Properties of New Low GWP Refrigerants

Transcription

1 Kungl. enisa Högsolan Institutionen för Energiteni Stocolm ttp:// Properties of New Low GWP Refrigerants Slutrapport till projet nr. P4 inom energimyndigetens program Effsys

2 Contents. Abstract 3. Sammanfattning 3 3. Nomenclature 3 4. Introduction 4 5. Evaluation of alternative refrigerant wit low GWP 4 6. HFO-34yf p-diagram 6 7. HFO-34yf cycle data 7 8. Pressure drop and eat transfer in plate eat excangers 9 9. Experimental data 0. Discussion of results 3. Suggestions for future wor 3. References 3 List of figures. Saturation pressure temperature curves 5. Volumetric refrigerating effect vs. evaporation temperature at = 40 C and no supereating or subcooling 6 3. Pressure entalpy diagram of HFO-34yf 7 4. Evaporator pressure drop 5. Evaporator eat transfer 6. Condenser eat transfer List of tables. Experimental data for drop-in tests at standard conditions 6. HFO-34yf cycle data 8 3. Evaporator figures of merit 0 4. Calculated ratios of pressure drops and eat transfer coefficients of HFO-34yf to R34a for similar evaporating effects 0 5. Condenser figures of merit 0 6. R34a experimental data 7. HFO-34yf experimental data 8. Evaporator experimental results at t = 4 C 9. Condenser experimental results at t = 4 C 3

3 P4 Properties of New Low GWP Refrigerants Sad Jarall P. D E, Energiteni, KH, Stocolm Abstract ermo-pysical properties of low global warming potential (GWP) refrigerants were obtained to study and compare teir performance wit R and R34a. eoretical cycle data, pressure drop, and eat transfer of HFO-34yf were calculated and compared wit tose of R34a. Drop-in tests were carried out using two refrigeration units of plate eat excangers: one for R and R90, and te oter for R34a and HFO-34yf. e results were compared to tose concluded teoretical. (Sammanfattning: ermodynamisa egensaper för öldmedier med låg påveran på den globala uppvärmningen (GWP) ar insamlats för att jämföras med R oc R34a. eoretisa cyeldata, trycfall oc värmeöverföringstal för HFO-34yf ar beränats oc jämförts med motsvarande för R34a. Drop-in tester utfördes i två olia ylsystem med plattvärmeväxlare; ett för R oc R90 samt ett för R34a oc HFO- 34yf. De experimentella värdena utvärderades sedan mot de teoretist framtagna.) Nomenclature COP d Coefficient of performance of cycle Condenser outlet entalpy wit no subcooling, J/g,is Outlet entalpy after isentropic compression of supereated vapor, J/g,is,o Outlet entalpy after isentropic compression of saturated vapor, J/g Compressor inlet entalpy of saturated vapor, J/g Compressor inlet entalpy of supereated vapor, J/g H eat transfer coefficient m r Measured refrigerant mass flow rate, g/s m w Water mass flow rate, g/s p Condenser pressure, bar p Evaporator pressure, bar p red Reduced pressure = actual pressure/critical pressure Q (r) Condenser effect based on te refrigerant data, W Q (w) Condenser effect based on te water data, W Q (r) Evaporator effect based on te refrigerant data, W Q v Volumetric evaporator effect based on te refrigerant data, J/m 3 s Entropy, J/g K, sat Condenser saturation temperature, C (in) Condenser inlet temperature, C (out) Condenser outlet temperature, C Condenser outlet temperature of saturated liquid, C 3

