Energy efficiency in HVAC industrial applications

August 12th, 2015, Published in Articles: EE Publishers, Articles: Energize, Articles: Vector

 

Several industrial HVAC requirements differ from commercial building requirements and are not often considered in terms of energy efficiency.

Heating, ventilation, and air conditioning (HVAC) is a major energy consumer  in commercial buildings and energy efficiency measures have been developed in this field. Industrial premises have special requirements which are more complex than commercial buildings and require customised solutions.

In addition to general air quality, air of defined quality may be required for specific processes within the production area or factory.

Industrial premises can be divided into offices and administration, production areas and storage and warehousing areas.

Offices and warehousing areas which do not require accurate temperature and humidity control will use standard systems beyond the scope of this article.

The production area will, however, have a set of requirements which vary with the industry and the specific site, and energy savings must be tailored to suit each application.

Some of the specifics to manufacturing area may be:

  • High heat load due to the machinery in use.
  • Air quality problems due to the materials being used in the process (paint bays, grinding, sanding, welding, cutting, powders, dry material and others).
  • Accurate control of temperature and humidity in specific manufacturing areas (clean room requirements).
  • Explosion-proof environments.

All of these requirements can exist in a single facility, and the energy account must take them all into account if the overall design is to achieve the desired result. Manufacturing and industrial facilities tackle a varying load of heavy-duty projects during their day-to-day operations so that HVAC system designers face a number of challenges to achieve sustainable, energy-efficient solutions.

A manufacturing or industrial facility will typically have a large outside-air requirement to offset the various air, heat and pollution loads throughout a plant. Innovative ways to provide energy savings and energy recovery in these systems are required and the control systems must function optimally.

Industrial environments have a wide range of HVAC requirements. Where commercial and retail facilities are focused on comfort cooling, an industrial facility may need explosion-proof ventilation in one area and a clean-room environment in another. Process air streams may require large volumes of outside air and use specialised systems to clean the air. All of this may require ancillary air handling and conditioning systems over and above the facility plant, but still require a supply of quality air intake to function properly [1].

In addition, although the air circulation systems may be simpler than for commercial buildings because of the larger open spaces involved, the volumes of air and the tightness of controls make energy savings more complicated.

Fresh air intake, air circulation rates

Facilities require a higher circulation rate and a higher fresh air intake than commercial buildings. The heat load ensures that the main requirement will be for cooling of ambient air rather than heating, irrespective of the season, and this provides an opportunity to combine economiser cycles and the use of free cooling in the fresh air intake. Control of air flow rates and air changes depending on conditions offer the first savings opportunity.

Free cooling

The heat load in an industrial facility can necessitate constant cooling, not only for human comfort but also to ensure that the equipment does not exceed specified operating temperature limits. This is particularly important in facilities with large quantities of computer or ICT equipment, such as data centres or telecommunications installations.

In such instances, energy can be saved by using the lower temperature of outside air (or the lower heat content) to supplement the air processing units. Economisers may operate simply on the outside air temperature or on outside air enthalpy, which is a measure of the heat content of air and depends on both temperature and relative humidity.

Instead of operating on a fixed minimum airflow supply, an economiser allows the HVAC system to use outside air by varying the supply airflow according to outdoor air conditions, usually by means of an outdoor dry bulb temperature sensor or return air enthalpy.

Enthalpy is a more efficient metric because it is based on the true heat content of the air. Although economisers and free cooling are used mainly for ITC and data centres at the moment, the range of equipment available makes their use ever more attractive for industrial and other high-heat load density applications. Economisers take two main forms: direct and indirect cooling.

Direct cycle

In the direct cycle, outside air is used directly to cool the inside of the building.

There are two possibilities:

  • Once-through cooling: Outside air is filtered and passed though the facility once. Hot air is vented at the opposite side of the building. This system may incorporate a direct expansion chiller in the air path to cater for occasions when the outside air temperature is too high to provide the full cooling load. This application is commonly used in data centres where heat density can reach up to ten times that of commercial buildings. Temperature is controlled by the rate of air flow through the facility. Variable speed drive (VSD) fans provide extra energy savings and allow a more accurate control of temperature.
  • Make-up air system: Outside air is filtered and combined with return air from inside the building in a proportion depending on the relative temperature or relative enthalpy of the outside and inside air. Mixing of air is regulated by dampers which control the air flow into the building (see Fig. 1).

Fig. 1: Direct free cooling system [4].

Fig. 1: Direct free cooling system [4].

Indirect cycle

In the indirect cycle, outside air cools the chilled water used in a conventional system. Several configurations are possible:

  • Precooling: The return chilled water is passed through a heat exchanger where it is precooled on its way to the evaporator. Outside air cools the heat exchanger.
  • Separate cooling loop: Outside air is used to cool water passing through a heat exchanger in the return water loop. There is no contact between the outside air cooling loop and the internal chilled water loop.

The heat exchanger usually takes the form of a conventional cooling tower.

