Other potential cooling methods might be vaporized nitrogen injection into the air inlet to lower air temperatures (beware of the asphyxiation hazard) or chilling the inlet air to the condenser. This wouldn't be cheap but might pay out in process throughput. It appears you only need to cool the inlet air to less than 105°F to maintain full production capability.
As always, thoroughly consider the process safety management risks associated with making changes before implementing any changes to your process equipment.
Pete Bisila, system reliability engineer-utilities
Flint Hills Resources, Inver Grove Heights, Minn.
CHECK INTO SEVERAL CHANGES
Here are a couple of suggestions that may help improve condenser performance until you can shutdown:
1. For a fixed UA, raising the column pressure increases the logarithmic mean temperature differential (LMTD) for the condenser and improves the ability to condense. Make sure your re-boiler has some margin for over-design because at a higher column pressure the re-boiler will become stressed due to an LMTD pinch.
2. If possible, add a cooling water supply, via spray nozzle distributor system to the condenser. Spray cooling water over the fins under the fan housing as the air is moving up and over the fins. This will provide an additional heat sink for the overhead stream of the column. You may need to collect this water into a portable sump that may need to get pumped on some frequency back to the cooling tower basin or the return headers.
3. Can your fan speed be adjusted higher to improve your heat transfer coefficient? If so, adjust higher if you have enough motor to do this.
When you have the ability to shut down, check the condenser for fouling. Have you been able to get the desired performance of the condenser in the summer months in the past or is this always the case in the summer? If the condenser performed reliably in the past, it may be more fouled now than before.
If the condenser has never performed, then it may be an issue with the design. Check the percent relative humidity you are currently experiencing versus the original design. If your relative humidity is higher than what was specified for a given rate, then the condenser may be under-sized for the duty required.
From an overall standpoint, check all current operating parameters with original design parameters and if something stands out that can significantly impact performance, e.g., flow, LMTD, blade tip angle, speed, etc., you may need to redesign the air cooler for your current conditions at the next available outage.
Eric M. Roy, principal engineer
Westlake Chemical, Houston, Texas
ASSESS THREE OPTIONS
Based on the description of the problem, the condenser seems to be working well. For a commercial unit to get a 20°F approach temperature on an air-fin is pretty good performance. Upgrading a unit with good performance to one with excellent performance is a challenge. For a conventional air-fin the temperature pinch normally occurs at the process outlet of the exchanger. Getting better performance comes down to three options: improve the approach; increase the saturation temperature of the process; or decrease the air temperature in.
Air-side fouling starts at the bottom of the air-fin where the air enters. Make sure that the bottom tube rows are as clean as possible. Pollen, fibers and dirt all increase approach temperatures.
Use a thermal imaging camera to look over surface temperatures. Cold spots show areas where fouling insulates the tubes. Additionally, the thermal imaging scan may show areas of low air flow (the tubes will be hot there). Look for what might cause this. Does the exchanger have air bypass due to leaks at the side, plenum damage or other factors? When pushing the equipment to the limit, good maintenance practices count for a lot.
Raising the tower operating pressure will increase the saturation temperature. This shifts the latent heat out of the cooling-water exchanger and into the air-fin. However, raising the pressure has other consequences as well. The higher operating pressure will increase the reboiler temperature. This may boost reboiler fouling or cause a pinch against available heat. At 155–160 psig, tower capacity may either increase or decrease. For hydrocarbons in this range, the balance of typical tray designs coupled with vapor and liquid density can create situations where capacity changes aren't obvious. If you do raise pressure, watch tower hydraulic capacity carefully.
If the tower is over-reboiled, see how much the bottoms duty can be turned down. Any extra duty into the bottom must be removed from the top as well. This is less likely to be successful than changing the tower pressure, but may still help.
On the air side, check local air currents with a ribbon strip — basically a streamer attached to a PVC pipe. Look for recirculation of air around the exchanger. Strategically placed barriers can dramatically lower air recirculation and keep air temperatures down.
There's one option to decrease the air temperature in – using a high pressure drop across a spray nozzle, mist underneath the exchanger. This isn't intended to put bulk water on the exchanger. The object is to create humidification cooling when the water evaporates. The water needs to evaporate before it gets to the exchanger. Pressure drops of up to 400 psig may be required on the spray nozzles. The target is to get roughly 50–100-micron water particles. This gives quick evaporation. The evaporation then cools the air. Depending upon local humidity, a temperature drop of 5–10°F may be possible. Unless the water is used for only short periods of time, very clean water is required. The exchanger will collect much of the solids created by material in the water.
Brute force increases in air capacity are less likely to be successful. Fan changes, speed changes, more motor power and other modifications may increase air rates. However, a 20°F approach temperature is already pretty good for a large commercial air-fin. Extra air will not move the outlet pinch very much under these conditions.