Cooling water filtration is necessary for sound operation in any process. Without this protection, buildup of material can cause problems such as plugged system nozzles, a reduction in heat transfer, poor quality production and, in the worst case, a plant shutdown. This article will offer some solutions for selecting filtration equipment for contact and noncontact cooling process cooling water loops.
Two Types of Process Cooling Water Loops
Noncontact cooling water is water that is recirculated and used for the purposes of cooling, but it does not have direct contact with the process. Contaminants enter this water system through the air — in the makeup water itself or by failure of a seal or barrier, thereby allowing contact to the process. The system installed must be able to remove particles 10 microns and larger to prevent fouling of the cooling element. In addition, the system should be sized to a scale where it requires maintenance only when a significant upset event happens and the water becomes contaminated. In effect, the filtration system is a sacrificial item that will alert plant operations to a water quality issue within the cooling water loop should it fail. The filtration system should be sized to remove the contaminants and bring the system back to equilibrium.
By contrast, contact cooling water comes into direct contact with the process to provide cooling of a production piece or production system. In many cases, the cooling water is used to cool or quench a metal, so the contact cooling water can contain oil, grease, suspended solids and powders as well as any solids that are present in the system makeup water.
Equipment in this service must be able to continually remove the contaminants as they are introduced to the water stream during each production batch. The filtration equipment also must be able to filter down to small particle size because many systems have engineered spray nozzles or orifices that must remain unclogged.
Technology Selection and Sizing
In noncontact cooling water, the load of contaminants generally is low and stable (5 mg/l), and bag filtration systems are often the technology of choice. The capital cost of these systems is lower than automated back-washable models, and the elements can be adjusted for seasonal or flow variances. Proper sizing also means selecting the model that will in no way become a maintenance issue for plant operators, with the aim being something that requires service at weekly or monthly intervals. The general rule for sizing is one-to-four turnovers per day when calculated as a ratio of daily flow rate to total system volume.
In contact cooling water systems, the load of contaminants can be high (20 to 80 mg/l), change quickly and contain oil. Many technologies are used, including sand, anthracite and induced-gas floatation cells and back-washable strainers. This article will explain how another technology — automated back-washable walnut-shell filters — can be used in contact cooling waters.
Features of a walnut-shell filter include:
- It is designed for oil service where sand filters would foul.
- The media is permanent, will not require replacement and is non-hazardous. Sand and anthracite filters are a silicosis hazard and can require replacement annually.
- A polishing filter, the walnut-shell filter is designed to filter down to 1 micron to help ensure that spray nozzles remain free of debris. Strainers and floatation cells cannot filter down to this level.
- The waste (backwash water) is 1 percent of the forward throughput, which helps minimize reprocessing and cost to treat. Other technologies can be as much as 10 times.
- If a system upset occurs, a walnut-shell filter will keep the process water clean and backwash; others may require manual intervention.
- When sizing the system, contaminant load and variance of load should be top considerations, but in general, these systems are sized at four-to-eight turnovers per day when calculated as a ratio of daily flow rate to total system volume.
Aluminum Plant Pilot Study
A case history can help demonstrate how a walnut-shell filter can help with oily cooling water. An aluminum plant in the Midwest produces aluminum billets and redraw rod. The facility uses recirculation water in various plant processes, which contaminates the wastewater with oil and suspended solids. The contaminated water gravity feeds to the recirculation area, where it is treated and cooled, before being pumped or recirculated back to the plant for reuse.
The aluminum plant has been using a dissolved air floatation (DAF) unit with the addition of chemical treatment to remove contaminants from the water. Personnel at the aluminum plant contracted with a manufacturer of walnut-shell filtration to trial and demonstrate the performance of an industrial walnut-shell water filter. The DAF unit is reaching the end of its useful life — increasing the likelihood it will require operator attention and operate less efficiently — so a trial can demonstrate alternatives prior to replacement.
The walnut-shell filter was evaluated alongside the DAF unit to determine:
- How floatation and filtration would differ.
- The level and range of contaminants in the water stream.
- How much water would need to be filtered to have quality water for process recirculation.
- What size commercial unit would be required for the recirculation area.
Following testing at the aluminum plant, the results showed that the level of contaminants — oil and total suspended solids added — varied between 6 and 25 mg/l. This is the normal operating range for a filter and low for a floatation cell. In addition:
- The walnut-shell filter provided more than 90 percent removal of contaminants with one-half of the coagulant feed.
- The walnut-shell filter provided approximately 80 to 85 percent without coagulant feed.
- DAF maximum removal was 60 percent and lowest was 10 percent.
Due to the improved efficiency for the aluminum plant cooling water filtration, the walnut-shell filter could achieve better overall removal of solids and oils while processing 50 percent of the water flow of a DAF unit. Based on these results, the aluminum plant has initiated plans to install a new plant filtration system in the 2014 calendar year.
When selecting a cooling water filtration system, it is important to know the contaminant load and to what level the contaminants may vary for your cooling water. Once you have this information, you can engineer a system that meets the objective of your plant operation and maintenance cycles.