Thermal Conductivity of Liquids and Gases Unit H471

Transcription

1 Thermal Conductivity of Liquids and Gases Unit H471 Figure 1: H471 Rapidly Stabilises, Allowing Multiple Measurements in a Single Laboratory Period. Easily Cleaned to Allow Measurement of Different Materials Manufactured from Corrosion Resistant Materials. Optional Computer Linking Upgrade. Two year Warranty. Page 1 of 5

2 Figure 2: H471 Shown with HC471A Optional Computer Linking Upgrade Introduction When predicting the rate of heat transfer in all heat exchangers, a knowledge of the thermal conductivity of the fluid(s) involved is essential. The Hilton Thermal Conductivity of Liquids and Gases Unit has been designed to enable students to easlily determine the important property of a wide variety of liquids and gases. The unit will be of interest to everyone involved with heat transfer, particularly in: The unit will be of particular interest to those studying: Building Services Chemical Engineering Marine Engineering Mechanical Engineering Plant and Process Engineering Refrigeration & Air Conditioning Experimental Capabilities Calibration of the unit to establish the effects of incidental heat transfers. Determination of the thermal conductivity of any suitable gas or liquid compatible with materials on construction. Description The fluid whose thermal conductivity is to be determined fill the small radial clearance between a heated plug and a water cooled jacket. The clearance is small enough to prevent natural convection in the fluid and the fluid is presented as a lamina of face area πd m L and thickness r to the heat transfer of heat from the plug to the jacket. The plug is machined from aluminium (to reduce thermal inertia and temperature variation) and contains a cylindrical heating element whose resistance at the working temperature is accurately measured. A thermocouple is inserted into the plug close to its external surface, and the plug also has ports for the introduction and venting of the fluid under test. Page 2 of 5

3 The plug is held centrally in the water jacket by O rings, which seal the radial clearance, but which allow quick dismantling for cleaning. The jacket is constructed from brass and has a water inlet and drain connections and thermocouple is carefully fitted to the inner sleeve. Due to the positioning of the thermocouples and the high thermal conductivities of the materials involved, the temperatures measured are effectively the temperature of the hot and cold faces of the fluid lamina. A small console is connected by flexible cables to the plug/jacket assembly and provides for the control of the voltage supplied to the heating element. An analogue voltmeter enables the power input to be determined and a digital temperature indicator with 0.1K resolution displays the temperature of the plug and jacket surfaces. An optional Computer Linking upgrade HC471A is available, which can be factory fitted prior to delivery or supplied later as a user installed upgrade. Hard copy printout of both raw and processed data can be obtained allowing students to compare their results with those generated by analysis on the computer. Data can also be stored to disk for subsequent review outside of the laboratory. Experimental Results Calibration Curve ~ Relationship Between Incidental Heat transfer and Plug/Jacket Temparature difference. Operation Calibration The incidental heat transfers in the unit are determined by using air (whose thermal conductivity is well documented) in the radial space. One calibrated in this way the results may be carefully preserved and used in subsequent determination of the thermal conductivity of other fluids. Cooling water is passed through the jacket and the heater input adjusted to give small temperature difference across the air lamina. When stable, the voltage applied to the heater and the temperatures are noted. Page 3 of 5

4 The rate of heat transfer Q & through the air lamina is then calculated from kair dml t Q& Π = r 2 The difference between the heat input Volts Resistance and that calculated above, is the incidental heat transfer at the given temperatures. The above may be repeated at other plug and jacket temperatures and a calibration curve showing incidental heat transfer against plug/jacket temperature difference, may then be drawn and kept for reference. Determination of Thermal Conductivity The unit is cleaned and reassembled. The fluid to be tested is then introduced into the radial space (ensuring that no bubbles exist if a liquid is used). Water is then passed through the jacket and the heater adjusted to give a reasonable temperature difference and heat transfer rate. When stable, the rate of heat transfer and plug and jacket temperatures may be observed. After deducting the incidental heat transfer at the given temperature differences, it is known that the remainder is that passing through the fluid lamina. The thermal conductivity of the fluid may then be readily calculated. A typical calibration curve and calculation is shown below/ Determination of the Thermal Conductivity of Mineral Oil Observation data Plug temperature 25.8ºC Jacket temperature 14.6 C Voltage applied to heater (V) 64.5V Heater resistance (R) 56.4Ω Radial clearance ( r) 0.3mm Heat transfer area (A= πd m 1) m 2 } = t 11.2K Determination Heat input 2 2 V 64.5 = = R W Incidental heat transfer (from graph) = 4.3W Heat conducted through oil Q & = 69.5W Thermal conductivity of oil Q& r k = = A t = Wm K Page 4 of 5

5 Specification General Bench top unit for the determination of thermal conductivity of liquid and gasses, and complete with a console for the control and display of temperature and heat input. Detailed Plug/Jacket Assembly: Cylindrical water jacket constructed from nickel plated brass and fitted with K type thermocouple to measure inner surface temperature. Mounted on stainless steel stand. Cylindrical plug, machined from aluminium anodised. Housing heating element and K type thermocouple. Provided with ports for introduction and venting of fluid under test. Mean diameter (d m ) 39 mm Effective length (l) 110 mm Radial clearance ( r) 0.3 mm (note: The actual radial clearance and the resistance of the element are stated on the plug) Console: Aluminium and plastic coated steel console hosing: variable transformer; Voltmeter; Digital temperature indicator with selector switch Resolution 0.1ºC. Safety Features: All electrical components fused and earthed. Heater input limited to 100W. Heater voltage limited to 70V. Dimensions Height: 250mm Depth: 280mm Width: 435mm Weight: 7kg An optional computer linking upgrade containing an electronic logger and transducers enables all parameters to be logged by menu driven software, and copied to printer and/or disk for subsequent analysis. Accessories and Spares Unit supplied with: One experimental operating and maintenance manual in English, Spanish or French. Accessories and spares for 2 years normal operation. List available on request. Services Required Electrical: A: 100W Volts, Single Phase, 50Hz(With earth/ground). B: 100W Volts, Single Phase, 60Hz(With earth/ground). Water: 3 litres per minute at 10m head (min). Ordering Information Order as: Thermal Conductivity of Liquids and Gases Unit H471 And: Optional computer linking upgrade. HC471A Electrical Specification Either: A: Volts, Single Phase, 50Hz(With earth/ground). Language Either: B: Volts, Single Phase, 60Hz(With earth/ground). English, Spanish or French. Shipping Specifications Net Weight: Approximate Gross Weight: 7kg. 37 kg. Packing Case Dimensions: 62 x 59 x 0.55 cm Packing Case Volume: m 3 Also Available On Request Further detailed specification. Additional copies of instruction manual. Recommended list of spares for 5 years operation. P A Hilton Ltd. Horsebridge Mill, King s Somborne, Stockbridge, Hampshire, SO20 6PX, England. Telephone: National (01794) International Fax: (01794) sales@p-a-hilton.co.uk Page 5 of 5