Heat Transfer Measurements with Ice Slurry

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Kim G. Christensen and Michael Kauffeld Danish Technological Institute DTI Energy, Refrigeration and Heat Pump Technology _ ^-uS) Teknologiparken, DK-8000 Aarhus, Denmark 4 0? OOC^L

1. Summary

In order not to increase energy consumption by introducing a secondary refrigerant, the use of ice slurry may be very interesting. Heat transfer coefficient and pressure drop for ice slurry flowing in a horizontal pipe have been measured. The stainless steel pipe (ID 21.6 mm) is heated by condensing R134a outside the pipe. The heat input is supplied by an electrical heater evaporating R134a. The heat flux is assumed to be constant over the whole surface and the heat transfer coefficient is calculated using the temperature difference and a known heat transfer coefficient for condensing R134a. The measurements show high heat transfer coefficients with melting ice slurry. The heat transfer coefficient increases with increasing ice concentration and increasing velocities. A correlation for calculating the heat transfer coefficient is developed.

2. Introduction

More and more refrigeration systems are designed with secondary loops, thus reducing the refrigerant charge of the primary refrigeration plant. In order not to increase energy consumption by introducing a secondary refrigerant, alternatives to the well established single phase coolants (brines) have to be evaluated.

One prospective alternative is ice slurry - a mixture of water, a freezing point depressing agent (antifreeze) and ice particles as melting secondary refrigerant. Some of the greatest advantages by using ice slurry have been known for years: high transport energy density and practically constant temperature during heat exchange. But other properties of ice slurry have to be examined to evaluate ice slurry as a secondary refrigerant.

A thorough research and development program initiated 3 years ago has resulted in a well equipped ice slurry laboratory placed at the Danish Technological Institute (DTI). Figure 1 shows the cascade plant schematically. The plant uses CO, and NH3 as primary refrigerants in the low temperature loop (-30°C-> -5°C) and high temperature loop (-5°C-> 30°C) respectively. A storage tank separates the two plants and is used to accumulate ice during the night. Ice slurry is used for cooling at -5°C and evaporating C02 is used for cooling at low temperatures.

IIR/IIF International Conference, HR Commission Bl, November 1997 Heat Transfer Issues in Natural Refrigerants DISCLAIMER

Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. m

Ice storage

Ice/Water/Ethanol

Low temperature

Cooling Machine room

Figure 1: Use for NH3, CO, and ice slurry in a cooling plant with storage tank

Aspects of practical use of ice slurry have been examined, and this paper will present a description and the results of the heat transfer and pressure drop measurements carried out. An attempt to calculate pressure drop and heat transfer coefficient will also be presented together with experience in a commercial ice slurry cooled shell-and-tube condenser working with CO,.

3. Experimental equipment

The total test plant consists of a NH3 part, a CO, part, an ice generator (IG), a storage tank, a heater and a test loop (Figure 2).

Figure 2: Test plant.

2 The NH3 plant works with a Sunwell IG as a pump circulation-type evaporator controlled by a high pressure float-type expansion valve. The condenser is a water-cooled shell-and-tube condenser.

The C02 plant is equipped with a flooded evaporator, a separator and a shell-and-tube condenser. The injection of refrigerant is controlled by a manual expansion valve. An agitator is mounted in the ice slurry storage tank, hence the ice slurry has a constant ice concentration in the entire tank.

3.1 Heat transfer measurements with ice slurry

Ice slurry from the IG is circulated to the test loop (Figure 3) and through a pipe mounted horizontally in a vessel containing R134a. The ice slurry is heated by condensing R134a outside the stainless steel pipe (ID 21.6 mm). The heat input is supplied by an electrical heater which evaporates R134a. The heat flux is assumed to be constant over the entire surface of the tube and the heat transfer coefficient is calculated using the temperature difference and a known heat transfer coefficient for condensing R134a.

Massflow ---- —1X1 Massflow R134a

DS 540 Dn20, DIN 2440 Sight glas (Plexiglas/Vacuum) OD 21.3 mm

Figure 3: Test loop.

