Customizing environmental control with unitary rooftop packages

This article describes the benefits of installing a tightly controlled unitary rooftop package HVAC system to control a plant environment. Key topics include how packaged units save installation costs, zone controls to improve indoor air quality, and simplified maintenance.

07/06/2001



Plant Engineering - January 2001

ON-LINE EXCLUSIVE
HVAC

Feature: Customizing environmental control with unitary rooftop packages
Dean Clark, Unitary Products Group, York International, Norman, OK


Key concepts

  • Packaged units save installation costs.

  • Zone controls improve indoor air quality.

  • Maintenance is simplified.


    While a manufacturer of Beanie Babies and a maker of vitamins have different criteria for controlling the manufacturing conditions in their facilities, they do have something in common %%MDASSML%% the need to guarantee product quality and production levels by maintaining an environment that is conducive to the process. The best way for a plant engineer to achieve that is to install a tightly controlled unitary rooftop package ( Fig. 1 ).

    This type of HVAC system is a packaged unit with standard indoor and outdoor components inside a single box, requiring only power, a temperature control system, and duct connections to operate.

    Unitary rooftop package benefits include:
    • lower installation costs

    • efficient controls

    • process-enhancing options.


      • Low installation costs
        One of the most persuasive reasons to install a heating and cooling system using rooftop units is that the installation costs are much lower than those of a chiller plant.

        A chiller plant is a heating and cooling system located in a central area. It employs a chiller to cool a sealed loop of water that is distributed by pumps to one or more air handlers or fan stations.

        Chiller plants require an outside condenser or cooling tower to operate. Each chiller may cost a few hundred thousand dollars for a 400-ton machine and the associated piping, valves, controls, and hardware. Several chillers may be needed for a large manufacturing facility that also includes office areas.

        The upfront cost for a unitary rooftop package is about $10,000 per unit. If a large plant requires several units, the upfront cost is about half of the chiller plant cost. Additional savings accrue because only roof curbs or equipment mounts are required to install this type of equipment. Additional costs %%MDASSML%% anywhere from $100 for a low-end thermostat to $100,000 or more for a complex system in a manufacturing environment %%MDASSML%% result if digital controls are added. But the price is well worth it, due to benefits such as enhanced indoor air quality and customized zoning options.

        Advanced controls
        Digital controls for unitary equipment ( Fig. 2 ) operate through a complex processing network that distributes information about specific areas or zones, outside air, space, and supply-air temperatures from each unit through a Local Area Network (LAN). The network then decides how much and which locations need air to evenly maintain the proper temperature and humidity of the environment. These advanced controls make unitary rooftop packages ideal for manufacturing environments.

        Since most plants operate longer than typical office hours, savings generally do not result from conserving energy but from equipment efficiency.

        Preventive maintenance
        If a plant has a mission-critical area that must be kept operating, the network system can alert personnel if a piece of HVAC equipment requires maintenance or has a problem that may affect the output or quality of the manufacturing process. While this function doesn’t provide a direct energy savings, the savings from preventing downtime and production loss can be significant.

        Improving indoor air quality
        Unitary rooftop packages allow manufacturers to take advantage of air sensing controls and high-quality filter accessories that improve air quality. Sick building syndrome, which can cause health problems from headaches to life-threatening allergic reactions, is often related to poor air circulation. The controls associated with unitary rooftop packages can be programmed to continually ventilate the building with fresh air and ensure proper airflow.

        An economizer can be added to each rooftop unit. This component monitors outdoor air to determine if fresh air can be used in place of mechanical cooling or heating. Controls on the economizer provide the option of performing a building purge or morning ventilating. Using this function, the unit is programmed to open the economizer in the morning, providing outdoor air temperatures aren’t too far from the desired set point, and ventilate the building for a specified period of time.

        Customized zoning
        Zoning capabilities make a unitary rooftop package with network controls especially appealing for manufacturing plants with office areas because it can accurately heat and cool several different areas without the need for autonomous systems.

        Zoning, which uses sensors to regulate temperature across the floor or area, monitors each designated area and rooftop unit independently. The network shares this information in order to maintain temperature at the lowest possible cost.

        Most rooftop systems have two modes of operation, 100% on and off. But through the use of a zoning system, each piece of unitary equipment can be controlled differently; some cooling, some heating, and some just blowing air. This ability to "police" itself multiple times per second, and react accordingly, is what makes a zoning system ideal for manufacturing facilities with office areas and production areas that have different heating and cooling needs.

        In a typical manufacturing plant, each process area could have its own rooftop unit programmed for that zone to keep the temperature and humidity levels exactly where they need to be. The network system can perform multiple zoning, individual zoning, or any combination of the two within a single rooftop unit.

        Office areas, which typically suffer from hot and cold spots, can also benefit from the zoning capabilities of unitary packages. Uneven air distribution or changes in the use of office space cause the all-too-familiar hot spot/cold spot syndrome. But zoning eliminates this problem by monitoring designated zones in the office area as stand-alone spaces.

        The controls can directly vary airflow and temperatures in very small areas. If the zone-control system senses that it needs full capacity or airflow, it simply tells the rooftop to slow down the fan or turn off compressors to reduce energy usage and maintain the desired temperatures.

        Simple maintenance
        As an added bonus, unitary rooftop systems are easier to maintain than chiller equipment because they have fewer components. Typically, filter changes, standard maintenance, and quarterly coil cleaning is all that is required to keep the equipment running in top-notch condition.

        How does it work?
        Programmable controls that rely on sensors are the intelligence behind the benefits that are possible with unitary rooftop packages ( Fig. 3 ). Sensors placed in various areas throughout each zone communicate through LAN cables to the main computer, which makes a decision based on the information the sensors provide about what task it should perform, such as reducing temperature or letting in air.

        To obtain the highest possible efficiency, a little tweaking of the controls is usually necessary. Programming changes such as start and stop times can be helpful, but a good zoning system has the ability to vary these times depending on factors such as outdoor conditions, time of year, and building occupancy.

        For instance, the unit can be programmed to remember how long it took to meet the setpoint from previous days. It can automatically adjust its start time to the optimal time to start, based on outside air temperatures, and not use additional electricity.

        The ability to tweak controls is especially important for plants, because incremental changes in temperature and humidity can often improve the manufacturing process. For instance, temperature and humidity levels are critical factors in paper manufacturing. Plant engineers in that industry start by tweaking setpoints, then allow the network to make adjustments to fine-tune the levels in a given space to improve manufacturing output or to guarantee product quality.

        The financial side
        While the benefits provided by controls are impressive, the prompt return on investment is another reason more manufacturers are turning to unitary rooftop packages for their heating and cooling needs. If a manufacturing environment currently has a system that is running long hours and doesn’t have controls, the ROI could be reached in as little as a year because of the significant increase in efficiency with controls.

        Utility contracts
        Tight controls permit manufacturers to regulate and budget monthly utility usage, which allows them to negotiate with the local utility to guarantee a fixed cost per kilowatt-hour. Normally, upon contracting with the utility, the manufacturer agrees to use a set amount of kilowatts per month or a certain level of kilowatts during peak demand times. In exchange for signing such a tightly regulated contract, the manufacturer gets a significant discount on each kilowatt of electricity.

        However, there is a downside to these attractive contracts -- very large fines or charges are often applied when a manufacturer exceeds the monthly contract demand limits. A typical fine can exceed a normal monthly electric bill.

        - - Edited by Joseph L. Foszcz, Senior Editor,
        630-320-7135, jfoszcz@cahners.com

        More info
        The author is available to answer questions about rooftop packages. He can be reached at dean.clark@york.com .


    Plant Engineering

     

     





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