The goal of a steam and water analysis system (SWAS) in power generation is to properly condition the sample so that it is representative of the process. Secondary sample cooling provides a way to eliminate measurement uncertainty issues by removing the variable of temperature from the measurement. Secondary cooling also provides a temperature “cushion” when primary cooling systems are undersized or underperforming.
controlling sample temperatures
Sample temperatures are controlled by cooling water temperature, cooling water flow and sample flow rate. However, cooling water temperatures vary wildly between seasons, regions and sources. While analyzers have standard temperature compensation factors defaulted in the software, solution temperature compensation is difficult to predict with varying temperature and chemical feed concentrations.
These factors can be controlled with the appropriate equipment, eliminating analysis compensation sources of uncertainty in measurements. Eliminating uncertainty in critical analysis improves confidence in measurements and chemical feed accuracy, often resulting in significant cost savings.
ensure Accurate readings with sample coolers
A sample cooler is a compact, high-efficiency heat exchanger used to condense steam samples and to cool steam and water samples. These are the first step in temperature control to get accurate readings on high-temperature samples. There are two types of coolers to use in your sample panel:
- Primary Coolers – Primary cooling is normally the first step in the sample conditioning process. A primary sampler’s goal is to reduce the steam or water sample to a safe and constant temperature, generally between 100°F-122°F (30°C–50°C). Primary sample coolers are designed to remove the majority of the heat from incoming samples, and can reduce the incoming sample temperature to within 5-10°F (3-6°C) of the cooling water supply temperature.
- Secondary Coolers – These help you control the temperature of the sample to what is standard for chemistry measurements, removing variables that could be a source of uncertainty.
importance of secondary sample cooling
Secondary coolers are located after the pressure reduction valve to provide a consistent sample temperature to the on-line analysis equipment downstream. They’re used to adjust the final sample temperature from as high as 140°F (60°C) down to 77°F ± 1°F (25°C ± 0.5°C). A secondary sample cooler can reduce the sample temperature to within 1°F (0.5°C) of the cooling water supply temperature, despite variations in flow rate and incoming temperature of each line.
The secondary cooler bank is supplied with chilled water typically controlled to about 75-76 °F +/- 1°F (23.9 – 24.4 °C). Special chillers with hot gas bypass are supplied to maintain very close temperature control and have the ability to add heat into the system in the case of sub-cooled samples, such as in the winter months.
In general, individual high-efficiency sample coolers provide much better control of final sample temperature than other secondary cooling methods.
Secondary cooling systems help:
- Maintain sample temperatures
- Ensure more accurate analysis results
- Negate any potential inaccuracies with the analyzers’ algorithms
- Bridge gaps in primary cooling water systems
temperature ALGORITHMS are not created equal
Temperature compensation algorithms assume a specific background matrix. Therefore, the variability of the sample to be analyzed should be considered.
The accuracy of temperature compensation under various conditions should also be addressed. In general, the greater the difference between the sample temperature and 77 °F (25 °C), the greater the potential error in the correction factor.
An example of this can be found in measuring conductivity in high purity water for the purpose of monitoring for a condenser leak. Since leak constituents may vary day to day, it is not possible to predetermine all of the background conditions, which may mask early detection of leaks.
Isothermal baths vs. secondary sample coolers
Two widely used methods for secondary cooling are isothermal baths and individual sample coolers.
The isothermal bath uses coils immersed in a chilled water bath. In one scheme, the chilled water is controlled by a hot gas bypass system. Another bath variant uses thermal bypass valves that splits a portion of the sample flow and mixes it back into the sample coil effluent to maintain temperature.
Isothermal baths typically provide larger variations in sample temperature than other methods due to inadequate bath temperature uniformity, plugged thermal bypass valves and large chiller temperature variations. Because of the complexity and maintenance associated with bath systems they are rarely used in current SWAS designs.
In a secondary cooling system, a high-performance cooler is piped for counterflow, allowing a temperature crossover. That means that the sample can be cooled below the temperature of the final cooling water. The cooler is tightly baffled, creating high turbulence and very high rates of heat transfer. The cooling water inlet temperature is constant across a bank of coolers.
In general, individual high-efficiency sample coolers provide much better control of final sample temperature than other secondary cooling methods. Secondary cooling ensures analyzer readings are accurate and can meet the needs of your processes.
Ready to learn more? Download our Heat Exchangers 101 eBook.