The test tube has the following dimensions: Outside diameter OD: 26.9 mm Inner diameter ID: 21.6 mm Length L : 1 m Material Stainless steel, seamless, ANSI 304, DN20, DIN 2440

On the inlet and outlet of the vessel a Coriolis mass flow meter is placed. The temperature of ice slurry at the inlet and outlet and the condensing pressure of 134a is measured. The heat input is measured by a power transducer.

Table 1: Instrumentation in the test loop.

Instrumentation Type / make Place Accuracy Temperature PT-100 Inlet/outlet of vessel, ice slurry side 0.05 °C (abs) Pressure Gauge transducer Vessel, R134a-side 0.02 bar (abs) Pressure, difference Gauge transducer Pressure loss vessel, ice slurry side 0.001 bar (abs) Mass flow/ density Danfoss, Mass 2100 Inlet/outlet of vessel, ice slurry side 0.15 % Power Power transducer Electrical heaters in vessel 0.5 %

3 3.2 COz condenser with ice slurry The condenser is a shell-and-tube condenser with condensing C02 outside the tubes. The plain tubes are placed triangularly with a pitch of 40 mm. The condenser is originally designed ID= 17 mm for 150 kW, thus having 102 tubes in 4 OD = 20 mm passes on the secondary side. The capacity of the IG and the ammonia plant is 50 kW, thus the C02 plant will run on part load (25 % - 50 %). To compensate for this 38 tubes are blocked.

Modified condenser: Figure 4: C02 condenser - tube configuration. Number of tubes 64 Length of tubes 1.95 m Passes, secondary side 4 Number of vertical columns of tubes 22 (^column) Number of tubes in a vertical column ~ 3 (average) O^tube.column)

Table 2: Instrumentation on the CO, condenser.

Instrumentation Type/make Range Accuracy Pressure transducer (abs) Danfoss ASK 32R -1 - 59 bar 0.1 bar Pressure transducer, difference Mobrey 0 - 500 mbar ±0.5 mbar (0.1 %) Temperature PT 100 - ± 0.02 °C Mass flow meter Danfoss 2100 0 - 3000 kg/s ± 0.15 % of max.

4. Experimental procedure

When the system is in steady state operation, data are collected for about 10 min. for each experiment. The flow of ice slurry is especially difficult to stabilise due to a small partial blockage of the flow. However, during the measurements the variation of the flow has been less then 5 %.

4.1 Heat transfer measurements with ice slurry The heat transfer coefficient for condensing R134a was first found by running tests with water at high velocities through the tube. A model for condensing R134a was developed.

0)C 4000 j u 5= 3000 -• o £5" a. =6016.4- Re^™ U) 2000 -- 14

Figure 5 Heat transfer for condensing RJ34a

4 Assuming that all the power from the electrical heater is transferred to the ice slurry (the vessel is well insulated) and using the developed correlation for condensation of R134a, it is now based on measurements possible to calculate the heat transfer coefficient for ice slurry. Tests were carried out in the following area: Ethanol weight concentration: 10 % Ice concentration: 0-30 % Heat flux: 6-14 kW/m2 (3-8 K) Velocities: 0.7 - 2.5 m/s

4.2 Measurements on an industrial condenser with ice slurry At first the insulated condenser was tested with pure water to obtain some reference measurements. Using the known heat transfer coefficients on the water side, the heat transfer coefficients with condensing C02 was found. After this, tests with ice slurry were carried out.

Ethanol weight concentration: 10 % Ice concentration: 0-30 % Heat-flux: 4-10 kW/m2 Velocities: 0.7 -1.5 m/s

5. Data processing

Based on all collected data over a period of 10 min., mean values are calculated for all data points. Based on the measured pressure the condensation temperature is determined. Together with the temperatures of the ice slurry at the inlet and outlet, the log mean temperature difference can be calculated:

(T.-T„,) — (T.-Tm)

Tc ~Tb2 J

The ice concentration of the ice slurry coming to the vessel is determined by ice concentration in the storage tank, and the velocity is adjusted by a variable speed pump (centrifugal type). The load (Q) is measured, and now the overall heat transfer coefficient can be isolated.

Q = K-A-dTln

It is now possible to calculate the heat transfer coefficient on either side knowing the heat transfer coefficient on the other side.

