Evaluating your process will help you specify a plate-and-frame heat exchanger that is right for your application. Use these 10 tips to specify your next plate heat exchanger and maximize its service life.
It is no surprise that plate-and-frame heat exchangers (PHEs) offer good heat transfer. But, there are several important factors you must keep in mind as a specifier or user of PHEs. For instance, if the PHE is to be used in conjunction with a cooling tower, it should be designed for the water treatment chemicals being used to treat the tower water. Also, it is easier to prevent a heat exchanger from leaking than to stop a leak once it occurs. And, did you know that you can minimize erosion in the port area by lowering nozzle velocities?
Consider the following 10 tips when choosing and using your next plate-and-frame heat exchanger.
Figure 1. Two liquid streams enter and exit a heat exchanger from the hot side and the cold side. As a result, there are four temperatures to consider when sizing the heat exchanger: hot side inlet, hot side outlet, cold side inlet and cold side outlet.
TIP 1: Tell Them Everything
Be sure to communicate all pertinent information regarding your application to the manufacturer when you request a quotation. Two liquid streams, referred to as the "hot side" and the "cold side," go into and come out of a heat exchanger (figure 1). As a result, there are four temperatures to consider when sizing the heat exchanger: hot side inlet, hot side outlet, cold side inlet and cold side outlet.
The minimum information needed to size the heat exchanger for both the hot and cold sides are: fluid names, flow rates, inlet temperature, outlet temperature, operating pressures and maximum pressure drop allowed across the unit. For fluids that are uncommon or proprietary, physical properties such as viscosity at inlet and outlet temperatures, thermal conductivity, specific gravity and specific heat will be needed. Most manufacturers have a design questionnaire available for you to use when collecting data for a heat exchanger application and would be eager to furnish you with a copy.
TIP 2: Check Compatibility
Check with your manufacturer to ensure chemical compatibility among the heat exchanger components and your process liquids as well as any possible cleaning solutions. An overwhelming number of PHEs contain 316 stainless steel plates; 316 stainless is compatible with and corrosion resistant to many chemicals commonly found in processing plants. One chemical that is not friendly to 316, however, is chloride. Chloride's effect on 316 stainless steel is temperature related. A rule of thumb for chloride's compatibility with 316 is as follows: 180 ppm (parts per million) at 122oF (50oC), 120 ppm at 170oF (77oC), and 50 ppm at 212oF (100oC). Some manufacturers use a 316 stainless steel with a higher nickel content, which will be more resistant to chloride, but it is wise to check with them before implementing any chemical change.
Plate-and-frame heat exchangers commonly are used in conjunction with cooling towers. The heat exchanger should be designed for your needs and the water treatment chemicals being used to treat the tower water. If you are thinking of switching to more aggressive chemicals to remedy a problem such as algae within the cooling tower, check with the heat exchanger manufacturer to confirm its compatibility with the plates and gaskets.
Another typical situation occurs in the food industry, where it is a common practice to clean-in-place (CIP) the unit. This involves circulating a caustic cleaning solution throughout the system. Again, make certain that the cleaning solutions you use are compatible with the heat exchanger plates and gaskets.
TIP 3: Avoid Situations Where Pressure Spikes Can Occur
Although manufacturers make extra allowances in the design engineering, it is important to stay within the pressure limits of the heat exchanger's rating. If a pressure spike within the system piping is possible due to the quick closure of a valve, water hammer, etc., you must take the necessary precautions to protect the heat exchanger or suffer the leaky consequences of a blown out elastomeric gasket. Avoid pressure changes of more than 150 psig/min and temperature changes of more than 20oF/min (11oC/min). Remedies for this situation include relief valves, rupture discs, pulsation dampers and arrestors. Check with the manufacturer for recommendations.
TIP 4: Use a Strainer or Bypass the Unit During Startup
During startup, it is advisable to include a temporary strainer on the cold or hot water inlet as well as the inlet of the process liquid. Altern- atively, bypass the heat exchanger altogether during startup. Even if your liquids are clean, with no particulates present, a temporary strainer or bypass configuration around the PHE is recommended for new, startup applications. During construction, it is all too common to see dirt, weld beads and other debris get into tanks and piping, and then get pumped through the system. (I once found a work boot in the suction line of a large pump.) If this debris finds its way into the exchanger, it may get trapped, causing an increase in pressure drop and a decrease in heat transfer efficiency.
Figure 2. The plates in a plate-and-frame heat exchanger typically are in a chevron or V-shape. Every other plate is reversed, causing a waffle-type grid between those plates. Most PHEs will allow 0.0625" or smaller particles to pass through without fouling. For liquids containing larger particles, wide-gap plate designs are available.
