Energy Reduction Process Heating and Cooling

Energy Efficiency: The energy efficiency of any process cooling system depends on the system’s heat balance. The exact amount of heat generated by the process has to be removed by the chiller. If too much or too little heat is removed, production may drop and quality may be affected or, alternatively, too much energy may be consumed for a given amount of output.

Applied Energy and its partners can measure and compare existing performance of refrigeration systems against theoretical benchmarks. Differences between them can be analyzed and changes made to improve performance. If necessary, modifications to the system design can be made to achieve optimum energy efficiency relative to production requirements.

Heat Balance: The main issue in process cooling is heat balance and there are 5 key areas that can be adjusted for optimum heat balance and control of refrigeration capacity:

• Compressor cycling
• Multiple compressors
• Cylinder unloading
• Gas bypass
• Condenser fan cycling & Water flow control

Compressor Cycling: Compressor cycling is one method for controlling the amount of cooling in a chiller. Most systems use a thermostat with a predetermined refrigeration temperature - the ‘set point’. When the amount of cooling reaches the set point, the thermostat stops the compressor from circulating refrigerant through the system, and while the water circulation pump continues pumping, the temperature of the process water rises. The thermostat detects this rise and turns the compressor back on.

The drawback of compressor cycling is the additional stress on the compressor motor windings when the compressor is started repeatedly. With the inrush current spiking up to 600-800% of its normal full load amps, compressor cycling over an extended period can cause premature failure unless a soft start is built-in to the system.

Multiple Compressors: There are some applications where refrigeration equipment failure could result in serious financial loss, e.g. frozen food, above and beyond the repair expense. In such cases, it is advisable to consider a multiple compressor chiller system.

For part-loads, one or more compressors may be started or stopped as required. This also provides redundancy should one of the compressors fail. Although the system operates at a lower capacity if a compressor fails, it should not be allowed to run in this condition for long periods of time to prevent potential damage to the other compressors.

Cylinder Unloading: Capacity can also be controlled through compressor cylinder unloading. A thermostat controls the solenoids (assuming the compressor has multiple cylinders) that force the discharge valve to stay open. Since the cylinder is now open to the discharge manifold, no refrigerant gas compression occurs. The result is a drop in refrigeration capacity in direct proportion to the number of cylinders being “unloaded.” The torque on the electric motor falls and results in lower power consumption. Cylinder unloading is the most desirable method of controlling since it balances a chiller’s refrigeration capacity with the process load and saves power consumption.

The cylinders typically unload from six to four and then two. This capacity reduction is then followed by gas bypass. For screw compressors, partial unloading can be accomplished because they have two stages.

Gas Bypass: As the compressor satisfies the process load, the chilled water from the evaporator begins to over-cool. A thermostat senses this drop in water temperature, and, at a preset temperature, the thermostat opens a solenoid hot gas bypass valve, allowing part of the refrigerant to bypass the condenser and the expansion valve.

Without heat removal by the refrigerant, the water in the chiller begins to warm up and, at a pre-established temperature, the thermostat closes the bypass valve when the chilled water reaches the set temperature. The hot gas bypass prevents the compressor from rapid cycling when the chiller operates under partial loads. This valve is particularly important when operating a semi-hermetic compressor since the compressor must receive a full amount of refrigerant for the motor windings to remain cool.

Condenser Fan Cycling and Water Flow Control: Condenser water flow control for water-cooled chillers, or condenser fan cycling for air-cooled chillers are other methods of capacity control. Both methods can only partly control a chiller’s capacity.

If the incoming air or water temperature drops seasonally, the colder water (or air) will increase a chiller’s capacity. If the process load remains constant during this change, throttling the condenser water flow or cycling the condenser fans will reduce the chiller’s capacity to a certain extent.

There are 4 main ways to maximize the efficiency of your furnace or oven operations.

• Organize production scheduling
• Adjust product positioning
• Maintain equipment to specification
• Recapture exhaust gas heat losses

A commonly overlooked factor in energy efficiency is scheduling and loading of the furnace. “Loading” refers to the amount of material processed through the furnace or oven in a given period of time. It can have a significant effect on the furnace’s energy consumption when measured as energy used per unit of production, (e.g. Btu/lb).

Certain furnace losses (wall, storage, conveyor and radiation) are essentially constant regardless of production volume; therefore, at reduced throughputs, each unit of production has to carry a higher burden of these fixed losses. Flue gas losses, on the other hand, are variable and tend to increase gradually with production volume.

Equally, if the furnace is pushed above its design rating, flue gas losses increase more rapidly, because the furnace must be operated at a higher temperature than normal to keep up with production. Total energy consumption per unit of production will follow a bell curve, with the lowest cost at 100% of furnace capacity and progressively higher costs the further throughput deviates from 100%.

The lesson here is that furnace operating schedules and load sizes should be selected to keep the furnace operating as near to 100% capacity as possible. Partially loaded or overloaded furnaces are less efficient.
 

