Proper Fluid Selection and Maintenance for Heat Transfer Applications

Technical Note Technical Proper fluid selection and maintenance for heat transfer applications Scott Pratt, Thermo Fisher Scientific, Newington, New Hampshire Key Words Fluid types, corrosion risk, temperature range, toxicity, evaporation, viscosity, expense, bio-fouling, inhibitors, disposal, thermal properties, compatibility, electrical conductivity, density, flammability, fumes, bath, bath circulators, chillers and heat exchangers Goal Whether your temperature control unit (TCU) is a water bath, refrigerated/ heated bath circulator, cooling/heating recirculating chiller or recirculating chiller, a fluid will have to be selected. If you are operating between approximately 5 ºC and 95 ºC, then water seems the likely choice. You might find yourself close to a source of tap water, filtered water, distilled water, reverse osmosis (RO) water, or deionized (DI) water. However, just using the nearest convenient source may not be the right choice. While these water types all have their uses, not all are suitable for your TCU needs. Regardless of how it has been treated, all water starts out with the same basic properties, advantages and disadvantages. Before we look at the properties associated with water, let’s define the various fluid properties important to use in a TCU: Viscosity Viscosity describes a fluid’s resistance to flow due to the friction between its molecules. It is important when selecting a fluid for use in a TCU because the fluid must be pumped either around the bath or out to the application and back. Fluids with a higher viscosity are naturally harder to pump. Most fluids, other than water, also become more viscous as the temperature is decreased. Increased viscosity Thermo Scientific™ Polar Series Accel 250 LC Cooling/Heating makes the fluid harder to pump, reducing flow, and may Recirculating Chiller form an insulating boundary layer–both of which reduce its capability to move heat energy. For a TCU, kinematic viscosity is the preferred measure- ment, as highly dense fluids will be harder to pump and less There are two ways viscosity is measured: dense fluids will be easier to pump. This consideration is • Dynamic viscosity measures the force required to shear not reflected in dynamic viscosity. the fluid and is reported in centipoise (cP) • Kinematic viscosity is dynamic viscosity divided by density and is reported in centistokes (cSt) 2 Density Specific Heat Density is the mass per unit volume typically expressed in Specific heat capacity is the quantity of heat required to 3 grams per cubic centimeter (g/cm ) and pounds per gallon raise the temperature of a substance’s unit of mass by a unit 3 (lb/gal) or pounds per cubic foot (lb/ft ). change in temperature. It is specified as: Specific Gravity • Calories per gram, per degree Celsius or Cal/g. °C Specific gravity is the ratio of the density of the liquid to the • One calorie of energy will change the temperature of density of water at a specified temperature. When using the one gram of water one degree centigrade metric system, density = specific gravity. Because it is a ratio of densities, it does not have a scientific unit (SI) or physical • British thermal units per pound, per degree Fahrenheit dimensions but is commonly referred to as “SG”. or BTU/lb °F • One BTU will change the temperature of one pound of Temperature Range water one degree Fahrenheit 1 BTU/lb °F = 1 Cal/g. °C A manufacturer’s specified temperature range for a given • Kilojoules per kilogram, per degree Kelvin or kJ/kg. °K fluid is likely to extend to the pour point and the flash point or boiling point for non-flammable fluids. For use with a • One kilojoule of energy equals 1000 watts for TCU, considerations such as viscosity and fire point almost one second always narrow the usable temperature range. • 1 kJ/kg. °K = 0.24 Cal/g. °C or BTU/lb °F • Pour point: The lowest temperature at which an oil or Lower specific heats (metals, plastics, oils, alcohol) take less other liquid will pour. energy (Kcal/hr) per kilogram to increase or decrease • Flash point: A substance will ignite briefly, but vapor temperature. Higher specific heats (water) take more energy might not be produced at a rate to sustain the fire. per kilogram to increase or decrease the temperature. For use in a TCU, this means that fluids with higher specific • Fire point: The fire point of a fluid is the temperature at heats have a higher capacity to carry heat either into or which it will continue to burn for at least 5 seconds after away from the application. ignition by an open flame. Most tables of material properties will only list