sintered powder provides in the evaporator region of the heat pipe [3–5]. It is also the unique heat pipe with the fastest heat recovery after the heat pipe crisis phenomena. But the efficient sintered powder wick needs to be optimized in order to ensure the high heat flux performance capability. The problem of the wick structure optimization is related to structural porous wick parameters: the particle size and its form, wick porosity, specific surface of porous wick, pore diameter, wick thickness [5]. MHP wick is a key element in mHP design, because the driving force for the working fluid circulation is the capillary pressure developed at the liquid vapor interfaces of the menisci in porous wick. For the successful operation of the heat pipe, the mHP wick has to provide capillary head, which is equal or exceeds all pressure losses in the system. The wick structure of mHP used in gravity field also has an impact on the radial temperature drop at the evaporator zone between the inner mHP hot envelope surface and the liquid– vapor surface. Thus, an ideal mHP wick should have a high level of permeability in radial and axial directions, as well a high capillary pressure difference between mHP evaporator and condenser zones.
Dpc ðDpsum Þ Dpl þ Dpv Dpgr þ Dpphc ;
Fig. 1. Cross section of hermetic refrigeration compressor.
ð4Þ
where Dpc means the capillary pressure losses which are the function of liquid surface tension forces, pore radii r pr and solid surface wettability:
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Fig. 2. Heat pipe application for cooling the heat loaded parts and the oil inside the hermetic compressor.
2r cos h r pr
60
It is important to note that the pressure loss due to phase change Dpphc is not significant for such kind of mHP and should be not taken into account. Thus, the pressure balance equation ought to be expressed as:
Dpc Dpl þ Dpv þ Dpjj þ Dp? þ Dpd
ð6Þ
where Dpl and Dpv are frictional pressure losses along liquid and vapor paths, Dpjj and Dp? are pressure losses along and across the heat pipe due to the action of the gravity force and Dpd are pressure losses due to working fluid trapping by the wick in the mHP condenser. The pressure losses in the mHP vapor path are evaluated by the Poiseuille equation:
!
Cðfv Rev Þlv
Dpv ¼
2ðrh;v Þ2 Av qv hfg
Leff Q
ð7Þ
where Leff = 0.5Le + La + 0.5Lc and values C and fv Rev are dependent on the vapor flow regimes. Following the Darcy equation, the pressure losses in the MHP liquid path could be found as:
Dpl ¼
ll
KAw hfg ql
Leff Q
ð8Þ
where K represents the wick permeability. Hydrostatic pressure head across and along the mHP could be accordantly expressed as:
Dpþ ¼ ql gdv cosu;
Dpjj ¼ ql gLsinu
ð9Þ
The capillary force by action of which the working fluid trapped in the wick on the mHP condenser zone could be determinate as:
Dpd ¼ 2rcosh=r h;d :
ð10Þ
As abovementioned, the strictest limitation in the mHP cooling capability is the capillary limit:
Qc ¼
Dpc Dpþ þ Dpk Dpd Dpv =Q þ Dpl =Q
ð11Þ
and, finally, by taking into account Eqs. (2)–(7), the Eq. (8) could be expressed in as follows:
Qc ¼
1 rc
80
ð5Þ
r 1 2r cos h ql gdv cos u þ ql gL sin u h;d Cðfv Rev Þlv ll Leff þ 2 KAw h q 2ðr Þ A q h h;v
v v fg
ð12Þ
fg l
For mHP Q depends on two capillary structure parameters – the mean hydraulic pore diameter and the inner diameter of the porous wick. To find the Qmax we need to analyze Eq. (9) in order to
Q max
Dpc ¼
40 20 0 0
50 100 150 Temperature, deg C
200
Fig. 3. Qmax as a function on T.
find the extreme function. Due to the temperature dependence of the thermo-physical properties of the working fluid, the maximum heat flow Qmax for different saturated vapor temperatures Tsat in the heat pipe transport zone will be different, Fig. 3. The heat pipe considered for the analysis is the copper/water mHP with copper sintered powder wick. The heat pipe has a length of 200 mm and a diameter of 4 mm. For different angles of heat pipe inclination to the horizon it shall be necessary to determine Qmax at the worst situation under the point of view of the heat transfer, when the heat pipe evaporator is disposed above the heat pipe condenser (inverted heat pipe disposition). So, the analysis shows the possibility to transport a heat flow Q = 50–60 W along the suggested heat pipe at the saturated temperature near 100 °C. 4. Experimental set-up The experimental set-up to determine mHPs (flat and cylindrical) parameters and mHPs predicting software was presented in [6], Fig. 4. The general objective of this set-up design was to determine the temperature distribution along the heat pipe for different heat loads, estimate the maximum heat transfer rate in any position, and evaluate the dependency between a heat pipe thermal resistance and heat dissipation. The cylindrical mHP, Fig. 5, has the following dimensions: L = 200 mm, Le = 70 mm, Lc = 85 mm, La = 45 mm. Outer diameter Dp = 4 mm, mHP wall thickness – 0.2 mm, diameter of the vapor channel Dch = 2 mm, size of the copper powder particles – 40 W, in vertical position >50 W, in vertical inverted position >20 W, with DT between the evaporation zone and the middle of the adiabatic zone