K-A = (3)

The accuracy of the measured heat transfer coefficient depends strongly on the accuracy of the correlation to determine the heat transfer coefficient on the other side. However, it is always important that the known heat transfer coefficient is larger than the one that has to be calculated.

It is now possible to convert the collected data into heat transfer coefficients and carry out a comparison with calculated data from various models.

5 6. Calculations

6.1 Heat transfer with melting ice slurry Referring to 111, 111, /10/, /13/, /15/ the thermodynamic and thermophysical properties for ice slurry can be calculated. The viscosity and the pressure gradient for a suspension are always greater than that for the carrier fluid and the difference increases as the degree of heterogeneity increases. As particle size or solid density decreases or solids loading and/ or velocity increases sufficiently, the ice crystals remain suspended in more-or-less homogenous distribution. As in this case the particles are small (10 - 100 pm), the velocity is kept higher than the minimum deposit velocity (0.2 m/s, ID = 16) and at the same time the solids loading is under 35 %, it is assumed that the suspension may be adequately described as a single uniform continuum (pseudo homogeneous) with properties which are a function of both fluid and solid characteristics.

The definition of the Reynolds number is;

(4)

The transition between laminar and turbulent flow may not be sharp and difficult to determine. Here Re = 2100 is used as a realistic guess.

Laminar flow (Re < 2100) The velocity distributions in laminar flow depend on the physics of the fluid. Ice slurry with more than 15 % of ice behaves no longer like a Newtonian fluid, but more like a Bingham fluid. In this case the shear stress exerted in the z-direction on a fluid surface of constant r by the fluid in the region of lesser r can be expressed by 191:

T A Non-Newtonian (pseudoplastic) Bingham-fluid r

■-----Newtonian fluid

» z Non-Newtonian

—k— ------►

du/dy

Figure 6: Bingham fluid vs. Newtonian fluid Figure 7 Flow profde for a Bingham fluid

du (5)

The equation (5) reduces to the expression for a Newtonian fluid for T0 = 0.

6 The flow profile of a Bingham fluid is not only parabolic, but contains also a “plug flow region ” in the central part of the tube. The shear stress closer to the wall is therefore greater compared to a Newtonian fluid for the same average velocities 191. u(r) = - J-. ^. [(r22 - r,2) - 2 • r, • (r2 - r)] 4 p dz Sr^,r,=2.i (6 )

dz Pressure loss measurements have been made at Danfoss ’ Refrigeration Laboratory on different tube sizes. Water containing 10 % ethanol was used and ice concentration velocities were varied. From these measurements it is possible to find the critical shear stress for ice slurry. Data from measurements on a tube with an inside diameter of 16 mm are presented in Figure 8. ‘

Ice-concentration [%] du/dr [s'1] Figure 8 Shear stress dependence on velocity Figure 9: Critical shear stress depending on ice gradient perpendicular to the flow concentration direction (10% ethanol, ID = 16).

The ice slurry behaves partly as a Bingham fluid and partly as a pseudo-plastic fluid.

The critical shear stress depends on the ice concentration and is found where the straight line crosses the y-axis in Figure 8. From the pressure drop measurements it is possible to calculate the critical shear stress at different ice concentrations (Figure 9).

Fitting these measurements with a 3rd order polynomial leaves equation 7 where “i” is the ice concentration (% wt.).

To = 0.00059 * i3 - 0.00701 * i2 + 0.08700 * i - 0.02498 (7)

The ice slurry behaves very much like an Newtonian fluid for ice concentrations below 15-20 %, which is in agreement with other investigations /14/ where it is stated that an ice slurry can be treated as Newtonian for ice concentrations below 25 %.

7 Turbulent flow (Re > 2100) For turbulent flow conditions it has been shown experimentally that the time-smoothed quantities Ttz(r)anduzaverage are roughly given by: ■SsW =('i--L|X-n’-* vmg =1 (g) c,.m» ^ V n=imx 5 No simple relation exists between the shear stress field and the mean velocity field. Velocity fluctuations in turbulent flow exchange momentum between adjacent layers of fluid, thereby causing apparent shear stresses that must be added to the stress caused by the mean velocity gradients. To calculate the head loss, a friction factor is defined and its value is determined experimentally.

