Recirculation rates have long been a designer's quandary. Without a full understanding of how evaporators work, many system designers grossly oversize ammonia liquid recirculation lines. Unfortunately, the indiscriminate use of high recirculation rates can compromise operating efficiency and greatly affect the system's safety. Knowing how parameters such as pressure, circuiting and orifice size affect evaporator performance can help designers specify the optimum recirculation rates for their application.

Flow Basics

The flow of liquid and gas through a typical evaporator (an air unit with vertical or horizontal headers and generally orificed circuits) varies from 100 percent liquid to whatever the quality may be at the exit of the circuits. For a circuit without orifices, the flow also is influenced by the pressure and volume of the liquid being forced into the circuits. In theory, as the liquid transforms to a high vapor content, the velocity will continue to increase. The two-phase flow will experience many different transformations such as slugs, wavy, annular and dispersion. Each transformation has unique heat transfer characteristics and influences how well or continuously the evaporator's coil surface is covered with liquid refrigerant.

In top-in/bottom-out evaporators, the liquid often separates and lies in the bottom of the tubes. As a result, a higher recirculation rate (above 1:1) will probably be required to obtain the maximum amount of heat transfer. Liquid separation is unlikely to occur in bottom-in/top-out evaporators because the liquid is being “pushed” uphill rather violently by gas molecules vibrating at their sonic velocity. The quantity of the gas (the liquid not “boiled”) influences the process in several ways, including the amount of wetted surface, the pressure drop, and the velocity of the gas and liquid as they pass through the circuit. While gravity influences the liquid, the dominant forces on the liquid and gas are friction, separation characteristics and, eventually, the dispersion of droplets when the liquid becomes entrained in the gas.

Software that provides good indications of pressure drop can be purchased or found online. Alternatively, pressure drop information can be obtained from the evaporator manufacturer.

Evaporator Circuits

While knowledge of evaporator circuits is highly proprietary with each manufacturer, recirculation rates can influence circuit lengths, evaporator temperatures and defrost capabilities when circuits are used for hot gas defrosting (ice melting). Some manufacturers will provide larger orifices on the bottom circuits to make sure enough hot gas condensate can pass through the lower circuit to complete the defrost.

Evaporator systems using 1:1.5 and lower recirculation rates have been successfully designed for this application for years. Some designers think that coils are not efficient at lower recirculation rates; however, it is the condition of the coils during operation that is important in determining efficiency. Coils that frost evenly to the top of the circuits are not working near optimum levels. Increasing the recirculation rate from 1.5:1 to 4:1 can cause the circuits to experience higher pressure drops and higher temperatures. Pressurizing the circuits with a higher volume of liquid will then raise the average boiling temperature of the refrigerant in the circuits. This actually occurred in one case, in which “live brine” circuits had to be used to keep the liquid ammonia under pressure and thereby increase the velocity to enhance the heat transfer rates. This solution required a higher volume of pumped liquid (and therefore more horsepower) and a more sophisticated piping design.

Some evaporator designers using coils with no orifice buttons have requested recirculation rates of 7:1, only to find out that evaporator temperatures could not be obtained because the circuits were being “pressurized” by the pumps. In other cases, plants have requested a recirculating pump for 4:1 while the evaporators could only handle 1.5:1, making the pumps grossly oversized. Without large bypass regulators, the pumps were unable to handle the low flow levels without cavitating.

Orifice Size

The amount of flow that will pass through a given orifice size depends on pressure, which is a function of the orifice elevation. Assuming a 4'-high circuit (headers), it would be reasonable to expect 1 psig of pressure difference between the upper and lower circuits if the driving pressure at an evaporator inlet after the heat exchanger regulator valve is 5 psig and at the other circuits is 4 psig. Except for some lower circuits that are kept oversized for defrost conditions, most manufacturers vary orifice size as a function of elevation.

Knowledgeable manufacturers can design evaporator circuits such as these air units for specific recirculation rates.

The Pressure Drop Connection

Computer software and field trials can help system designers identify the optimum recirculation rates for their operation. Tom Cooper of RCI in Grand Rapids, Mich., uses an air unit computer program (along with years of experience and a lot of common sense) to help design refrigeration evaporators for specialty meat chillers manufactured by Marlen Research Corp., Overland Park, Kan.

Through field tests, Cooper has observed that the maximum heat transfer occurs when zero superheat is reached using a recirculation rate of 1:1 in a bottom-in, top-coil configuration. He also has seen that little or no pressure drop occurs in suction risers as high as 15'. Conversely, appreciable pressure drops have been observed as the recirculation rate is increased above 1:1 (except in systems with top-in/bottom-out evaporators, as noted previously).

Knowledgeable manufacturers can design evaporator circuits for specific recirculation rates. If an application requires a 1.5:1 ratio, an evaporator with a 1.5:1 recirculation rate will be at least as efficient as an evaporator with a 4:1 ratio - and probably even more so. It might also have longer circuits and should therefore cost less than coils that use more circuits and higher recirculation rates.

Because recirculation systems operate with low liquid pressure and use liquid without much flash gas content, they are inherently safer than the “old” high-pressure liquid line and surge drum setups. However, this safety is compromised when liquid recirculation lines are grossly oversized. By specifying systems with “right-sized” lines, designers can optimize the safety and operating efficiency of ammonia refrigeration systems.

Want to learn more?

For more information...
From Bonar Engineering Call (703) 414-5320.

From RCI Call (616) 785-7335.

From Marlen Research Corp. Call (913) 888-3333.

Sidebar: One Man's Advice: Don't Be Afraid To Ask Questions

Several years ago, I began asking manufacturers for larger and larger air units - sizes that were not even in the catalog. One manufacturer was willing to make them. At the time, the manufacturer used vertical headers, and the air units approached 6' in height. After startup, it became evident that the lower circuits of the freezer coils were difficult to defrost. I visited the facility and asked whether the lower circuits could have an enlarged orifice hole to facilitate defrosting. However, I was told this was not a plausible solution.

A few years later, after the air unit manufacturing company had been sold, I was at lunch one day with the company's national sales manager. During the course of our conversation, he happened to mention that the company had begun enlarging the orifice buttons in lower circuits of coils to facilitate defrosting. (He was unaware that I had suggested this several years earlier.).

Whether you're sizing recirculation systems or troubleshooting a cooling problem, don't ever hesitate to ask what might appear to be a stupid question. You never know when it might lead to a solution. - Henry Bonar

Links