Flooded evaporator in small refrigeration- and heat pump systems. Ericsson S

Transcription

1 ID: 913 Flooded evaporator in small refrigeration- and heat pump systems Ericsson S Bubble Expansion Valve Corporation, Lonnrunan 54, Torslanda,Sweden info@bxv.se fax ABSTRACT This paper describes tests of a system to that obtain flooded evaporation in small refrigeration and heat pump systems. With flooded evaporation the heat transfer in the evaporator can be improved, which increases the evaporation temperature as well as the cooling and heating coefficients of the system. (Prof Dr.-Ing. Michael Kauffeld 30%) (Rizzvi Z and Dr Peter Heggs 16-48%) The system is based on three main newly developed system components: An automatic self-acting bubble expansion valve to secure a bubble free condensate. New technique to recirculate compressor oil from evaporator/accumulator, New strategy to recover the pressure drop losses by circulating the cold refrigerant several times between the condenser and the evaporator. Energy savings with this method has been evaluated and an improvement of 10% compared with a standard system using superheated gas strategy has been verified. Keywords Flooded evaporator, ejector pump circulation, bubble expansion valve, super heat. State of the art Background The mechanisms when the refrigerant boils and evaporates, forms the basis for the design and development of a new method of heat transfer solution which is presented in this paper. Standard cooling system efficiency is dependent on a thermostatic expansion valve to achieve a superheated gas. The temperature of the incoming coolant or air heats the evaporated gas to a preset overheating temperature level that is higher than the evaporating temperature. If overheating occurs in the vital evaporator, this results in reduced heat absorption. For example, if the expansion valve superheat is adjusted to 6K overheating to make a stable running of the system, the temperature difference between the incoming coolant to heat the superheated gas from the evaporating temperature must in most cases be above 6.5 K. Each degree change of the evaporation temperature level affects compressor efficiency by 3.5 to 4%. Project goals No gas overheating in the evaporator. More efficient mixing ratio between gas and liquid in the evaporator. Ensure an effective refrigerant circulation with an even distribution of coolant inside the evaporator. The oil return from evaporator to the compressor is made with heated oil and without interference of refrigerant liquid. Design of a system capable of operating at low condensation temperature. Design of a refrigerant control for flooded evaporating system and condensate sub-cooler without need of condensate-tank (normally used on high pressure float valve level system)

2 Figure 1: Overview of system with flooded evaporation. Technology Briefly the system works in the following manner. The evaporator is fed from an accumulator tank, which also acts as a liquid separator. After the evaporator, a mixture of liquid and gas return back to the same accumulator. The compressor suckss saturated gas from the top of the receiver. The refrigerant is circulated through the evaporator by an ejector pump, driven by the pressure difference between condenser and the evaporator. The refrigerant liquid flow to the evaporator is controlled via a bubble expansion valve. The signal to the valve is created by the flow through choke 1 and is controlled by the refrigerant state at the condenser exit. If the condensate after the condenser contains gas bubbles, the mass flow through the restriction decreases. The flow through choke 2 is not affected by the state before choke 1. If bubbles pass through choke 1 the pressure decreases between choke 1 and 2, the pressure signal to the valve drops and the valve closes. An ejector effect drives the flow from the accumulator back into the evaporator via the ejector pump.

3 Compressor oil is pumped by ejector pump pressure and returns back to the compressor through the condensate sub cooler. Figure2:: Overview of system with flooded evaporation liquid iquid / Gas proportion affect heat transfer.

4 Figure3: Overview of bubble control system with flooded evaporation Tests Two identical heat pumps were used to evaluate ejector pump circulation and precooled liquid. The first phase of the project verified by accurate measurements that the two heat pumps had the same performance. One of the heat pump units was then equipped with new developed components as the above principle sketch describes. Finally comparative measurements were made on the modified heat pump and the unmodified one. Tested units Both of the heat pumps had a nominal heat capacity of 8 kw and used R404a as refrigerant. The two heat pump units were fitted with: Evaporator SWEP B25x24= 1, 39 m2 with a long thermal length which is adapted to use for direct expansion system. Condenser SWEP B25x40. Copeland Compressor Scroll type ZB21KCE. Thermostatic expansion-valve Danfoss TUAE carefully set to 6K work superheat value, at brine in temperature/ out radiator water 0 / 35 C. The brine for the evaporator was water with 35 weight percent of ethylene glycol.

