Heat Exchanger Design Q & A
But, how does a user provide that information? Answering the following series of questions when requesting a heat exchanger quotation can help you get a quote for a correctly sized unit.
1. What Is the Process Flow? This question sounds simple, but it is important. Is the process flow air, nitrogen or something else? This also is the time to address composition for multicomponent flows. When cooling conveying air, this can be as simple as considering the humidity in the ambient air at the job site. If the application is more complicated -- condensing chemicals venting off a storage tank, for example -- the answer to this question should include the amount of each component in the process stream.
Summer conditions usually are considered when sizing a heat exchanger that will cool ambient air used for conveying because this is when air will be at its warmest temperature. In general, if the exchanger can provide the desired cooling in the middle of summer, it will perform even better on a cooler day.
2. What Is the Flow Rate? The flow rate can be expressed as a volumetric or mass flow rate. Blower manufacturers usually express performance curves in volumetric flow rates. Most heat transfer calculations are based on standard cubic feet per minute (scfm). If you are providing actual cubic feet per minute (acfm) or inlet cubic feet per minute (icfm), make sure you know the temperature and pressure at which this value is measured.
3. What Must Be Done to the Flow? Fully define the process by answering a series of questions about the flow. What is the desired outlet temperature for the process flow? Does it require cooling? If so, is the goal merely to cool the process medium, or is there a component in the gas stream that also needs to be condensed?
4. What Is Available to Provide the Required Cooling? In order to cool process flow, another medium that is colder than the desired process outlet temperature is required. Depending on what must be done to the flow, ambient air may be able to provide the desired cooling. If not, a water source or another cooling medium may be required. A good rule of thumb is to have a cooling medium that is at least 10oF colder than the desired process outlet temperature.
5. Are There Any Other Heat Exchanger Requirements? Once the process flow is defined, consider the exchanger itself. Does the process flow require certain materials of construction such as stainless steel to withstand a corrosive environment? Are there restrictions on the maximum pressure drop across the exchanger? Can the blower withstand another psi of pressure drop or only a few more inches of water column?
In addition, for air-cooled exchangers, consider: Does the environment require specific electrical requirements? Some harsh environments may require an explosionproof motor. Other countries use different voltages or frequencies than the United States. If this is known at the outset, the correct motor can be included in the heat exchanger design.
Applying the AnswersA simple case history can help demonstrate how defining the process adequately ensures good performance. A cereal plant in Cincinnati unloads rail cars of sugar using a pneumatic conveying system. The blower compresses 600 scfm of ambient air to 9 psig, raising the air temperature by 117oF (47oC). To prevent the sugar from carmelizing, the air must be cooled below 120oF (49oC). Ambient air and cooling water are available for cooling.
When applying the recommended five questions to this example, it is learned that:
- Flow is ambient air.
- The blower can supply 600 scfm of air at a 9 psig operating condition.
- According to the data for the ASHRAE reporting station in Cincinnati, the 1% summer design point temperature is 92oF (33oC). Factoring in the 117oF (47oC) temperature rise across the blower, the air leaves the blower at 209oF (98oC).This air must be cooled below 120oF (49oC). The conditions given are exceeded 1% of the year, or approximately three to four days per year.
- The 92oF (33oC) ambient temperature is 28oF less than the 120oF (49oC) process outlet requirement. Therefore, an air-cooled exchanger can be used to provide the necessary cooling.
- Process flow is clean air. Therefore, no special materials or food-grade finishes are required. If the blower and heat exchanger will be located outside, the motor enclosure can be totally enclosed and fan cooled. An air-cooled exchanger will be able to cool the process air at the blower discharge to 100 to 110oF (38 to 43oC) on the hottest days of summer. On cooler days, the same exchanger will provide more cooling to the conveying air. This cooling will protect the sugar from high temperatures and keep the conveying line from plugging due to carmelized sugar.
If cooling water also is available on site, another heat exchanger option is available. With this option:
- Flow remains air at 9 psig and 209oF(98oC).
- Process air remains 600 scfm.
- Conveying air must be cooled below 120oF (49oC).
Most water-cooled exchangers can cool air to within 10oF of the water inlet temperature. Therefore, any water temperature under 110oF (43oC) can be used because the air only needs to be cooled below 120oF. Most water supplies are cooler than 110oF (43oC) and can provide more than the minimum cooling required. Besides providing colder conveying air, this cooling also will condense some water vapor out of the airstream.
Tabulated ASHRAE data used to determine ambient temperature also includes relative humidity at the 1% summer point (table 1).
As with the air-cooled example, the exchanger will not be in contact with the sugar so no special materials will be re-quired. The water-cooled heat exchanger does not have a motor, so any requirements for the motor enclosure are not applicable. If cooling water is available inside the plant, the exchanger also could be located indoors. An indoor location usually is preferred for a water-cooled exchanger to protect against freezing.
These examples show you how supplying thorough answers to the five initial questions can help determine a heat exchanger's design. Other applications may have different answers or more questions that must be answered, but the five questions are a good starting point for all applications. PH