4 , sat Evaporator saturation temperature, C is Compressor discarge temperature, C Copressor inlet temperature, C b(in) Evaporator brine inlet temperature, C b(out) Evaporator brine outlet temperature, C ex emperature before te expansion valve, C s Condenser outlet temperature of subcooled liquid, C w(in) Condenser water inlet temperature, C w(out) Condenser water outlet temperature, C Room temperature, C UA Overall eat transfer coefficient multiplied by te surface area, W/ C v Specific volume at compressor inlet, m 3 /g Wv Volumetric compressor wor, J/m 3 η cd Δp Cycle Carnot efficiency pressure drop (bar) Introduction e influence of fluorocarbon refrigerants on te global warming as prompted countries trougout te world to pass legislation preventing or pasing out te use of suc refrigerants. In EU regulations ave already been enacted to replace R34a in mobile air conditioning systems by a new low global warming potential (GWP) refrigerant, suc as HFO-34yf wic as very similar termodynamic properties as R34a in comparison to oter environmentally less armful candidates. e purpose of tis project is to collect information about alternative low GWP refrigerants, suc as HFO-34yf, R5a, and propane (R90), and investigate te feasibility of using tem as replacements for previously used refrigerants. e selected refrigerants ave te advantages of providing te same cooling effects, efficiencies, durability, and oter performance factors as te original refrigerants do. No modifications are necessarily needed to te existing system. R90 may replace R in refrigeration installations wit low evaporation temperature if safety measurements are to be considered due to its ig flammability. R5a or HFO-34yf may replace R34a in small, low pressure refrigeration units. HFO-34yf is favorable due to its milder toxicity and flammability in comparison to R5a. Evaluation of alternative refrigerant wit low GWP Wen selecting low GWP refrigerant, te first ting to investigate is its vapor pressure curve. It as to be similar to tat of te original refrigerant if te original compressor is to be used, as fluids wit similar vapor pressure curves will give similar capacities and require similar size of te compressor motor. Fig. presents te saturation pressure vs. temperature of HFO-34yf, R90, R5a, R77 as well as R34a and R. 4

5 30 5 Pressure (bar) 0 5 HFO-34yf R34a Propane R5a R R emperature ( o C) Fig. Saturation pressure vs temperature e figure sows tat at te same temperatures R90 as te same or lower pressures tan tat of R. At condensing temperature of 40 C R90 as 0.7% less pressure tan tat of R. Ammonia as similar pressure curve to R and from tis respect it can be used as an alternative to R. Yet, it is less favorable tan propane as a replacement. It reacts wit copper and bronze in te presence of a little moisture wic demands te replacement of copper and bronze parts of te system wit iron and steel parts. It as a ig latent eat but low density. It is also irritating to breate, somewat flammable, and wit te proper proportions of air may form an explosive mixture, altoug suc an incident is rarely appened. HFO-34yf as similar vapor pressure curve as tat of R34a, wile R5a as lower pressure tan tese two refrigerants. However, HFO-34yf is considered more frequently tan R5a as a replacement for R34a as it poses less toxicity and flammability. e refrigerants cooling, or eating, capacities tat can be acieved wit a given compressor and a given swept volume rate can easily be calculated in detail. Comparison between te refrigeration capacities for different refrigerants can be done by comparing te teoretical volumetric refrigerating capacity at defined evaporation and condensation temperatures. Fig. sows te teoretical volumetric refrigerating capacities of different refrigerants versus te evaporation temperatures at condensation temperature of 40 o C and no supereating or subcooling. 5

6 Q v (J/m 3 ) HFO-34yf R34a R5a Propane R R ( o C) Fig. Volumetric refrigerating effect vs. evaporation temperature at = 40 C and no supereating or subcooling e figure indicates tat propane as less cooling capacity tan R by 4.8% and 6.3% at evaporating temperature of -30 and 0 C, respectively. It also sows tat HFO-34yf as less volumetric cooling effect tan R34a by 7.%. At condensing temperature of 50 C te percentage of propane increases by approximately % wile tat of HFO- 34yf increases by 3%. Drop-in tests for some different standard conditions performed at KH lab using a refrigeration unit designed originally for R gave te results sown in table below. able Experimental data for drop-in tests at standard conditions ( C) ( C) Super eat ( C) Q R (W) Q R90 (W) e experimental data sow less difference in te cooling effects tan tat indicated by Fig. due to te influence of te supereat. HFO-34yf -log(p)-diagram HFO-34yf p-diagram is drawn using RefProp wit reference state of = 00 J/g and s = J/g.K for saturated liquid at 0 C. e density data are converted to specific volume data. 6

