ACHRNEWS

Don’t be fooled; Process cooling is far different from comfort cooling

September 18, 2000
It is critical to have the best possible load information when sizing a chiller for comfort cooling. It is even more critical to have all the load variables and time constraints for a process cooling application — and the information may not be available.

The primary differences between a comfort cooling chiller and a process cooling chiller are the sanitary requirements and temperatures at which they operate. However, their specific design elements are such that chillers currently on the market are usually not suitable for both applications.

A falling-film chiller is one of the chiller designs especially suited for process cooling. This chiller allows you to cool most fluids to within 1°F of the freeze point without the fear of chiller damage, and this can be done with a closer evaporator temperature than you might expect (5° to 7° or less).

In a falling-film chiller, the fluid being chilled falls in a thin film over vertical evaporator plates into an insulated reservoir that is a part of the chiller. It is pumped from the reservoir to the process, where it absorbs heat.

In some processes, such as makeup water for a food or beverage product, the fluid is consumed in the product; in others it is returned to the chiller for removal of the absorbed heat.

The principal advantage of a falling-film chiller is the low temperatures to which the fluid can be cooled. And because of its design, it is easy to clean and is easily expandable, many times without the need for a costly second installation.

A regulatory body (USDA, FDA, and 3A Standards) dictates many chiller design requirements for process chillers in the food and beverage industries. These regulations often specify design requirements that make one type of chiller particularly well suited for an application.

One of the biggest problems in the selection of a chiller is the lack of an understanding of these application-specific sanitary needs.

Sanitary requirements include that the product-contact side of the evaporator be easy to clean and inspect. They dictate construction features such as minimum radii, where every two surfaces meet at a 90-degree angle and have no cracks or crevices in the construction.

They also specify material types and surface finishes acceptable for an application. Stainless steels with a smooth finish are the most common materials used for the construction of chillers in process industries. Falling-film chillers are commonly built of stainless steel for ease of cleaning and corrosion resistance.

Load requirements

Next to the sanitary requirements, to properly size a process chiller, you must understand the application-specific load demands and time constraints, which vary for every process.

Falling-film chillers are used heavily in the produce-processing industry. To properly size a chiller for this application, you will have to consider the condition of the return water and final product temperature desired.

You will have to deal with produce, foam, and dirt returning to the chiller, no matter how well you filter the return water. You will also have to deal with a large volume of system water that will be changed several times during the day, with a short time period allowed for cooling the replacement water.

In some cases, the water refill-cooling load could be 10 or more times greater than the normal operating load.

In most produce applications, the water is used for both cooling product and moving the product through the system, so water flow velocities become critical. In this situation, you have high water flows with low temperature differences. In other applications, the makeup water load becomes a major factor to consider.

There also are times when the proper choice could be multiple chillers, one for makeup water and one for the process load.

Understanding the real load and time constraints of a process cooling application is the number-one problem in delivering satisfactory performance.

To properly size a process chiller, you must perform several load calculations and add several different loads together to arrive at the true total load.

Start with the product load, then look at the makeup water load (always consider summer water temperatures), and add in the ambient temperature load factors from the process area. Don’t forget the load from the pump, and finally, any and all other miscellaneous load contributors.

Bakeries are another place where falling-film chillers are used, mainly for cooling water used in making the dough. Most bakeries produce multiple batches of various baked goods — many times multiple batches of product per hour. Batch cooling for bakery-ingredient water, unlike constant-flow chilling used in produce cooling, adds a few different considerations to the equation:

  •  A falling-film chiller selected for batch cooling in a bakery must have a properly sized storage tank. Too large a tank will increase the compressor horsepower requirement as much as a storage tank that is too small.

  •  The next critical consideration is the amount of time it takes to move the water from the storage tank to the mixer(s), and the amount of time it takes to refill the storage tank.

  •  Another important factor is that makeup water cannot enter the storage tank during a batch draw cycle for the mixer. This would affect the final water temperature delivered to the mixer and damage the product.

Once you have considered all of the factors and time constraints affecting the load requirements, you can start the condensing unit selection process.

Condensing unit choices

In a process application, you might have determined the load is 20 tons but due to a time constraint, the full load is required during 40 min instead of 1 hr. This means the condensing unit must have a 30-ton capacity, not the 20-ton load originally determined.

Sometimes the project specifications may be incorrect. You may have prior knowledge of an application and be aware the product temperature coming in is going to be warmer than the specified design temperature. And, you may know the supply water is warmer than the specified design temperature, at least for a major portion of the year.

How you tackle this problem can get down to the basics of how you run your business.

You can quote exactly to the specifications and count on being among the lowest bidders. Or, you can quote the specifications as you know them, when providing that exact quote might put you out of the bidding process.

But if you bid to the specifications knowing they are not correct, you also know you will end up trying to explain during the next peak-load condition why the system can’t keep up.

The design engineer on the project seldom has prior knowledge of a contractor who has been keeping the equipment running, or may not know all of the peculiarities of an operation. He/she may not know the water supply line runs 500 ft under a large asphalt parking lot that increases the water temperature 10° on a sunny day.

And if the design engineer did know this, where would he find the information to accurately predict the effect?

A concept I am comfortable with is providing a quotation to the specifications and an alternate which, to the best of my knowledge, will meet the true performance. Then, I start a dialog with all those involved until we reach an agreement on what is actually needed.

A whole slew of operating conditions

Produce cooling and bakery ingredient water chilling are just two examples of falling-film chiller use. Both of them are dealing with cooling water.

Falling-film chillers are frequently used to cool liquids other than water. Each of these liquids presents its own challenges for the person sizing and selecting the equipment. Technical information on the customer’s product may not be available, and sizing may have to be based on assumptions.

But even with water, the difference between winter and summer temperatures may present another consideration. These differences in supply water temperature frequently make it necessary to plan on unloading the system when cooler-than-normal water temperatures are encountered.

Other seasonal or operational changes may also make this necessary. Process chillers usually operate year round. A produce chiller may need to operate under vastly different seasonal conditions. It is too late when the chiller is on-line and you figure out the system must operate at 10% capacity — and you only have one compressor.

In process cooling applications, you must understand and size for the real load under all operating conditions. A falling-film chiller is an excellent chiller for process cooling. It is one of the most flexible chiller designs available.

But, use of a falling-film chiller is not flexible enough to make up for an improperly calculated cooling load.