Apparent viscosity of ice slurry The primary factors influencing the viscosity of suspensions are: solids loading, particle size, distribution and shape. The phenomena are very complicated and the viscosity must be determined experimentally. At DTI various equipment to measure viscosity have been examined /!/, Ill, /3/. Here a rotational visco-tester from Haake (VT550 with a VM1 sensor) was chosen. Apparent viscosities similar to the model presented by Thomas /4/, /10/, /111, /13/ were measured, hence this model is used.

p = pr/-(1 + 2.5- (ivol /100) + 10.05 • (z;,0/ /100)2 + 0.00273 • exp(l 6.6 • ivol 1100)) (9)

To calculate the viscosity for the carrier fluid the concentration of the additive (here 0.014 ethanol) must be known. The crystals being 0.012 created will only contain water and leave the 0.010 remaining liquid more rich in ethanol. The concentration of ethanol in the liquid 0.008

c = c0 • 100 / (100 — i) (10) 0.002

Ice concentration [%] Here c0 is the overall concentration of ethanol (%). Figure 10: Viscosity for ice sluriy depending on ice concentration 6.2 Condensation with CO, The Reynolds number for the condensate film on the tubes can be calculated as follows:

2 • mref R-efiJm — (ID ** column " Fliq ^tube

The Prandlt number for condensate film is: ^-'p.liq ' M'liq Pfriim — (12)

Nusselt’s film theory IS/ can be applied when the vapour velocities are small (shear stress between vapour and liquid is negligible) and the condensate film is assumed laminar.

8 ( 3 X 0.25 8 ’ Pliq ' (Pliq — P gas) ’ ^lg akond =0.729' (13) X^tube.column M'liq (^sat "^liq) ^oj

The equation (13) contains a correction for the vertical number of tubes. The heat transfer coefficient depends on the properties of the refrigerant in liquid form.

The conductivity is the most Temperature = 0 °C CO, NH, R22 important parameter for the Conductivity FW/m*Kl 0.111 0.523 0.0962 condensation (a ~ X0-75 ), but also Dynamic viscosity |"tiPa*sl 105.1 175.8 210.1 the viscosity of the film is Evaporating heat fkJ/kgl 230.5 1261.8 204.9 important (a ~ (l/p.)0,25). Surface tension fmJ7m2l 4.43 26.76 11.79 Table 3: Properties for refrigerants in the liquid phase Figure 11 shows heat transfer coefficients for R22, NH3 and C02 during condensation. 12000 12000

10000 — 10000 The model does not take the surface O—NH tension of the liquid film into consideration, which actually will give C02 an advantage because of the very low surface tension of this substance. 2000

Figure 11 Heat transfer coefficient for condensation (Tc = 0°C).

7. R esults and discussions

7.1 Measured heat transfer coefficients with ice slurry Ice slurry with an overall weight concentration of 10% ethanol was used in all experiments. The heat flux was between 6-14 kW/m2, corresponding to temperature differences of 3-8 K between the condensing refrigerant and the melting ice slurry.

Figure 12 shows measured and calculated heat transfer coefficients with ice slurry at 3 different velocities. Increasing heat transfer coefficients with increasing velocities and increasing ice concentrations can be found. However, the ice concentration seems to have a greater influence on the heat transfer coefficient than the velocity.

Similar to evaporation, the heat flux was expected to have an influence on the heat transfer coefficient of melting ice slurry, but no such influence could be found for heat fluxes varying from 6 to 14 W/(m2 K). 10000

8000 O Measured (2 m/s) 7000 o' «*• X Measured (1.5 m/s) 6000 • Measured (1 m/s) 5000 — — Calculated (2 m/s) - - - - Calculated (1.5 m/s) 3000 Calculated (1 m/s) 2000

Ice concentration [•/•]

Figure 12: Selected measurements of heat transfer coefficients for ice slurry of 10% weight ethanol