7 7 Fig. 3 Pressure entalpy diagram of HFO-34yf HFO-34yf cycle data Cycle data are calculated, based on fluid properties taen from RefProp, for te teoretical cycle wit isentropic compression and no subcooling of liquid or supereating of vapor. e data can be used to predict te basic performance of a new refrigerant and compare it to oter nown refrigerants. Influence of subcooling and supereating on te calculated volumetric cooling effect and te coefficient of performance is presented in te form of te factors y, y, y 3, and y 4, wic are defined as: y (factor defining te influence of (external) subcooling on Q v and COP d ) s s = ' 00 ) ( y (factor defining te influence of te internal supereat on Q v ) ' ' 00 ) ( v v = y 3 (factor defining te influence of te internal supereat on COP d ) ',,, ' 00 ) ( o is is = y 4 (factor defining te influence of te external supereat on COP d ),,, 00 ) ( is o is =

8 able HFO-34yf cycle data p p Q v w v y y y 3 y 4 ( C) ( C) (bar) (bar) p /p is ( C) (J/m 3 ) (J/m 3 ) COP d η cd (%per C) (%per C) (%per C) (%per C)

9 e table indicates tat HFO-34yf as less pressure ratio and discarge temperature tan R34a, wic is an advantage, but it as less COP d and Carnot efficiency wic is a disadvantage. See [] for R34a cycle data. At t = 40 C and t = -0 and 0 C, HFO- 34yf as a pressure ratio less by 9.5% and 5.%, and discarge temperature by 6.3 and 3. C, respectively. At t = 40 C and t = -0 and 0 C, HFO-34yf as less COP d by -7.4% and 4.5% and η cd by 7.3% and 4.4%, respectively. e table also indicates tat HFO-34yf is significantly more influenced by subcooling and supereating tan R34a. At t = 40 C and t = -0 and 0 C, HFO-34yf as iger y and y by nearly 30% and 50%. Pressure drop and eat transfer in plate eat excangers Pressure drop and eat transfer in te evaporator and condenser may be predicted if all te transport properties of te refrigerant and te secondary coolants, mass flow rates, and te construction of te eat excangers are nown. If adequate information is not available or a general prediction is needed, a comparison between refrigerants performances under te same conditions may be used. is can be done by separating te independent properties of te refrigerant in correlations for pressure drop and eat transfer to form wat is called figures of merit. o compare te pressure drop in te evaporator wit HFO-34yf to R34a, Blasius correlation for pressure drop in turbulent single pase may be used. According to tis ρw L correlation Δp = f, were ƒ =0.36 Re -0.5 d Substitute te velocity in terms of te mass flow rate m r, and ten m r in terms of Q / fg, te Blasius equation for different refrigerants and similar Q, L, and d yields: 0.5 μ Δ p = const = const x FOM.75 Δp, were FOM Δp is figure of merit for pressure drop. ρ fg Figure of merit for eat transfer may be calculated using Dittus-Boelter equation for turbulent single pase eat transfer: Nu = 0.03 Re 0.8 Pr 0.4 wic yields, FOM SPH = Pr 0.4 /( fg μ) 0.8 Boiling figure of merit may be derived from Cooper s pool boiling correlation: 0.-0.log H pb = 55 p Rp red 0 (-log 0 p red ) M -0.5 q 0.67 aing te surface rougness R p as μm, te figure of merit for boiling will be FOM pb = p 0. red (-log 0 p red ) M

10 Condensation figure of merit can be derived from Nusselt s film teory applied for laminar condensation in vertical tube. Average condensation eat transfer c = const ( 3 l ρ l (ρ l - ρ g )/μ l Γ l ) /3, were Γ l is te condensation rate at lengt l divided by te inside tube circumference. Γ l = Q cond / fg πd i Substitute tis value into te equation of te eat transfer coefficient and consider ρ l NNρ g and Q cond constant for te two compared refrigerants: c = const l (ρ l fg /μ l ) /3 = const FOM cond Ratio of te pressure drops and te eat transfer coefficients of HFO-34yf and R34a may be predicted by dividing teir relevant figures of merit. Figure of merit for pressure drop is calculated for vapor. e tables below present te calculated data. able 3 Evaporator figures of merit HFO-34yf R34a FOM SPH liquid FOM SPH vapor FOM pb FOM Δp FOM SPH liquid FOM SPH vapor ( C) FOM Δp FOM pb -0.44E E E E E E E E E E E E E E E E able 4 Calculated ratios of pressure drops and eat transfer coefficients of HFO- 34yf to R34a for similar evaporating effects Liquid Vapor ( C) Δp ratio ratio ratio pb ratio able 5 Condenser figures of merit ( C) HFO-34yf FOMcond R34a FOMcond fc ratio

