Industrial Strength Solutions (September 2000)
The engineering of an hvac system in heavy industry can present its own unique set of challenges. Industry processes such as foundries, plating, coating, and producing chemicals all offer their own obstacles to the typical hvac engineer. They usually have high-heat source equipment, and large exhaust systems for fume and dust control, as well as operations that can be susceptible to cold air drafts. Of course, the checklist also includes occupant comfort, which has to be considered as with any other hvac project.
However, if you ask the right questions at every stage, from client meeting through startup, the result can be an efficient, productive system.
The Client MeetingThis type of project should start out by having a kickoff meeting with the client. Every project has a unique set of circumstances to it. Every hvac system has to be custom-designed for each particular situation. A kickoff meeting is the best method for receiving the data required to design a successful hvac project. It is important to have the right personnel attend this meeting, primarily so you can get full and accurate information to work with. Be sure anyone essential to providing a piece of this information puzzle attends this meeting.
Examples of some questions that need to be addressed:
- What is the process in this plant?
- Does temperature or humidity affect the process?
- Is the process constant throughout the day, week, month and, year or does it vary?
- Are there any areas that are sensitive to drafts, temperature, or humidity?
- Does the process create pollutants? Is it constant, or does it vary in the process cycle? Are the pollutants captured by an exhaust system? Is it constant exhaust or does it vary? Are there areas where drafts from the makeup air system can cause problems with the exhaust system?
- Can air exhausted from dust control systems be returned to the room with or without HEPA filtration?
- Are there personnel comfort issues?
- Do the personnel work at fixed workstations, or do they move around?
- Are there areas of high-heat loads or moisture loads in the area?
If these questions are raised during the meeting, the client’s representative normally will bring up any other relevant unusual circumstances for this particular project. As the industrial plant becomes more computerized and sophisticated, the need for an hvac system to control the environment becomes an increasingly high priority.
Heating and Cooling CalculationsThe engineering of these type of projects begins with the basics. Cooling and heating load calculations must be completed for all plant process areas included in the project.
Unfortunately, process loads do not blend well with canned computerized programs for load calculations. Many of the load calculations will require contacting the various equipment manufacturers. They also may require utility audits of the equipment, and energy balance calculations to determine how much energy is being released into the cooling zone.
As an example, if you have a gas-fired aluminum melt furnace, you may have to do an energy balance on the furnace. You will need to audit how much natural gas is being fed into the furnace and how much aluminum is being melted in a typical hour of production, plus how much heat is being removed in the exhaust air stream. The following are questions that will need to be answered during the calculation of the energy balance.
- How is the molten aluminum being transported out of the area? Is this method open to the room where it will give up heat to the room? If it is being piped, how much heat is being released through the piping to the room?
- What are the effects of radiant heat on personnel comfort? Do we need to consider heat shields?
- If the aluminum is used for making castings in the area, are they stored in the area while still hot? If they are, you will have to figure the energy transferred into the space from the hot castings as they cool off.
- Is the aluminum being continuously heated in the furnace to keep it in a molten state?
- Is this a continuous operation, or are there so many batches per process day?
Collecting this information will provide a summary of the variables necessary to make detailed calculations, which represent the only way that a good cooling and heating load calculation can be created for the industrial plant. This method will give you the best chance of designing the proper hvac system for your client.
During this calculation phase, the temperature, humidity, and air quality that needs to be held in the space must be discussed with the client. This final agreement is the design basis of the project. These three factors will be very important on the final selection of the air-handling unit for the space. It has to be remembered that in an industrial process, there may be times during the process where high-temperature air, high humidity, or pollutants may be given off. You will need to observe the process and ask questions of the client to make sure you have a good reading on the field conditions.
Return Air or 100% Outside Air?Next, the exhaust systems that pull air out of the space must be analyzed. In most circumstances, these exhaust systems are removing air pollutants such as fumes and dust from the space. This volume of exhaust must be considered in your calculations, since this volume must come from the outside and be conditioned before entering the space.
At this point, it must be decided whether the exhaust systems are capturing all the pollutants in the area. If any pollutants are missed by the exhaust system, they will be returned to the air handler and finally will be fed back into the space. If there is any chance that a pollutant will be returned to the air-handler system, then a 100% outside air system should be considered for the space.
At first glance, the decision may appear to be to return the air to the air handler. However, if there is no guarantee that there will be no pollutants in the air, this decision must be analyzed in detail. Some questions that need to be considered and answered:
- If pollutants are brought back into the space, what health issues are encountered with personnel?
- Will products of combustion be returned to the space?
- Does the return air need to be filtered before returning to the unit? What type of filtration is needed? What type of maintenance is required?
- Are there corrosive materials in the return air that will affect the unit?
- How much outside air must be brought in to dilute the return air to a safe level? Will code allow you to return air from this process area?
The decision to return air to the air handler or use 100% outside air is a very important choice in an industrial hvac project. Usually, 100% outside air is safest, however it can be a costly method. It has to be remembered that this system will need to handle all the air from the outside. Most standard air-handling equipment is designed to handle a maximum of 30% outside air, so the capital cost will be higher than standard. In addition, the everyday cost of running this system will also be higher.