The increasing heat transfer coefficient with increasing ice concentration is explained by the melting ice crystals close to the wall eliminating the temperature gradient through the boundary layer. Stewart et. al. /11/ describe various phenomena in the ice slurry flow, increasing the heat transfer coefficient. A correlation was developed from the experimental measurements. i > 5% Nu, icesluny = J + 0103‘i — 2 003 • Re -O.I92(30-i)/30 _ .OJSgfRc^^/lOOOO) (14) Nu ' ' icesluny

Nu,icesluny i < 5% = 1 (15) Nu u : average velocity [m/s] 0.7 - 2.5 m/s i : ice concentration [%] 0 - 30 %

The heat transfer coefficient is calculated as;

Nu icesluny ^ icesluny a icesluny - (16) In Figure 13 the measured and + 30% calculated values for the heat « 6000 t transfer coefficient are compared I 7000 • • and the accuracy of the model can be | determined. " 6000 •' I % 5000 - - The model estimates the measured 2 | 4000 - -

values within 35 %, and 75 % of the | E 3000 * • measurements are estimated within •§ 2000 • • 30 % by the model. | «o 1000 • • The correlation used to calculate the 0 1000 2000 3000 4000 5000 6000 7000 8000 heat transfer coefficient for the single phase substance (water/ethanol) is Measured heat transfer coefficients [W/mA2*K] found in 111. Figure Comparison between measured and calculated heat transfer coefficients with ice slurry The validity of the correlation is limited, though it is advisable only to use the correlation for tubes with an inner diameter between 18 and 25 mm.

7.2 COz condenser with ice slurry Tests using water as coolant were made. The accuracy of available correlation for heat transfer coefficient for single phase substances (water) is much higher than the developed correlation for ice slurry. Hence, tests with water were used to find heat transfer coefficients for condensing C02.

3000 x

5 2500 -•

3 2000 • •

1500 - •

♦ Nusselt y = 12310%-0.4207 A Measured

o 500 - Mens (Measured)

Figure 14: Calculated and measured heat transfer coefficients for condensing CO: (Tc =0°C).

The calculated heat transfer coefficient is 15 - 20 % higher than the one measured. This can be caused by oil carried with the hot gas to the condenser or the surface tension causing the refrigerant liquid to increase film thickness on the tubes.

li Nusselt’s film theory gives a fine estimate of the heat transfer coefficient (± 20 %). However, a new correlation is developed from the measurements.

a = 12310-Refilm"°-4207 [W/m2*K] (17)

The correlation can be used on plain tubes with approximately 3 tubes in a vertical column.

The performance of the condenser is examined with ice slurry for different heat load capacities, ice concentrations and velocities. During all tests no signs of blockage of ice in the heat exchanger were observed.

The measurements showed a strong dependence between overall heat transfer coefficient and the ice concentration of the ice slurry. However, the dependence on the velocity of the ice slurry is not so strong.

At a constant flow velocity (1.4 m/s) selected measurements are presented in Figure 15. The load is varied by using a bypass of the hot gas to the separator. Total capacity^

• (32 kW) ♦ (42 kW) A (64 kW)

£7 1400 r CM E 1300 ■-

♦ • 5 1200 -- O

1100 e A A 1000 ♦ 900 --

800 O 0 5 10 15 20 25 30

Ice concentration [%] Figure 15: Overall heat transfer coefficient for the condenser.

At higher rating loads the heat transfer coefficient for the condensing C02 limits the overall heat transfer coefficient. On the other hand increasing ice concentrations increase the overall heat transfer coefficient significantly. Deviation and difficulty in reproducing some of the measurements indicate lack of stability or thermal balance in some cases. However, the tendency is estimated to be a usable result. Heat transfer between condensing COz and melting ice slurry is an effective process with high heat transfer coefficients on both sides. Flow velocities for ice slurry around 0,8 - 1,2 m/s create heat transfer coefficients in the area of 2500 - 5000 W/m2*K, however for condensing C02 it is somewhat lower (1700 - 2500 W/m2*K). The area ratio (area inside/outside s 0,85) equalise some of the difference, but an extended/ high performance surface outside could be useful. Heat transfer coefficient rW/m2*Kl Table 4: Heat transfer coefficients inside and outside the condenser tube Outside (condensation) 1700 - 2500 Inside (melting ice slurry) 1 m/s 2000 - 5000

makes it possible to run with very low velocities on the secondary side (ice slurry side). Still, the advantage of a very small temperature glide and high heat transfer coefficients can be utilised. The work of the pumps is at the same time decreased (lower velocities and lower pressure losses).

