A sample cooler for steam & water sampling within a power plant is used to cool a sample from a process stream and is simply a small shell and coil heat exchanger. The sample to be cooled flows through the tube side of the cooler, and the cooling fluid, usually water, flows through the shell side. The cooled sample then is taken to a laboratory for analysis or, in some cases, piped to in-line process instrumentation for continuous monitoring of certain properties such as conductivity, pH or chemical constituents.

Sample coolers generally range from one to five square feet of heat transfer surface. Selecting a one-size-fits-all cooler for all samples is uneconomical, and can be hazardous to personnel and/or equipment. It’s important that power plant managers use the right sample cooler for the needs of the plant, considering specific temperature, pressure and flow conditions.

Inefficiency issues

Power plants throughout the world use many different types of sample coolers. Some are homemade, others are shell and tube type heat exchangers, and others are the more sophisticated single helical tube, spiral tube or tube-in-tube cooler types.

Unfortunately, many coolers in use are undersized or too inefficient for the desired service, leading to these safety issues:

1. If the sample becomes too hot to handle, the operator will throttle the flow to unacceptably low levels, which means the sample is no longer representative or acceptable.
    
2. Analyzer accuracy suffers as the temperature strays from 25°C.
    
3. Plastic parts such as cation columns can be damaged.
    
4. If secondary cooling is being used, more load is dumped onto the refrigeration system when the primary coolers do not keep up. This issue commonly occurs when secondary cooling is added to an older system – and many power plants are older. In that case, the older primary coolers need to be replaced to reduce the load that otherwise would need to be picked up by the chiller.

What size sample cooler is needed?

A truly representative sample requires adequate sample flow. Electric Power Research Institute (EPRI) recommends a flow rate of 6 feet (1.8 m) per second on water samples. Generally, a 2.4 square foot (0.22 square meter) cooler is adequate for water samples of 1200cc per minute, while a 3.5 square foot (0.33 square meter) or larger cooler should be used for water samples of 3300cc per/minute.

For steam samples, flow rates are lower due to the high velocity of the steam as it flows to the sample panel. Generally, flow rates of between 500-1000cc per minute are adequate. The cooler size is determined by the required flow rate and the steam inlet pressure. As the source pressure decreases, the cooler performance decreases – and a larger cooler is required. This is due to the relative densities of different steam pressures. Pressure drop also is a major consideration when sizing a cooler for a steam application.

Consider the specific requirements for each sample line when selecting a sample cooler to ensure adequate flow rates and optimize cooling water usage.

Sentry^® sample cooler designs are offered in the widest selection available from any manufacturer. See them here.