As an example, if we have a typical standard air handler rated at 10,000 cfm, that unit should be capable of 3,000 cfm of outside air and 7,000 cfm of return air from the plant. However, if it is decided that we need to use 100% outside air, the cooling and heating sections, as well as the outside air filtering sections, need to be designed to handle over three times as much as the standard unit. As you can well guess, the capital cost of the unit goes up as the sections increase in capability and, of course, the everyday energy to run this unit increases.
If the decision is 100% outside air, installation of an energy reclaim system should be considered between the exhaust air streams and outside air streams. This can sometimes be a problem since the exhaust air stream may be multiple exhausters throughout the plant instead of one centralized exhaust. In all cases of energy reclaim studies, three basic questions have to be answered:
- Is there a chance that the pollutants in the exhaust air stream will be transferred with the energy to the incoming air stream?
- What type of maintenance will be required to keep the system running?
- Will the additional capital cost as well as everyday running cost be able to pay itself off in a reasonable time frame?
In the final decision, if it is decided to use 100% outside air, design weather conditions become very important to the air-handling unit manufacturer. It must be remembered that outside weather conditions now impact 100% of the air that the unit is handling, not the typical 30% of standard units. Therefore, there must be an agreement with the client on which weather design conditions to use in the design. The comparison between ASHRAE design conditions and local climatological data available from NOAA should be presented to the client for discussion.
The final agreement will be based on the answer to the question: What happens to the plant process on the days when outside conditions are worse than the design conditions for outside weather? The important fact that has to be remembered is that the hvac system may not only affect people comfort; it may affect the efficiency of the industrial process and, therefore, the bottom line.
A good parameter to use when dealing with the air handler manufacturer is to put a return air section on the unit, even if it is designed for 100% outside air. The process in the plant today may change, where return air can be used in the future. Today the return air can be blanked off, but tomorrow it may be able to be used in the new process. If the return air section is just blanked off, it takes very little effort to convert it to a return air system. This can save the client a lot of rework in the future.
The Air Distribution SystemNow that the decision has been made on the air handler, we need to look at how we are going to distribute the air into the plant. This again can be a difficult decision in an industrial environment. Again some questions that will need to be answered:
- What height can be used for my distribution system? Can it be at 10 to 15 ft above the floor, or does it need to be up higher to keep from conflicting with cranes or the process requirements?
- Are there issues of air stratification in the ceiling area that need to be considered in the design?
- Does the ductwork need to be easily removable for access for major rebuilds or maintenance of process equipment?
- Can we use round ductwork for the horizontal ductwork runs, since there is less flat area on top to collect dust in an industrial environment?
- Are there areas where my supply system will blow pollutants away from the exhaust system, and keep the exhaust systems from capturing the pollutants?
- Are there areas of the process where drafts can cause problems with overcooling of the process?
- Are the personnel located in workstations or do they travel throughout the area?
The resulting answers should lead to a concept of a distribution system design that can achieve the desired effect in the plant without affecting the industrial process.
Pressure ZonesIf all industrial plants were one large room, it would be a simple question concerning whether you want the plant under positive or negative pressure to the outside. However, most industrial plants are a multitude of rooms, with a special process or one step of the process running in each.
Since most products need to be transferred from room to room, or personnel need to travel between rooms, it becomes necessary to pressurize zones between rooms or within rooms. This again is an area where client input is suggested. You will need to ask the client which way the air is to travel when a door is opened between two rooms. You then can set up the airflow in the rooms.
This can be a tedious task, since many plants will have numerous rooms and types. It may lead to damper arrangements in the ductwork controlled by static pressure controllers to vary volume if the pressure in the rooms change. It may require putting more cooling air into an area that is needed for the load in the area, so reheat may be needed in the ductwork for temperature control. It may require adding exhaust to a room to get it under a negative pressure. It may end up that your supply system is just following the exhaust system to keep the room under a certain pressure; however, you have to remember you also have to deal with the cooling and heating loads in the room. As you can see, sometimes the design of the pressure zones in a complex industrial plant can become a project in itself.
SummaryIn summary, the engineering of an hvac system for industry can present some unique problems and solutions, however, if we remember some rules throughout the project, it should be a success.
- Start the project with a kickoff meeting with the client. Get some understanding of the process as well as the unique features that need to be considered for the hvac system.
- Perform a thorough investigation and calculation of heat and cooling loads for the industrial plant. This is the basis for a successful system.
- Investigate whether this system should return air to the air handler or be 100% outside air. This impacts air quality, project cost, and everyday running cost.
- Design field flexibility into the system. Industrial plants seldom perform the same process for many years. You can be a hero in the future.
- Don’t count on a great deal of maintenance. If it is on the roof, you can be fairly certain that “out of sight, out of mind” will apply. Try to design for as little maintenance as possible.
- Design the air distribution system to get air where you need it, but not where it is always in the way. Your clients will be happy if they don’t even know where the ductwork runs in their plants.
- Look at the pressure zones and how the outside or surrounding rooms will impact your system. This is a tedious task, but it can make or break your overall system.
- Remember an industrial hvac system doesn’t just impact occupant comfort, it may impact the industrial process and, therefore, the bottom line.