Thermoelectric Total Enthalpy Flow Calorimeter

The constant-pressure flow calorimeter operates between 200 and 700 K at pressures to 20 MPa. It is well-suited for measurements of the total enthalpy of fluids of interest to the energy industry, including reactive, corrosive and thermally-labile substances. It will provide benchmark-quality data with uncertainties of approximately ± 0.2 % at temperatures to 500 K. The system uses a thermoelectric cooling element to remove energy from the calorimeter at a precisely controlled rate. Energy is supplied to the calorimeter either by an electrical heater or by the sample fluid flowing through the apparatus. The enthalpy of the fluid is measured by observing changes in the power delivered by the electrical heater required to maintain the calorimeter at constant temperature.

The construction of the apparatus has been divided into three sections: pre-heater, connecting tube, and calorimeter. These sections are illustrated in the figure. The sample fluid flows inside 316 stainless steel tubing which passes through these sections in the order listed. The flow tubing first enters the pre-heater section. The principal part of this section is a solid aluminum cylinder with parallel spiral grooves machined on the surface for the flow tubing and coolant tubing. It is surrounded by a tight-fitting clamshell that supports five high-power heaters. This section is capable of heating or cooling a sample to an outlet temperature between 200 and 700 K, as verified by a standard platinum resistance thermometer.

When sample fluid exits the pre-heater, it enters the connecting tube section. The chief purpose of this section is to effectively separate the two temperature zones of the apparatus in a way that permits no exchange of heat between them. The connecting tube is made of stainless steel, a material chosen to minimize heat transfer by conduction from the pre-heater section to the calorimeter section. The flow tubing at the center of the connecting tube is surrounded by tubular aluminum guides on which are mounted low-powered heaters which provide the fine control of the fluid temperature as it enters the calorimeter section.

As the fluid enters the calorimeter, the fluid temperature is observed with a specially designed microthermistor probe which resides inside the flow tubing. Its cross-sectional area is less than 20% of the total area of the flow tubing, and hence does not restrict fluid flow. The inlet temperature is measured with a resolution of ± 10^-4 K. A second thermistor probe measures the outlet temperature. The principal part of the calorimeter section is an aluminum disk sandwich with a machined planar spiral groove into which the flow tubing is pressed. Temperature gradients on the disk are observed with differential thermocouples. Both a heater and a Peltier cooling element are stacked underneath the disk assembly to control the heat leak path. In practice, the cooling element always removes energy at a constant rate, whereas the heater power is varied to maintain the outlet temperature at the reference temperature of 298.15 K. A static experiment at the same pressure as the subsequent flowing fluid experiment provides a measurement of the baseline heat to the matching heater. The difference between this baseline heat and the heat input in the flow experiment is the enthalpy increment of the fluid.

Computer programs in the C language were developed which achieve a high level of automation. Calibrations were carried out for the oscillating quartz crystal pressure transducer using a high accuracy oil-lubricated piston gauge. A calibration check of the platinum resistance thermometer confirmed the vendor's value of the resistance at the water triple point of 273.16 K. Performance tests with liquid water and steam are underway. Comparisons with internationally accepted standard reference data for water and steam will be made to verify the apparatus performance.

Physical & Chemical Properties Division	Chemical Science & Technology Laboratory	NIST Boulder Labs	NIST Homepage