In addition to optimizing production rates, the workload must be properly positioned for exposure to the incoming heat. In high temperature furnaces and infrared heated ovens, where radiant heating is the main heat transfer method, the work should be positioned to present as much surface area as possible to the heat source. In batch heating operations, this may mean decreasing load sizes to avoid burying pieces deep in a pile. In continuous processes, it may help to space products farther apart or arrange them so more of their surface is exposed to the heat source.

In lower temperature ovens using convection heating, the most efficient heat transfer requires close contact between the heating gases and the work. If the products are densely packed, the gases can’t circulate easily around and through them. Wherever possible, locate the products as close as reasonable to the incoming hot airflow. Heat transfer rates are strongly affected by the velocity and turbulence of the hot gases at the point of contact. Excessive distances between the hot air outlets and the product allow the velocity to decrease quickly, and heat transfer efficiency will suffer.
 

Process Heating: Maintain Equipment to Specification

Allowing equipment to deteriorate and slowly deviate from its specified performance levels is VERY expensive. You might save a little on maintenance costs in the short run but you will pay through the nose in energy terms. Particular attention should be paid to the following:

Wall Losses: Ensure that existing insulation or refractory lining is doing its job. Discolored paint, blistered sheet metal or other signs may indicate that the lining is breaking down. A systematic survey of exterior skin temperatures should be done at least annually. Finding losses before they become obvious will save a lot of energy and minimize repair costs.

Radiation Losses: In high temperature processes, radiation losses can result in huge energy costs. Furnace doors should not be opened any longer than necessary. If certain openings must be left open for access by material-handling equipment, (conveyors, fork lifts etc) they should be fitted with radiation shields of made of flexible ceramic fiber or textile.

Shutdown Losses: When equipment is shut down and restarted in batch operations, some of the heat required to bring the unit up to operating temperature will be lost and must be replaced. If shutdown is unavoidable due to lack of product, maintenance or less than 3 shifts, fuel consumption should be monitored to compare the effect of a complete shutdown with maintaining a low temperature.

Exhaust or Flue Gas Losses: The single biggest heat loss in furnaces and ovens is exhaust gas. The goal is to keep the volume and temperature of those gases to the minimum consistent with your process.

Controlling the volume of exhaust gases can involve several different parameters, depending on the process. For high temperature furnaces, setting the combustion ratio for no more excess air than necessary, is important. The amount of ambient air drawn into the furnace by stack draft should also be limited, because this air behaves the same as excess air going through the burners, absorbing heat from the furnace and then leaving the stack without doing any useful work.

On processes with exhaust fans, the flow through the exhaust system has to be balanced against the incoming flow of combustion products and makeup air. If the incoming flow doesn’t satisfy the exhaust system, the oven chamber will operate at a negative pressure, pulling ambient air in through any available opening, wasting fuel and creating cold spots at the entry points. The opposite situation is when an exhaust system is unable to remove all the gases entering the oven. This results in high positive chamber pressures forcing hot gases out to heat the surrounding area, instead of the product.

To keeping exhaust gas temperatures in balance, two parameters should be monitored carefully - product loading and heat transfer inside the oven or furnace. Processing too little product increases energy consumption per unit of production. Processing too much product is worse. As hot gases enter the heating chamber, they begin transferring heat to the product. The longer this takes, the more complete and efficient the transfer. In an overloaded furnace or oven, the product can only be heated properly by over-firing the combustion system, either by raising the burners’ firing rate (higher gas flow, shorter residence time, less efficient heat transfer) or by raising the set-point temperature, thus forcing the exhaust gases to leave at higher temperatures.
 

Kilns used in the brick industry are a good example of this. These continuous furnaces operate at temperatures of 2,000oF (1,100oC) and higher, but their exhaust gases don’t leave the stack until the temperature has dropped to about 600oF (320oC). The result is high thermal efficiency despite high operating temperatures. Combustion gases from the high temperature areas are forced to pass over cooler incoming product. Thus the heat is transferred to the product, bringing it part-way up to full processing temperature. As the exhaust gases cool on their way out, they continue to encounter cooler and cooler incoming product, so heat transfer continues. If you have a furnace or oven that operates at moderate to high temperatures, it is worth investigating design modifications that would route some or all of the exhaust over the cold incoming product. The energy efficiency benefits can be substantial.

Never Waste Heat: Even if it’s not practical to use exhaust gases to preheat product, it may still be possible to salvage some of the energy in the exhaust by sending it to a lower temperature process. Obviously the donor and recipient heating equipment must be situated fairly close to each other so large amounts of heat are not lost during transfer, and the two pieces of equipment have to operate on compatible cycles. It doesn’t make sense to run Furnace 1 just to keep Furnace 2 going, and Furnace 2 should be operating frequently enough to make good use of Furnace 1’s exhaust. If you can’t get a perfect match-up of operating cycles, an auxiliary heater on Furnace 2 can be used to fill the gaps.

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