5 Test results A new system with bubble expansion valve, ejector pump circulation, oil return system, subcool- condensate, has been tested and verified and offers 3.5K less temperature differences in the evaporator heat exchanger, than in a conventional system. This resulted in 10% lower power consumption. Improvement 3,5K evaporating temperature * C 0 Glycol in to evaporator / Water out from condenser Evaporating temperature /45 * Bubble -9,3 DX system -12,43 0/35 * +5/35 * -4,95-0,44-8,41-3,9 Figure 3: Evaporating improvement is 3,5K higher temperature Logarithmic mean temperature difference Evaporating LMTD K /45 * Bubble 2,84 DX system 6,05 0/35 * +5/35 * 3,14 3,44 6,82 7,07 Figure 4: improvement of logarithmic mean temperature difference

6 Operating levels -5/45 0/35 5/35 unit System DX BXV DX BXV DX BXV WW To radiator 45,1 45,1 34,4 34,8 35,0 35,1 C WWDt radiator 7,83 8,05 10,15 9,84 11,63 11,69 K Warmwaterflow l/h Brine to evaporator -5,3-5,3 0,0 0,0 5,1 5,2 C Dt brine 2,0 2,1 3,0 3,1 3,5 3,8 K Brineflow l/h Evaporating pressure 3,96 4,41 4,55 5,11 5,29 5,92 bar Evaporating temperature -12,4-9,3-8,4-5,0-3,9-0,4 C Returngas temperature -6,0-3,1-1,1-0,1 3,9 3,6 C Hotgas temperature 68,5 66,2 53,5 52,0 53,4 50,0 C Liquid before expvalve 36,8 16,9 24,9 11,0 24,7 11,5 C Logarithmic mean temperatur of Evaporator 6,05 2,84 6,82 3,14 7,07 3,44 K Condensereffect W Increase condenser effect W Cond increase in % 11,4 9,5 11,5 % Table 1: Test results. Direct expansion system (DX) and Bubble expansion valve system (BXV) CO CLUTIO S A bubble expansion valve system has been shown to reduce the temperature difference by more than 50% in comparison with a conventional system with a direct expansion valve. At the same time the heat transfer effect in the evaporator has improved, which means that the efficiency of the heat transfer, expressed as average heat transfer rate increased from 616Wm 2 K to 1579Wm 2 K or even to 245 % after the modification to a flooded evaporator system. The improved use of the evaporator surface may be attributed to one or more of the following factors: The need for overheating has been eliminated, resulting in a significantly higher heat transfer rate and smaller temperature differences in the evaporator. (Prof Dr.-Ing. Michael Kauffeld) The need of superheating is eliminated which may reduce the sensitivity of the uneven distribution of refrigerant flow between the parallel refrigerant channels of the plate heat exchanger. Distortion is considered to be a common cause of loss of performance for this type of evaporator. (Stefan S. Jensen)