TIP 5: Keep Large Particles Out
The plates in a PHE typically are in a chevron or V-shape (figure 2). Every other plate is reversed, causing a waffle-type grid between those plates. The space between the plates is called the channel and, in many cases, can be quite small to achieve higher levels of heat transfer efficiency.
Particles entering into the exchanger can potentially get caught in the plate channel, so it is important to ensure that no particulate larger than 0.0625" is allowed to enter the heat exchanger. If your liquids could contain larger particles, they should be removed with either a strainer or separator prior to their entering the exchanger. This is especially important if there is an open tank in the system where larger particles could be introduced. Increased velocity is not only important for efficiency, but also to help keep particles in suspension so they pass through the exchanger. Most plate-and-frame heat exchangers will allow particles 0.0625" or smaller to pass. For liquids containing larger particles than this, "wide-gap" plate designs are available.
Figure 3. The total dimension of a plate pack is based on the number of plates and grids in the unit. This distance is measured between the inside of the head and follower. You should check this dimension and tighten the unit, if necessary, at least once a year.
TIP 6: Periodically Check Plate Pack Dimensions and Frame Integrity
Generally, it is easier to prevent a heat exchanger from leaking than to stop a leak once it occurs. An important point to remember is that a plate-and-frame heat exchanger is not tightened to a torque specification but rather to a platage dimension. Between each pair of plates is an elastomeric gasket that, as compressed, seals the area between the plates and prevents leakage. Picture a large heat exchanger with perhaps 600 to 700 plates, each with a gasket between them, and you can imagine how much compression will take place when you start to close the unit by tightening the tie-bars.
The manufacturer will provide a total dimension of the plate pack based upon the number of plates and grids in the unit (figure 3). This distance is measured between the inside of the head and follower. It is recommended that you check this dimension and tighten the unit if necessary, at least once a year. At the same time, check the tie-bars and frame components for any damage or corrosion that could occur in chemical environments.
TIP 7: Use Good Piping Practices
As with all process equipment, good, common-sense piping practices should be used. A heat exchanger makes a lousy pipe hanger. Ensure that all piping is supported properly and does not put any undue stress on the connections to the heat exchanger.
In steam applications, ensure that all condensate lines are properly pitched away from the heat exchanger so that condensate will not mix with the steam and flash back into vapor. One cubic foot of water evaporated at 212oF (100oC) and 14.69 psig becomes 1,606 ft3 of dry saturated steam! Steam traps should be included and used in accordance with local codes.
TIP 8: Take Precautions to Minimize Port Erosion
A safe rule of thumb is to keep port velocity at the heat exchanger around 20 ft/sec. In addition, consider port liners of the same material as the plates, even on water applications. These liners, available in many different alloys, will protect the carbon steel head from erosion and corrosion in high flow, abrasive applications. As an example, in a sea (salt) water application, it would be common to use a carbon steel frame with titanium port liners and plates. This practice ensures that any components in contact with the seawater would be constructed of the corrosion-resistant titanium.
Figure 4. Several components make up a plate-and-frame heat exchanger. If your heating or cooling needs change, you can accommodate the new requirements by adding or deleting plates within the frame.
TIP 9: Design for the Future, But Purchase on Your Current Needs
Other than its heat transfer efficiency, one of the great benefits of a plate-and-frame heat exchanger is its modularity. If your heating/cooling needs change in the future, you can easily accommodate the new requirements by adding or deleting plates within the frame. With a little up-front planning, this flexibility allows you to purchase the equipment you need right now, with the knowledge that your future expansion needs can be handled. Figure 4 shows the components that make up a plate-and-frame heat exchanger. Once purchased, the head and follower will remain the same. Most manufacturers have several frame lengths available for each model.
Some plate heat exchangers include bolt-on top and bottom bars rather than welded construction. With the bolt-on design, you simply bolt on longer carrying and bottom bars, install the additional number of plates required for your new production needs and replace the tie-bars with longer ones. However, it is important to wait and do this only when your production needs change. Don't try to cheat and buy a larger heat exchanger now in anticipation of those increased requirements down the road. The efficiency of a plate-and-frame heat exchanger is dependent on several variables, one being velocity and turbulence across the plates. If your exchanger size exceeds your needs, the velocity will decrease, thereby increasing the possibility of plate fouling.
TIP 10: Purchase OEM Parts to Avoid Warranty Problems
As with all types of equipment, it is common to find companies willing to provide other than original manufacturers parts at discounted prices. While some of these companies provide adequate engineering and support after the sale, others do not. Carefully consider the potential advantages and drawbacks of buying non-OEM replacement parts before making any decisions.