Aerial Coolers

In a single aerial cooler tfe.-e mav be several fans and several tube bundles as shown in Figure 3-16, wl bay width, tube leng ... 

Louvers are probably the most common type of temperature control device on aerial coolers. They may be either automatically adjusted by sensing the process temperature or manually adju.sled. Blade pitch is probably second most common, and variable speed drive is third. 

The procedure for calculating the number of tubes required for an aerial cooler is similar to that for a shell- and-tube exchanger. Table 3-6 shows approximate overall heat transfer coefficients. Ub should be used when the outside surface area of the bare tube (neglecting fins) is used in the heat... 

In the interest of energy conversion, process heat can be obtained from a heat recovery unit in which heat is recovered from turbine or reciprocating engine exhaust. In a heat recovery unit, an exhaust gas flows over finned tubes carrying the fluid to be heated. The hot exhaust gas (9()0"F to I.2(K) F) heats the fluid in the tubes in a manner similar to that in which air cools the fluid in an aerial cooler. It is also possible to recover heat from exhausts by routing the exhaust duct directly through a fluid bath. The latter option is relatively inefficient but easy to install and control. 

Note that more than 30% of the heat duty was required to cool the water and condensate. If the liquids had first been separated, a smaller exchanger and lower seawater flow rate could have been used. In most gas facilities, where cooling is required, the cooler is placed downstream of the first separator for this reason. Often an aerial cooler is used for this service. 

In this example we selected a final outlet temperature of 100°F, This would be sufficiently low if the gas were only going to be compressed and dehydrated. For our case, we must also treat the gas for H2S and COt removal (Chapter 7). If we chose an amine unit, which we will in all likelihood, the heat of the reaction could heat the gas more than 10° to 20 T. making the next step, glycol dehydration, difficult (Chapter 8). In such a case, it may be better to cool the gas initially to a lower temperature so that it is still below 110°F at the glycol dehydrator. Often this is not possible, since cooling water is not available and ambient air conditions are in the 95°F to 1()0°F range. If this is so, it may be necessary to use an aerial cooler to cool the gas before treating, and another one to cool it before dehydration. 

These cool the interstage gas. They may also be required to cool the discharge gas prior to gas treating or dehydration or to meet pipeline specifications. Typically, aerial coolers are used in these situations. 

Forced coolant systems using a mixture of glycol and water are the most common for natural gas compressors. Normally, the compressor cylinder cooling system and compressor frame lube oil cooling system is combined. A single pump is used to circulate the coolant through the cylinders and the lube oil heat exchanger and then to an aerial cooler where the heat is dissipated. 

The process for absorbing acid gas takes place in two stages (1) absorption and (2) regeneration. The absorption takes place in a column where the sour gas is contacted with the lean solvent. The rich solvent is sent to a second column where the solvent is regenerated. Heat is applied to the system via a reboiler and the overheads are condensed, typically in an aerial cooler. The solvent regeneration is done not only at higher temperature, but also at lower pressure. Figure 1.1 is a schematic of the process. 

Compression increases the temperature of the gas and thus the fluid must be cooled after each stage of compression. The cooling is usually achieved using an aerial cooler. 

Interstage cooling is usually achieved using aerial coolers. The design of these cooler is such that the exit temperature of the gas is about 40°C (120°F). In warmer climates this design temperature maybe as high as 50° or 55°C. 

The design on an aerial cooler is the same as for any heat exchanger. As with the compressor, our design begins with the First Law of Thermodynamics, Equation (6.1). However, in the case of the cooler, there is no shaft work and it is the heat transfer that we wish to calculate. Thus equation (6.1) becomes ... 

Charts are available for the F for various flow configurations in the heat exchangers. These charts give the F as a function of the various temperatures. A chart for the typical configuration of an aerial cooler is given in Incropera and De Witt (1990) (and other textbooks on heat transfer). 

At first glance, it would appear that three or four times as much glycol circulation is required. But remember that the 120°F compressed gas is not saturated with water vapor it is really superheated. The compressed gas will have the same water content until it is cooled by the aerial cooler... 

As a rule of thumb, the glycol reboiler temperatures should be increased by 10°F for every 5°F increase in the equilibrium top tray temperature of the contact tower. Therefore, if you are drying gas from the discharge of an aerial cooler, you must raise your reboiler temperature by 10"F when the ambient temperature rises by 5°F to maintain a constant moisture spec in your dried gas. And remember, this is true regardless of the water content or flow rate of the wet gas. 

The required engine horsepower, but not the volumetric capacity, of a two-stage reciprocating compressor is affected by the intercooler outlet temperature. As a rule of thumb, each increment of 12 F increases the engine horsepower load by 1%. Frequently, all that is needed to minimize this problem is an occasional external water wash of the intercooler aerial fin-tube cooler. 

Imagine you are sitting in a meeting when the vice president asks, "Should we use an aerial cooler or colder seawater to condense the power plant turbine exhaust steam " Harry, the chief engineer, offers to run through the relative efficiency of the turbine for the two cases using his computer model and report back after lunch. You, the junior staff engineer, look up into space and after a few moments state, "Based on the Second Law of Thermodynamics I ve calculated an enhanced efficiency of 16.1 percent for the cooler seawater case." I always add that extra decimal for dramatic effect. 

As lower temperatures favor the absorption of CO by the Catacarb solution, I had expanded the capacity of the circulating Catacarb solution aerial fin-fan cooler to cool the absorber tower and thus reduce CO in the Hj product. Liz stood next to me at the panel when we started the Catacarb circulation pumps. I was totally confident. 

Liz said that I went deathly pale. I was soaked with sweat in the chilly control room. The operations chief and panel room operators beamed with pleasure. Confused, I asked them to turn on the last fan on the aerial fin-fan solution cooler. The absorber temperature dropped by 10°F and the CO in the hydrogen product went up from 3.5 to 4 percent CO. ... 

I shut off one of the four aerial cooler fans on the lean Catacarb solution (see Fig. 33.2), which helped a lot. 

So I shut off, one by one, over a period of days, all the aerial cooler fans. The absorber became quite hot and the CO dropped to 0.5 percent in the product. None of this made any sense. CO absorption in Catacarb solution should be favored by lower, not higher, temperatures. Equilibrium conditions for CO absorption are always improved by cooling the circulating absorption liquid. 

The catalyst was like a cold drink on a hot summer s day. 1 put all the variables I had changed in three weeks back to normal conditions. I turned on all the aerial cooler fin-fans. The CO dropped below 0.2 percent in the hydrogen product. 

A detailed discussion of tube and shell, aerial coolers, and fired heaters is found in... 

A pump around reflux system is provided at the draw tray to recover heat at a high-tanperature level and minimize the low-temperature level heat removed by the overhead condenser, which cannot normally be recovered by heat exchange and is rejected to the atmosphere through a water cooling tower or aerial coolers. 

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