7 The proportions between the refrigerant gas and liquid in the evaporator, is a higher amount of gas in the direct expansion than in the flooded evaporator system. Results reported in the scientific literature show that the heat transfer rate at flooded evaporator in many cases has a maximum for gas/liquid proportion around 70/30. (T.N.Tran, M.W.Wambganss) (Raja Balakrishnan,Mohan Lal Dhasan) (C.K Rice Ph.D) Mass flow rate of refrigerant in the flooded evaporator is higher because of the recirculation of the refrigerant. Higher mass flow generally increases the heat transfer rate with flooded evaporator. (T.N.Tran, M.W.Wambganss) At parallel flow couple of the evaporator with a small glide medium and with significant pressure drop, a more even temperature profile through the evaporator is obtained. Moreover, the parallel flow couple achieves higher heat transfer at the refrigerant inlet, which affects the heat transfer by stronger bubble formation at the evaporator inlet area. the refrigerant is evaporated more effective with increasing temperature-differences at the evaporators inlet and liquid / gas proportion decreases faster (motive see curve fig 2 ) Positively surface concentration of oil in the evaporator will be changed when switching from a direct expansion system to a flooded evaporation system. This change affects the thermal transition positively, not only in the superheating area of the evaporator. (J. Fei, W. Y. Hu, Z. M. Wang) Heat exchangers and compressor had original configuration during the tests.the flooded system showed better cooling effect (+16 %) and heating effect (+10%) than the original model. A completely fair comparison had demanded a smaller electric motor and compressor model for the bubble expansion valve system. This had increased the energy efficiency ratio difference even more in favour of the bubble expansion valve system. (Ericsson Svenning Palm Björn (measured data) Bilaga 1 korrigerad kyleffekt = cooling effect) References Prof Dr.-Ing. Michael Kauffeld Trends and Perspectives August 2007, Beijing, China 22nd IIR International Congress of Refrigeration (ICR2007) Supermarket refrigeration systems have an energy savings potential Flooded evaporator more at moderate costs 30% Energy savings. Rizzvi Z and Dr Peter Heggs 2003 Defrosting refrigerators 21th International Congress of Refrigeration, Washington, USA, IIR/IIF: paper ICR0660. Flooded Circuit cop % Stefan S. Jensen, B.Sc.Eng. MIEAust Scantec Refrigeration Technologies Pty. Ltd. Brisbane, Queensland, Australia Circuiting errors are most forgiving in liquid 6 IIAR 2009 Technical Paper # IIAR Industrial Refrigeration Conference & Exhibition, Dallas, Texas overfeed applications and potentially most disastrous in dry expansion feed cooling coils.

8 T.N.Tran, M.W.Wambganss, Boiling Heat Transfer in VCompact Heat Exchangers D.M. France 1994 San Fransisco (Forced convective boiling mass quality and mass flux) The heat transfer coefficient is dependent of mass flux and increased with quality. J. Fei, W. Y. Hu, Z. M. Wang, et al. Proc. 22nd int. Congr. Refrig., Beijing/C. R. Experimental study on pool boiling heat transfer of R-134a/oil mixtures The heat transfer coefficients with 100 ppm increase to % Raja Balakrishnan, Mohan Lal Dhasan 2008 Heat transfer Thermal Science Year 2009 Refrigerant composition on the heat transfer coefficient are investigated experimentally. C.K Rice Ph.D Oak Ridge National Laboratory The effect of void fraction 1987 Ashrae Winter meeting NY. Design consideration of flow control and charge inventory interactions mass quality Ericsson Svenning Palm Björn Flödande förångare i små värmepumpssystem. EFFSY2 (In Swedish) Ericsson Svenning Bubble Expansion Valve AB (BXV AB) Lonnrunan Torslanda, Sweden Mail: info@bxv.se Phone: Aim of the congress To bring together the most important and prominent industrial and academic stakeholders from all refrigeration fields To present and review the principal technology innovations and research findings accomplished over the last 4 years since the previous congress in Beijing, China, in 2007 To debate current hot topics, issues and future trends ANSWERE Dear Sirs Thank you very much for your reviews about flooded strategy paper. I have tried to do improvements by: i) Explanation about difference from use of hp float valve system / subcool the condensate for ejectorpump and liquid / gas optimise. ii) I have also done some changes for Abstract tests of and for References and explanation about Conclution see parallellflow text.

9 Question to the author without effect on paper acceptance: the flooded system with ejector could also work with ohter expansion valves, e.g. with high pressure float valve. Has the bubble expansion valve any other benefits? See Project goals, motive is now described in paper. Answere It is not so easy to subcool the condensate with float valve without a H.P receiver. paper version 1 - reviewer 765: Author did not prepare paper as required by the organizer. No reference is included in the work. This does not concern only the introduction (state of the art or as author call it background), but also results or statements. It is not clear how certain values were obtained (experimentally, calculation or from certain literature). Otherwise from the technical point of view paper shows interesting work Comments Author did not prepare paper as required by the organizer. No reference is included in the work. This does not concern only the introduction (state of the art or as author call it background), but also results or statements. It is not clear how certain values were obtained (experimentally, calculation or from certain literature). Otherwise from the technical point of view paper shows interesting work I hope that this will explain the technology better and straightens out some questions, please inform me if you think that i should do something more / else Thank you for yours very competent examinations. Sincerely Bubble Expansion Valve AB Svenning Ericsson M.D BXV