Cooler Fans - Half the Problem
With hot weather approaching Gas Plant Operators and Engineers face the annual problem of overheating engines, refrigerant condensers and process coolers. Each year the same ideas are tried and retried: pitch up the fan blades until the motors stall or until the belts slip; speed up the fans to maximum; change the fan to a high efficiency blade configuration; add blades, etc.
Whereas all these ideas may have some limited merit (which is discussed below) it is important to know that with aerial coolers, the fan (or airside heat transfer) is only half the problem. Tubeside or process-side, heat transfer may be half the problem. In the case of viscous fluid coolers (such as lube oil or amines), the process-side may be more than half the problem and therefore a remedy based on altering the process-side parameters may offer a more cost effective solution. Before discussing process-side issues, we need to understand the limits of airside or fan “fixes”.
Fan Speed
Airflow increases linearly with fan speed, but power increases with the cube of speed. Therefore to get an extra 10% airflow will require a 10% speed increase but at the cost of 33% fan power. Net heat transfer will increase approximately with the airflow to the exponent 0.3 which means that putting an extra 33% power into the fan will increase overall heat transfer by a mere 3%. In the case of overhead and refrigerant condensers, the extra airflow has a very beneficial primary effect on the cooler LMTD so this solution is not to be altogether disparaged. Most fans have a maximum safe running speed of around 13,000 feet/minute at the tip; otherwise tip speed is limited only by driver power.
Fan Pitch
Unlike speed, fan airflow will increase with pitch only up to stall point, beyond which the fan actually produces less airflow. Fan power, however, increases continually with increased pitch. It is therefore important to know the maximum pitch angle (or stall angle) for a given fan.
Fan Tip Clearance
API 661 states that this should be less than ½ % of diameter or less than ¾” (whichever is the smaller). The vast majority of commercial aerial cooler take exception to this very important parameter unless the fans are equipped with vortex tips or fan ring tip seals. This retrofit combined with bell-mouth ring entries, is one of the cheapest ways of improving cooler performance by a small margin.
Process-Side Parameters
In order to assess the most cost-effective way of improving cooler performance, a field test is usually advisable to assess whether the cooler is airside or tubeside controlling; i.e. which heat transfer component is effectively bottle-necking per-formance. If the answer is airside-control, then we pay attention to the fan; otherwise there are methods to improve tubeside heat transfer. These may include accelerator rods, turbulators, re-heading the cooler or even altering the fluid properties.
Most methods of improving tubeside heat transfer center on the fact that this transfer rate is primarily a function of fluid Reynold’s number, which depends on fluid viscosity and velocity and tube diameter. High Reynold’s number means high turbulence and better heat transfer. Installing accelerator rods or turbulator rods can alter tube effective diameter and fluid velocity. In one case an operator needed 35 MMBTU/hr of glycol/water aerial cooling for a large compressor intercooler. Standard 50/50 glycol/water is quite viscous and would have required a very large unit. By operating with 20/80 glycol/water in the summer and 50/50 only in the winter, operator saved $30,000 on the capital cost of the cooler and got 3° C better summer cooling operation. Bearing in mind that 3° C represented a 1% saving on compressor power; this translated to a 10-year lifetime power saving of $90,000 in utility costs for this machine.
Another operator improved the performance of an existing oil cooler by repiping two of the parallel bundles in series thus doubling the oil velocity which doubled the tubeside Reynold’s number. Bear in mind that ideas like this (as well as the retrofit of accelerator rods) will have a significant effect on process side hydraulics and may require reworking a process pump.
Noisy Coolers - A Summer Problem
Just as Gas Plant Operators may be speeding up cooler fans or increasing blade pitch to get improved summer performance, the other problem, which often peaks in summer, is that of gas plant noise complaints. There are two areas in Alberta where we see about 90% of gas plant noise related problems. These are two triangles bounded (roughly) in the SE by Edmonton/Calgary/Medicine Hat and in the NW by Grand Prairie/ Spirit River/Dawson Creek. Any gas plant or compressor in these fairly populated areas is statistically likely at some time in the plant’s life, to be subject to either a noise compliant due to existing operations or a noise concern when operator wishes to expand his facility.
Aerial cooler noise itself is likely to constitute 30% to 50% of any plant noise problem and this component becomes more dominant in summer when operator run coolers at maximum capacity and residents wish to sleep with windows open.
When an operator is faced with the dual problem of overheating coolers in summer and potential noise violations of the EUB ID 99-8, then solutions to overheating will preclude either fan speed increase or fan blade pitch increase – both of which significantly increase noise. Process-side solutions or the retrofit of high efficiency fans, tip seals, bell-entries, vortex blade tips and/or fan silencers become the potential solution. Sometimes a combination of the above is necessary to resolve the combined process/noise problems.