11 Experimental data A refrigeration rig wit R34a and a maximum cooling capacity of 3W is used as a drop-in unit. It as a plate evaporator tat is eated by a controlled brine loop, and a plate condenser wit water as a secondary coolant. ests were run, firstly wit R34a followed by HFO-34yf, at condenser temperature of 4 C and evaporating temperature ranging between -8 and 6. Overall eat transfer coefficient for te condenser is estimated only for saturation condition wile for evaporator a little supereat was considered. Results of te experimental wor are presented in te figures and tables below. able 6 R34a experimental data sat Q (in) p sat (in) (out) p (in) p (out) Δp (in) exp (out) w(in) w(out) (g/s) b(in) b(out) (g/s) (W) m w m r able 7 HFO-34yf experimental data p p Δp p sat (in) (in) exp (out) w(in) w(out) (g/s) b(in) b(out) (g/s) (W) sat Q (in) (out) 8.3 (in).4 (out) m w m r 0,06 0,05 y = -0,003x + 0,0338x - 0,09 Pressure drop (bar) 0,04 0,03 0,0 Poly. (HFO-34yf) Poly. (R34a) y = -0,0004x + 0,094x - 0,005 0, ,5,5,5 3 Cooling effect (W) Fig. 4 Evaporator pressure drop

12 0,35 y = -0,0058x + 0,07x + 0,0949 0,3 UA (W/K) 0,5 0, Poly. (R34a) Poly. (HFO-34yf) y = -0,0359x + 0,7x - 0,08 0,5 0, 0,5,5,5 3 Q (W) 0,5 Fig. 5 Evaporator eat transfer 0, y = -0,0045x + 0,0974x - 0,08 UA (W/K) 0,5 0, Poly. (R34a) Poly. (HFO-34yf) y = -0,06x + 0,566x - 0,06 0, ,5,5,5 3 Condensing effect (W) Fig. 6 Condenser eat transfer able 8 Evaporator experimental results at t = 4 C Δp R34a Δp R34yf UA R34a UA R34yf Δp UA Q (W) (bar) (bar) (W/K) (W/K) ratio ratio

13 able 9 Condenser experimental results at t = 4 C Condensing effect (W) UA R34a UA R34yf UA ratio (W/K) (W/K) Discussion of results Experimental results sow tat HFO-34yf as iger pressure drop and lower eat transfer coefficient tan tose predicted, particularly at lower evaporating temperature. is is due to te iger supereat encountered at tese temperatures wic accelerates te refrigerant and practically decreases te boiling eat transfer area. Condenser data sow tat HFO-34yf gives less increment in eat transfer coefficient tan R34a at iger eat capacity. is can be explained by te fact tat te teoretical prediction is based only on a film condensation correlation, ignoring by tat te convective eat transfer contribution. At significant condenser eat rejection te iger refrigerant mass flow rate induces greater convective eat transfer, and since te two refrigerants ave different properties related to te convective eat transfer, te variations of teir overall eat transfer coefficients wit condenser effects will be different. Suggestions for future wor Acquire more compressor data wit te new refrigerants to investigate te practical COP of te refrigeration system and te efficiencies and reliability of te compressor. References. Eric Granryd, Ingvar Erot, Per Lundqvist, Åe Melinder, Björn Palm, Peter Rolin Refrigerating Engineering KH, Energiteni, Stocolm Sweden. RefProp: fluid termodynamic and transport properties NIS, version 7 3. Björn Palm Hydrocarbons as refrigerants in small eat pump and refrigeration systems Direct Science, available on line at: 4. omas J. Lec, PD Development and Evaluation of Hig Performance, Low GWP Refrigerants for AC and Refrogeratopn Dul,Pont Fluorocemicals R&D, Wilmington, Delaware, 9880,USA 3