Brisbane Food Manufacturer

An energy assessment for a Brisbane industrial food processor with a large demand for heating, cooling, chilling, freezing and compressed air identified a range of opportunities to improve energy performance on site, and provided guidance about where to spend its capital investment.

Summary of Opportunities

Compressor Heat Recovery

Refrigeration for the rapid cooling and freezing of products is provided by three 375 kilowatt (kW) compressors in the ammonia refrigeration plant, one of the major users of electricity on site.

Refrigeration systems reject a considerable amount of heat from condensers and this heat can be used to replace other heat sources on the site. It is recommended to install a heat exchanger to take the heat rejected from the plant’s refrigeration system and utilise it for boiler water. The boiler water may achieve heats of up to 90°C.

Combustion Air Pre-Heating for Oil Heater and Boiler Water Pre-Heating (Economiser)

The processor is a large consumer of liquid petroleum gas (LPG) through its cooking process, via its thermal oil heaters. The burners are rated at 1.75 and 2.5 megawatt (MW) and a steam boiler has a rated duty of 1 MW.   

Two efficiency improvement options are recommended for consideration:

• Install air-to-air heat exchangers in the oil heater flues so that gases from the oil heaters are used to pre-heat the combustion air in the site’s 2.5 MW oil heater. With less LGP required to fire the main oil heater, savings of approximately 6,377 gigajoules (GJ) per annum and $148,884 in energy savings would be realised.
• Install an economiser (air-to-water shell and tube heat exchanger) in the steam boiler flue to provide an estimated 608 kW of thermal energy back to the steam boiler feed water to pre-heat it. This would significantly reduce the energy input required in the steam boiler, resulting in savings of 12,253 GJ in LPG consumption and $286,056 per annum.

Oxygen Trim on Boilers

The site’s three main LPG consumers (two oil heaters and the steam boiler) are about 20 years old and were originally intended to be fuelled by natural gas. It is likely that when changing to LPG the combustion air fans were uplifted to the highest set point to allow for a safe LPG fuel to air ratio.

Following the recent energy assessment, engineers determined that the air to fuel ratio could be dynamically controlled based on thermal load. To improve the efficiency of these units, the following is recommended:

• Firstly, equip the combustion air fan with a variable speed drive (VSD).
• Once a VSD is installed, an oxygen trim system can also be used to detect the loss due to unburnt fuel. The system then controls the fuel and air to reduce the loss. This would result in an estimated energy savings of 1,876 GJ and $43,806 in cost savings.

Solar

The financial case for a solar PV system installation was modelled on different size systems, based on current capital pricing and offsetting the site’s grid consumption.  As the business is a tenant and is currently on a commercial lease, any solar PV system should yield a quick payback, ideally less than seven years.  Modelling suggests that a 99kW system is the most economical, as it achieves the best net present value and a 5.7 year payback. The system would also benefit from small-scale technology certificate (STC) rebates.

Electrification of Gas-Based Heating and Cooling Processes

By switching to electricity and reducing its reliance on LPG, the business would benefit from:

• Lower electricity prices, which would be further reduced if solar PV is installed.
• A reduction in emissions as the electricity grid becomes ‘greener’.

On site, this could be achieved by using a bank of industrial heat pumps to replace the steam boiler for production of the site’s hot water. Heat pumps can use electricity to concentrate heat from various sources (e.g. waste heat from other processes and thermal energy underground) into a useful heat stream – in this case, water. The usable energy output for electricity input is around 300%, compared favourably to combustion boilers which rarely exceed 85% efficiency.