material flash Figure 2: Specific Heat Comparison Between Oil and Water points, but in general the fire points can be assumed to be about 10 °C higher than the flash points. However, this is no 1 kg. oil = 1.08 liter 1 kg. water = 1.0 liter substitute for testing when the fire point is safety critical! S.H. = 0.361 Cal/g ºC S.H. = 1.0 Cal/g ºC The highest working temperature as defined by the EN Apply 900 watts for 5 Apply 900 watts for 5 61010 (IEC 1010) must be limited to 25 °C below the fire minutes and the new minutes and the new point of the bath fluid. temperature is 199.1 ºC temperature is 87.4 ºC Thermal Conductivity Thermal conductivity is the heat energy transferred per unit thickness and is measured in watts per meter, per degree Kelvin (w / (m. °K)). When the thermal conductivity value for a material or fluid is used to calculate heat transfer, the answer will be in watts per square meter (w/m2). Figure 1: Thermal Conductivity Figure 2 shows what happens when you heat two fluids with different specific heat numbers. HIGHER LOWER TEMPERATURE TEMPERATURE Figure 1 shows how heat applied to one side is transmitted to the other side. How well it does this depends on the thermal conductivity of the material being heated. pH • A = Excellent 3 The pH scale goes from 0 to 14 and describes the level of • B = Good acidity or basicity of a fluid. Water is pH neutral at 7. Any value lower than 7 is acidic, while any value larger than 7 is • C = Fair basic (alkaline). It is worth noting that the pH scale is • D = Severe Effect logarithmic, where pH 4 is ten times more acidic than pH 5 and one hundred times more acidic than pH 6. This is a If a material has an A or B rating, it may be simple to say crucial consideration when checking and maintaining pH in that it is compatible. But if it is rated as a C or D, what your fluid. See Fig. 3. does that really mean? Is it going to last a month, a year or ten years? Without actual testing, it is not always Thermal Expansion reasonable to venture a guess. Since long-term testing is not often practical, short term testing may prove that Thermal expansion is the amount a given fluid expands a material is not usable, but may still fall short in proving when heated, or contracts when cooled. It is very low for (definitively) that it is usable. water–a liter of water at 20 °C expands to 1.04 liters at 95 °C (4%) and it is much higher for silicon oil–a liter of a typical Fluid Resistivity or Conductivity silicone oil at 20 °C expands to 1.10 liters at 120 °C (10%). Resistivity measures how strongly the fluid opposes the Thermal expansion must be considered when using a flow of electric current and is measured in Ohms (Ω). wide temperature range and not using water or the TCU • 1 million Ohms = 1 Megohm cm (Meg Ω) can overflow. Conductivity measures the fluids ability to pass electric Wetted Material Compatibility current and is measured in microsiemens (µs). Most any fluid will have some effect on the materials with which it comes in contact. It could cause any of the following: • Conductivity is the inverse of resistivity or 1/Meg Ω = µs (Figure 4) • Swelling: Will the fluid cause a wetted elastomer to swell? Ultrapure water does not do a good job of conducting • Corrosion: Does the fluid cause a wetted metal to electricity. It is the dissolved ions (salts) that make it the corrode through chemical or galvanic corrosion? conductive fluid with which we are all familiar. The more • Disintegration: Will the fluid cause a wetted elastomer salts present in the water, the greater its capacity for to disintegrate? conducting electricity. That is why sea water is more conductive than drinking water. To determine whether a particular fluid is compatible with the wetted materials of a particular TCU is not as straightforward as it may seem. That is because compatibility is not always a “yes” or “no” answer; it often has some sort of grading such as: Figure 3: pH Scale Hydrochloric Acid Toothpaste Ammonia Battery Lemon Tomato Acid Pure Sea Milk of Acid Juice Vinegar Juice Rain Milk Water Eggs Water Magnesia Bleach Lye ACIDIC BASIC 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 pH Figure 3 illustrates the pH of some common fluids. Figure 4: Conductivity/Resistivity Scale Ohms (Ω) 1 MegOhms cm (MegΩ) 10 100 1000 10000 100000 10000000 3 5 10 15 18.2 CONDUCTIVITY RESISTIVITY Microsiemens (µs) 100000 10000 1000 100 10 1 0.3 0.2 0.1 0.07 0.055 Sea Water Distilled Water Very Pure Water Tap Water Pure Water Ultra Pure Water Figure 4 illustrates the relationship between resistivity and conductivity for a variety of common water types.

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