Efficiency Gains Possible Through Optimal Heat Transfer

New and innovative technologies can mean significant energy, labor and parts savings for ethanol facilities aiming for greater profitability. Heat exchanger maintenance is one area producers can work to be more cost efficient.
By Gerek Foote | November 21, 2013

Today’s ethanol plant objectives are far different from what they were even just a few years ago. With Big Oil attacks and decreased margins, today’s plants must find ways to cut costs and find innovative ways to increase profit margins. Unfortunately, these costs are often accomplished by cutting maintenance budgets and shortening manpower lists. One costly area in the maintenance budget is the effective operation of plate heat exchangers. Many plants still spend tens of thousands of dollars on maintaining efficient heat transfer. However, this cost in itself is inefficient. 

Plate heat exchanger maintenance needs can vary by location, however it is still necessary. Many plants rarely open the units and thus save cost. These plants are typically located in areas with soft, clean water sources. Other plants must service units every year due to hard water or plate erosion or dirty cooling towers. Hard water is not an easy problem to fix, however plate erosion can be remedied. It has been said that a plate heat exchanger makes a great filter. This may be true if the particulates in the fluid are of a size that exceeds a significant portion—about 60 percent—of the channel spacing.  No heat exchanger will function properly if the majority of the available cross section and heat transfer surface is clogged, either due to the size of solid particulates or from scaling from precipitates.

Plate erosion, especially in the fermenter coolers, is caused by high port velocities. Several plants have been fighting erosion of the fermenter cooler plates since opening. The sad fact is that the plants know and understand the root cause of this costly problem. Typical process designs call for 12-inch piping on the fermenter coolers; however, the heat exchangers are equipped with 8-inch ports. This drastic reduction causes a spike in the port velocity and thus shortens the life span of the plate pack through excessive erosion in the high velocity field. 

Average costs of these plate packs range from $30,000 to $50,000. When a plant is forced to replace one or two plate packs just in the fermenter cooler per year, it becomes a major cost against the maintenance budget at $30,000 to $100,000 per year. This price is just for the new plate pack and does not include installation charges. In addition, onsite service can cost almost as much as the plate pack itself. 

The solution is to replace the fermenter cooler heat exchanger with a 14-inch port exchanger and thus lower the inlet port velocity. In some cases, the port velocity can be lowered from 50 to about 16 feet per second. This will exponentially lengthen the life of the plate packs. Plants should do their research to find the proper units to fit the needs of the plant. A few manufacturers have units that are easily serviced by the plant maintenance team without hiring outside labor teams. Training can also be provided by many manufacturers, if needed, increasing the maintenance team’s value to the plant by saving potentially significant maintenance costs. The same approach can also be adapted to other less-demanding plate heat exchanger systems in the plant, as well as those that require frequent maintenance.

Options for Consideration
As heat exchanger designs evolve, the added value of a gasket-free option is becoming increasingly popular. Today’s plants have a number of factors to consider when looking into welded-plate heat exchangers: Is the unit cleanable? Is the unit cleanable on both sides, for both fluids? Are there pressure limitations? And last, what is the cost? 

Welded heat exchangers are becoming more prevalent in today’s market as footprints get smaller and greater efficiency is needed. In the past, the only acceptable and viable option was a shell-and-tube unit. However, shell-and-tube exchangers can be costly and become very large, depending on the application. Finding the right welded exchanger for each specific application is critical to the successful operation of a plant. These units can be costly, and choosing the proper technology for the application is essential to maintain proper efficiency and longevity. Welded units, however, also require few parts with the lack of gaskets between plates. The accompanying chart illustrates different available technologies and their range parameters.  

When regular cleaning is necessary, the frequency depends on the application. These units can be cleaned using CIP (clean in place) or manually pressure washed, if they contain a removable core or are block style. Manual pressure washing may be enhanced by the type of geometry. For example, utilizing a double-dimple, block-style exchanger allows the user to directly hydroblast straight through the channel instead of at a reduced angle, making cleaning more effective and time-saving. Select models may even be cleaned using a brush when mechanically accessible with sufficient clearance. Some designs of welded units can only be cleaned from one side of the exchanger while a few designs allow access on both sides for thorough cleaning to maintain proper efficiency. 

Efficiency Follows Design
Proper heat exchanger design is the key to maximum efficiency. This is especially true regarding compact or plate-style heat exchangers. Plate-style exchangers are efficient heat transfer tools and also very sensitive to fouling. Heat exchangers are designed for specific applications with specific design criteria in mind. Flows, temperatures and media information (specific heat, viscosity, specific gravity, etc.) are critical information needed to properly size an exchanger. For instance, if the design specification is off a few percentage points on a thermal property such as specific heat or dynamic viscosity, it can have detrimental effects on the actual performance of the heat exchanger once installed. 

Plate-and-frame exchangers have the benefit of being expanded or reduced in the field to maximize efficiency. This is also a benefit when a plant is designed with a possible future expansion in mind. Welded and shell-and-tube heat exchangers do not have this benefit, so getting the design correct from the start is imperative. When considering design parameters to allow for fouling, it is very important to avoid using “canned” fouling factors such as those recommended by the Tubular Exchanger Manufacturers Association, as this can lead to seriously over-dimensioned compact heat exchangers. They may end up performing at a higher fouling rate due to reduced wall shear stress as a result of this requirement. An improved approach to this dilemma is to use a design margin that incorporates all uncertainties in the thermal properties as well as process parameters. There are several issues with using prescribed fouling factors: 

1) Published factors do not always reflect true service experience and are sometimes too high, sometimes too low.  

2) The factors are reported as static values, but in reality, they are dynamic, based on the mechanisms involved. 

3) Temperature and velocity of fluids most likely influence the level of fouling, but published fouling factors have limited at best accountability for these effects.  

4) Fouling factors often implicitly account for uncertainty in heat transfer methods, which can result in duplication of uncertainty effects.

Ethanol processes are continually evolving and increasingly involve different feedstocks and new technologies. A similar flexible approach should be used regarding heat transfer. Looking into waste heat/cooling recovery can save significant expense over time with minimal costs and quick return on investment. One product that is excellent for heat recovery is an inflated, plate-type exchanger. This product is a simple design that can be bolted to any vessel, tank or piping, providing a cost-efficient way to recover latent heat from waste (low pressure) steam or to heat a fluid inside a tank. The possibilities with this type of exchanger are virtually endless as it is readily customizable. A little creativity can bring large return on investment in a short time.  

Today’s modern heat exchanger manufacturers have produced innovative, maintenance-friendly products to ease the costs of the maintenance department at the plant level. Maintenance and plant managers should research and continue to explore the vast technologies available today. The ethanol industry is all about energy savings and advanced technologies. Heat transfer technologies follow the trend by continually evolving to improve methods and efficiencies. Utilizing and maintaining the same tired equipment can be costly; a little research into new and innovative technologies can mean significant energy, labor and parts savings. 

Author: Gerek Foote
Segment Sales Representative
GEA Heat Exchangers Inc., PHE Division