Energy savings with economiser cycle

Energy savings range from 25% to 100%, depending on the climate and the temperature range allowed within the facility. In some climates it is possible to use free cooling for the total cooling load for a large portion of the year, and substantial savings result. The potential is lower in South Africa, although free cooling could be used at night even during summer for sites operating continuously. It has been estimated that, for typical Johannesburg conditions, it would be possible to use free cooling for 55% of the time over a full year [2].

Free cooling in South Africa?

Free cooling opportunities are relatively abundant in the northern hemisphere. A data centre in London with a typical room temperature of 24°C can operate in full free cooling mode for as much as 95% of the year, generating potential energy savings of up to 50% compared with a conventional chiller-based system. In climates where summer ambient temperatures can soar higher than 50°C, the long-held view has been that free cooling opportunities are far more restricted, if not unachievable.

Raising supply and return air temperatures by as little as 1°C can, however, create a significant free-cooling window, even in such elevated temperatures [3].

In typical Johannesburg conditions, raising supply and return temperatures to 25 and 38°C respectively (still within industry guidelines) could generate annual energy savings of up to 110% using an air-cooled system alone, and 138% from a free-cooling system. This would result in an annualised energy efficiency ratio (EER) of 5,63 achieved through 99% of the year in free-cooling or partial free-cooling.

While raising supply and return temperatures enables data centres to capture extra free cooling opportunities, international opinion still varies on what constitutes an acceptable supply temperature before IT performance is affected. In 2008

The international body regulating data centre environments, Ashrae, revised its recommended dry bulb temperature from 20° – 25°C to 18° – 27°C. It did this to help reduce data centre energy consumption, with no evidence to indicate any negative impact on the reliability of IT equipment [2].

Humidity control

Humidity control systems add or remove water vapour from indoor air to stay within required humidity ranges. Humidity control is usually associated with thermal comfort for workers, but it also has a large effect on electronic and electrical equipment. Humidity levels can also affect product quality in the pharmaceutical or food processing industries. With automation increasing in industry, humidity control is becoming ever more important. Excess moisture in a building can lead to mould and mildew, causing problems for indoor air quality and affecting the operation of the equipment.

Humidity control is a large energy user. Although humidification does not consume much energy, dehumidification alone can constitute a quarter to a third of cooling energy in humid climates or seasons [3].

Mechanical dehumidification

Typical dehumidification is performed by systems which use the same basic mechanics as air conditioners. They are electrical heat pumps that dehumidify air by cooling it. Mechanical dehumidification is not the most energy-effective means, but it is the most common because it uses standard technology. Mechanical dehumidifiers over-cool incoming air to below the dew-point. As a result, the water condenses on the cooling coils. Afterwards, the cold, dry air is heated to the desired temperature again and/or mixed with untreated air to provide air at the desired temperature and humidity levels. The water, now in droplets, drips off the condenser coils so that more water vapour can condense there.

Waste heat dehumidification

Any energy savings plan starts with reducing the reheat process. The reheat process can consume the bulk of the energy used in the dehumidifier. Electrical heater elements were originally used for the reheat process. The first step is to substitute this energy-hungry source with another source of heat, preferably from a heat recovery system. The most obvious source here is the condenser coils of the cooling system. Where this source may not provide sufficient heat, recovered heat from other processes in the building can be used.

The dehumidification process is often applied to the fresh air intake only, and waste heat from the main air-conditioning plant may be available. Heat pumps forming part of the total installation can be used for this purpose. Fig. 2 shows the diagram of a hybrid dehumidifier system which uses heat from the unit condenser to reheat the air.

Fig. 2: Hybrid dehumidifier [6].

Fig. 2: Hybrid dehumidifier [6].

The load can be quite considerable where the whole plant is subject to humidity control. The actual load will depend on the difference between the temperature of the return air and the required exit temperature of the processed air.

Desiccant dehumidifiers

Desiccant dehumidifiers use a rotating wheel containing a regenerable desiccant to remove moisture from the air (see Fig. 3). Incoming air is passed through the wheel where the moisture is absorbed. Hot air is passed through the upper section as the wheel rotates and removes the water. Hot air containing water is exhausted.

The heat may be derived from electrical heaters or from a waste-heat source such as the condenser of the main air processing unit or other industrial process.

These dehumidifiers are available in sizes up to 8000 m3/h and are used for primary dehumidification, as well as for dedicated process air plants.

Fig. 3: Dehumidifier wheel [5].

Fig. 3: Dehumidifier wheel [5].

References

[1]    J Bauers, J Gerkel, M O’Connel: “Industrial-strength design: HVAC systems” Consulting-Specifying Engineer, 2015.
[2]    Stultz: “Direct free cooling for data centres”, http://za.stulz.com/products/precision-ac/cyberair-dfc2/
[3]    Airedale: “Vodacom case study”, www.airedale.com/web/About-Airedale/The-News-HVAC/Vodacom.htm
[4]    Energystar: “Air side economiser”, www.energystar.gov/index.cfm?c=power_mgt.datacenter_efficiency_economizer_airside
[5]    Autodesk: “Sustainability workshop”, http://sustainabilityworkshop.autodesk.com/buildings/humidity-control
[6]    CGC: “Hybrid heat pump system” www.cgc-group.com/dehumidification.php

Send your comments to: energize@ee.co.za

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