Conclusion

Heat transfer coefficient and pressure drop for ice slurry flowing in a horizontal pipe have been measured. The measurements show high heat transfer coefficients with melting ice slurry. The heat transfer coefficient increases with increasing ice-concentration and increasing velocities. However, the ice concentration seems to have a greater influence on the heat transfer coefficient than the velocity.

The increasing heat transfer coefficient with increasing ice concentration is explained by the melting ice crystals close to the wall eliminating the temperature gradient through the boundary layer.

The heat flux was expected to have an influence on the heat transfer coefficient of melting ice slurry, but no such influence could be found for heat fluxes varying from 6 to 14 W/(m2 K). A correlation to calculate the heat transfer coefficient with ice slurry was established, however the validity of the correlation is limited.

An industrial shell-and-tube condenser was tested with ice slurry. The overall heat transfer coefficient was measured and showed increasing value with increasing ice concentration. From the collected data the heat transfer coefficient for condensing C02 was calculated. The calculated heat coefficient transfer (Nusselt’s theory) is 15 - 20 % higher than the measured one. This can be caused by oil carried with the hot gas to the condenser or the surface tension causing the refrigerant liquid to increase film thickness on the tubes.

Surprisingly, no handling problems were observed in the ice slurry system during the tests. Plastic tubes and centrifugal pumps were used on the ice slurry side.

More work on ice slurry and C02 is planned for the future. Ice slurry has proved its great properties as a secondary refrigerant, and a new project will be initiated on this subject. Two demonstration plants are planned to work with C02 in the near future, and one of them will subsequently work with ice slurry as secondary refrigerant.

A cknowledgements

This work has been funded through different projects by the Danish Energy Agency and the Danish Environmental Protection Agency. Parts of the work have been performed in a close co-operation with Danfoss ’ Laboratory for Refrigeration, Sabroe Refrigeration and the Technical University of Denmark. Material contributions to our work by the Danish companies: Danfoss, Danisco, Grundfos, Hydro and Sabroe are greatly acknowledged. Nomenclature

A : area [m2] A0, A,- : total area, outer/ inner [m2] c : concentration of ethanol [% wt.] Co : overall concentration of ethanol [% wt.] Cp.Hq : specific heat capacity of C02 liquid [J/kg*K] d0, dj : outer/ inner tube diameter [m] dp/dz : pressure gradient in z direction [Pa/m] g : gravity [m/s2] N : heat of evaporation for C02 [J/kg*K] *voI : ice concentration [% vol.] i : ice concentration [% wt.] K : overall heat transfer coefficient referring to area A [W/m2*K] ^tube : tube length [m] mref : mass flow of C02 [kg/s] ^column : number of vertical columns of tubes H ^tube,column : number of tubes in a vertical column H Nu : Nu-number for the carrier fluid using the average velocity H Qc : capacity [W] ■‘■vv ice slurry : Re-number for the ice slurry where the average velocity is used H RCfilm : Re-number for condensate film H Re : Re-number [-] r : radial distance [m] R : radius of tube [m] Tbi» Tb2 : temperature of ice slurry, inlet/ outlet [°C] TAsat, TAwall : saturation temperature for C02 and wall temperature [°C] T= : condensation temperature [°C] dTln : log mean temperature difference [K] u : average velocity [m/s] u : velocity [m/s] K : time-smoothed velocity [m/s] z : distance [m] a 0, otj : heat transfer coefficient, outer/ inner [W/m2*K] ^"lube : heat conductivity for tube material [W/m2*K] % : heat conductance [W/m*K] P'Cf : dynamic viscosity for carrier fluid [Pa*s] p : dynamic viscosity [Pa*s] Pm : Bingham limiting viscosity [Pa*s] Pliq ^Pgas : densities for C02 liquid (film) and C02 gas respectively [kg/m3] P : density for ice slurry [kg/m3] X : shear stress [Pa] To : critical shear stress [Pa]