10 top tips for plate and frame heat exchangers

1. Be sure to communicate all pertinent information regarding your application to the manufacturer when you request a quotation.

There are two liquid streams going into and coming out of a heat exchanger. They are referred to as the “hot side” and the “cold side”. As a result, there are four temperatures to consider when sizing the heat exchanger. They are the hot side inlet and outlet and the cold side inlet and outlet temperatures. In order to properly size the heat exchanger, you will need to supply the manufacturer with any three of the four temperatures. In addition, there is some other basic data for liquids on both the hot and cold sides that are required when properly sizing a plate and frame heat exchanger. They include the fluids being used, the viscosity, vapor pressure, thermal conductivity, system pressure, specific gravity, and the allowable pressure drop for the system. 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.

2. Check with manufacturer for chemical compatibility between the heat exchanger components and your process liquids and 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 plants today. However, one item that is not friendly to 316 is chlorides. The effect of chlorides on 316 is temperature related. A rule-of-thumb for chlorides compatibility with 316 is as follows: 180ppm (parts per million) at 122?F, 120 ppm at 170?F, and 50 ppm at 212?F. Plate and frame heat exchangers are commonly used in conjunction with cooling towers. The heat exchanger should have been designed for your needs based on the chemicals you are using at that time. If you are thinking of switching to more aggressive chemicals to remedy a problem within the cooling tower, such as algae, verify the impact this will have on the level of chlorides in your system. Another typical situation occurs in the food industry where it is a common practice to CIP (clean-in-place) the unit. This involves circulating a caustic cleaning solution throughout the system. Make certain that the cleaning solutions you are using are compatible with the heat exchanger plates and gaskets. Some manufacturers use a 316 stainless steel with a higher nickel content such as 316L, which will be more resistant to these chlorides, but it would be wise to check with them before implementing any chemical change.

3. Avoid situations where pressure spikes can occur.

Again, remember that there are elastomeric gaskets between each plate in the heat exchanger. Although manufacturers have some extra allowances in their design engineering, it is important to stay within the pressure limits of the heat exchangers rating. If there is the possibility of a pressure spike within the system piping 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 gasket. Remedies for this situation include, relief valves, rupture discs, and/or pulsation dampers and arrestors. Check with the manufacturer for recommendations.

4. Use a filter on the inlet of the heat exchanger or by-pass the unit during start-up.

In most applications, it is advisable to include at least a temporary strainer on the cold or hot water inlet and/or the inlet of the process liquid as well. Even if your liquids are clear, with no particulates, a temporary strainer or bypass around the PHE is recommended for new, start-up 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 (even a work boot has been found in the suction line of a large pump). If this debris finds it’s way into the exchanger, it may get trapped causing an increase in pressure drop and a possible drop in efficiency.

5. Minimize erosion in the port area by lowering nozzle velocities.

A safe rule-of-thumb is to keep port velocity at the heat exchanger to around 20 feet/second. Any increase above this rate could possibly cause premature wear on the ports themselves. In addition, you should consider port liners, even on water applications. These liners, available in many different alloys will not only protect the carbon steel head from erosion, but from corrosion as well. For instance, in a sea (salt) water application. It would be common to use a carbon steel frame with titanium port liners and plates. This would ensure that any and all components in contact with the seawater would only be the corrosion resistant titanium.

6. Periodically check “plate pack” tightening dimensions and the integrity of the frame, tie-bars, etc.

It is typically much 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 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 600-700 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. Since there is so much compression of the gaskets, it would be impossible to determine a proper torque rating for the exchanger. Instead, by knowing the level of compression needed per gasket, and the quantity of gaskets in the unit, the manufacturer is able to determine the total dimension of the plate pack when properly tightened. 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.

7. Ensure that no particulate larger than the plate gap dimension is allowed to enter the heat exchanger.

The plates in a PHE are typically in a chevron or V-shape. Every other plate is reversed causing a waffle-type grid between those plates. The space between the plates is called the plate gap and in many cases can be quite small. Particles entering into the exchanger can potentially get caught in the gap. As stated in tip number two above, it is necessary to keep larger particles out of 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 to the product. Increased velocity is not only important for efficiency, but also to help keep particles in suspension so they pass through the exchanger.

8. Use good piping practices especially on steam applications.

As with all process equipment, good, commonsense piping practices should be used. A heat exchanger makes a lousy pipe hanger. Make sure that all piping is properly supported 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 212?F and 14.69psi becomes 1606 cubic feet of dry saturated steam! Steam traps should be included used in accordance with local codes.

9. Design your heat exchanger for future expansion capabilities, but purchase based on your current needs.

Other than it’s incomparable 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. In diagram (A) you can see 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. Higher quality PHE’s include bolt-on top and bottom bars as opposed to 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.

10. Purchase only 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 (pirated parts) at discounted prices. While some of these companies do provide adequate engineering and support after the sale, others do not. Plate technology and geometry is a rather complex equation and quality manufacturers spend a lot of time and money developing plates that are durable and efficient. What may appear to be a bargain may wind up costing you in the long run with either decreased thermal efficiency or a plate that will crack and leak in a short period of time. Spending a few dollars more by purchasing OEM parts will save you many more dollars down the road.

Article written by: APV Application Engineers
For further information please contact: apvproducts.us@invensys.com
Or visit the website: www.apv.invensys.com

APV Press Release, Nov 10, 2006 [doc]
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