Whether you are in a mature industrial plant or relocating to an existing facility, the plant engineer is faced with unique problems adapting to the existing infrastructure. However, through some simple design changes or use of additional hardware, he can find new means to supply the facility with the necessary utilities.
Potable Water SupplyFew plants are designed with adequate expansion with regards to their city water supply. As the demands grow due to expansion or process changes, the engineer is required to make changes to the system. Since the potable water supply directly affects both the process requirements in the facility, as well as the normal plumbing system, it is important that the engineer designs the system with adequate pressure-compensating elements.
A basic method to handle the supply requirements as well as the pressure concerns is to install booster pumps where the water enters the facility. In a larger facility, it would be common to split the main water line after the meter into two separate systems--potable, which would be for normal plumbing use, and process, which would supply the manufacturing process or all other uses that would not require potable water, such as cooling tower makeup. Each branch would have its own set of backflow preventers and booster pumps. The booster pumps greatly increase the capacity of the system and compensate for the pressure drop lost due to the backflow preventers.
Pressure control is easily obtained through the use of a Variable Frequency Drive (VFD). Some VFDs make the job simple by allowing the use of a controller card that accepts the signal from the pressure transducer, thereby eliminating the need to purchase a separate loop controller. The VFDs isolate the two systems so that major demand swings from the process side of the plant do not affect the flow and pressure on the potable side. It would not be uncommon for the potable side pressure to be greatly affected when a main line of the process side would open to fill a large tank. While it might not be a common occurrence, it is disturbing when there isn't even enough pressure to take a drink from the water cooler.
Another benefit of the use of the VFDs on the process side can also be seen with respect to water usage and process control. With an uncontrolled system, it is common to have extremely high pressures when the entire system is not up and running. This condition normally would occur during the startup periods and other times when selected portions of the plant are not running at capacity. Within some facilities, it might be common to use unregulated water in the manufacturing process. Without adequate pressure regulation, the flow out of these unregulated points would drastically change. While water might continue to flow through these points, it can lower to a level where damage could occur with critical equipment.
The use of the VFDs also saves both power and water usage. Without the controls, the open-ended parts of the system would discharge larger amounts of water when the entire system is not fully loaded, because of the higher line pressures. By controlling the discharge head at the pump to a consistent setting, it would result in less water usage and lower power consumption. This example is similar to a water line left open all the time for drinking water. When the pressure supplying the line is greater, the water shoots higher out of the discharge tube. When it is less, it is difficult to get a drink. If you always want to maintain the constant flow, the pressure needs to be regulated to a consistent level. The VFD is the simple solution to this problem, along with a properly sized pump.
Mechanical SystemsAdditionally, as facilities grow, new equipment arrives which adds more demand to the existing infrastructure. If the new equipment's load is small relative to the plant's capacity, there is a normal assumption by management that the piping to other mechanical systems can absorb this new load. Pumps, air handlers and air compressors are now asked to perform at a slightly higher level than previously. Soon, there comes a time when the equipment reaches its limit, and the facility suffers from its lack of capacity. It is very similar to the countless number of calls the hvac technician receives on the first really hot day of the summer. When this happens, it is an opportune time to evaluate the system and make changes that, in the long run, will lower the operating and maintenance costs.
I found one such example recently during the building of a plant. It consisted of many individual pieces of process equipment that required de-ionized water to cool the electrical equipment. It was interesting that the oem specification for the units only related to the inlet pressure requirements. It was assumed that the customer would pipe it with the same nominal pipe size, and the water would drain to a system with little or no discharge head. There was no specification relative to the heat load and pressure drop across the heat exchanger. This was a case of an oem being very capable of designing its equipment with little knowledge of the mechanical equipment that allowed that equipment to operate. In some of these cases, the engineer must do his best to back-calculate the design load by obtaining projected efficiencies for the units and then determine the heat load to the system.
However, it is important that the plant/facility engineers take the holistic approach. This oem built a packaged unit that didn't necessarily blend into the plant's existing infrastructure. The engineer then had to determine what the best fit would be dependent on site conditions.
In this particular example, individual process pumps from a cooling tower system were used to cool each piece of process equipment. As the plant expanded over the years with a similar type of equipment, this resulted not only in a slew of pumps (over 40), but also thousands of feet of pipe required to move the water from point A to point B and back. Needless to say, the pipe size used was the same as the size out of the heat exchanger and left little room for the pressure drop over the hundreds of feet between the water tank and the units. To add insult to injury, the pumps had higher discharge heads than required, and there was a complete lack of mass balance in the system.
The simple solution was to first do a cost analysis on the system. Each pump was examined, and an annual operating cost was assigned based on its usage and power consumption. Once it became apparent that there was a justifiable need on a cost savings basis, a new loop system was designed to provide a feed and a return line within close proximity to each individual use point. The loop system's capacity was also sized for future expansion and lower head loss. Once the loop was complete with its single operating pump, individual process equipment was transferred, depending on availability. Once again, VFDs came into play as the demand for water changes; the supply pump's discharge head can be regulated simply via the transducer and controller. In this case, all the smaller pumps were replaced by one larger pump. To ensure complete reliability, a parallel spare pump was also installed on the system, allowing periodic routine maintenance on both pumps without any disruption to the manufacturing process. It would not be uncommon to have less than a three-year direct payback on some systems as a result of lower utility costs.
Evaporative Cooling TowersMany times, cooling towers are one of the most ignored items in a manufacturing facility. It seems that most people are content just leaving them out of sight and mind. They are usually stuck on top of the roof or out in the "back forty." Without proper attention, they can be a source for major downtime or maintenance costs.
While the principles of evaporative towers are simply understood, the costs of operation vary greatly. You can pull the air in and push it up, or let it come in from the sides, but the most efficient way is to let it pull the air in all by itself. The induced draft-cooling tower has the ability to save 50-60% of the normal operating costs compared to other types of towers. No, it doesn't totally rely on complete natural convection, but also uses a fan to increase the capacity. In the northern part of the country, it is not uncommon to have the fan completely idled for periods of a time, thereby bringing the operating cost associated with electricity to zero.
Temperature discharge of the water out of the cooling tower is simply controlled by a VFD. If the fan operates at the minimum level for a set period of time, and if the water continues to be below the setpoint, then a smart controller will put the fan into what is referred to as a "sleep" mode. This mode completely shuts down the fan. With the dewpoint being very low during the winter months, it is not uncommon for the fan to run near or at the minimum level set by the controller. Once the demand increases, or the dewpoint rises, the fan "wakes" up and regulates the water temperature as required.
Within plants having a large, year-long cooling tower load, it can be very cost effective to change the towers to this type of design. Some manufacturers can provide side-by-side comparisons of operating costs, thereby giving the engineer objective operating data on which to base his decision.
For other existing units, the fan drives have proven to be a source of maintenance problems. Oems typically use a v-belt drive for the fan. These have a tendency to stretch over time, losing their efficiency. A simple solution to v-belt problems is to replace the belts with a toothed belt. Not only is the transfer efficiency higher, but they are more maintenance free than v-belts. If purchasing a new unit, inquire if the oem offers this option. Some manufactures offer a set of v-belts with a common backing, thereby eliminating the problems associated with a single belt stretching more than the others.