Reciprocating Engine Drives

Vapor compression chiller systems driven by reciprocating engines generally range in capacity from 20 tons (70 kWr) to several thousand tons (more than 10,000 kWr). Basic engine type distinctions include: cycle (i.e., Otto or Diesel), aspiration (i.e., atmospheric or supercharged/ turbocharged), fuel type, air-fuel ratio (i.e., lean or rich), operating pressures, coolant systems, and methods used to control emissions.

Over the past two decades, reciprocating engine-driven chillers have become a more mainstream cooling market technology. Packaged chiller systems are currently available in capacities ranging from about 40 tons (140 kWQ up to 3,000 tons (10,500 kWr). Figure 37-33 shows a

Fig. 37-30 Packaged Chiller with an Open Motor-Driven Centrifugal Compressor. Source: York International

3,000 ton (10,500 kWQ skid-mounted, packaged reciprocating engine-driven chiller. Figure 37-34 is a labeled illustration of a large-capacity packaged chiller featuring a gas-fired reciprocating engine-driven centrifugal compressor. Packaged gas-fired reciprocating engine-driven DX rooftop and condensing units are now available in capacities as low as 3 tons (10 kW).

Motor Driven Gas Compressor
Fig. 37-33 3,000 Ton Skid-Mounted Packaged Reciprocating Engine-Driven Chiller with Centrifugal Compressor. Source: Caterpillar Engine Division and York International

The most common reciprocating engines applied to larger vapor compressor drive service are those with maximum speeds ranging from 1,200 to 1,800 rpm. This speed is usually matched with compressors designed for operation at higher rpm, driving the compressor through a gear box. Higher speed automotive derivative engines can drive some higher speed compressors directly. Since reciprocating compressors operate at shaft speeds either the same or slightly lower than reciprocating engine speed, the gear box acts as an engine speed decreasing mechanism. With screw and centrifugal compressors, gearing is usually used to increase speed.

Capacity selection and operating strategy must be carefully chosen so that the system can reliably meet all peak load requirements, while operating in a thermally

Fig. 37-34 Labeled Illustration of Large-Capacity Packaged Reciprocating Engine-Driven Chiller. Source: York International

efficient manner over the majority of the operating regime. Moderate- to high-speed engine selections are usually made for seasonal space conditioning applications, while low-speed engines are more commonly applied to base-loaded applications.

Since space conditioning loads will commonly hit an absolute peak for relatively few hours over the course of a cooling season, variable speed capability usually provides operational benefits. Since variable speed-operated engines will operate at less than their rated speed, wear is reduced and thermal efficiency is increased.

Additionally, such load profiles make it possible to sometimes select a system with a prime capacity rating of about 90% of the peak load requirement. The system can then operate in over-speed mode to serve the 100% load requirement, as required. If a purchaser specifies Diesel Engine Manufacturers Association (DEMA) ratings, equipment will comply with operating requirements for 110% load for two hours per day without impact on engine life or maintenance cost. The advantage of this strategy is that a smaller, less costly system can be purchased. This may also further improve annual efficiency, since the compressor operates at optimal loading for a greater portion of the annual operating hours in the year.

Base-loaded, year-round refrigeration operations and larger space conditioning applications with substantial annual operating hours often use lower speed, industrial grade engines. Increased thermal efficiency, reliability, and engine life and lower maintenance requirement become far more significant factors as hours of operation increase. Therefore, the life-cycle economics may favor the more expensive lower-speed industrial grade engines and other components for such applications.

Heat Recovery Options

Commonly, engine heat recovery is used for hot water applications, particularly in smaller capacity applications. However, high- or low-pressure steam can be generated using exhaust gas heat recovery systems and low-pressure steam can be generated using ebullient engine cooling or systems that flash high-temperature circulated cooling system water. For facilities that have thermal loads that can be served by recovered heat, these systems can operate on standard cogeneration cycles.

For facilities that do not have alternative uses for recoverable heat, chilling can improve overall system ther-modynamic and economic performance. Either hot water or low-pressure steam can be used for add-on, single-stage absorption chilling. For example, for a 1,000 ton (3,500 kWr) cooling application, an 850 ton (3,000 kW^) recip-

rocating engine-driven chiller could be paired with a 150 ton (500 kWr) absorption chiller that is powered by the combined engine and exhaust recovered heat. One thousand tons (3,500 kWr) of cooling are provided with the fuel input required by an 850 ton (3,000 kWr) engine-driven chiller. This configuration offers a similar capital cost to the 1,000 ton (3,500 kWr) engine-driven chiller system. Alternatively, exhaust gas-generated high-pressure steam can be used to power a more efficient two-stage absorption chiller, and lower temperature recovered engine heat can be used for other purposes.

Figures 37-35 through 37-37 are schematic diagrams of heat recovery systems commonly applied to reciprocating engine-driven vapor compression systems. Figure 3735 is a closed-loop cooling system, Figure 37-36 is an ebullient cooling system, and Figure 37-37 is a forced circulation/steam cooling system.

Fig. 37-35 Schematic Diagram of Closed-Loop Cooling Heat Recovery System for Reciprocating Engine-Driven Chiller. Source: Waukesha Engine Division and The American Gas Cooling Center

Waukesha Engine Division
Fig. 37-36 Schematic Diagram of Ebullient Cooling Heat Recovery System for Reciprocating Engine-Driven Chiller. Source: Waukesha Engine Division and The American Gas Cooling Center

Fig. 37-35 Schematic Diagram of Closed-Loop Cooling Heat Recovery System for Reciprocating Engine-Driven Chiller. Source: Waukesha Engine Division and The American Gas Cooling Center

Performance

Typically, the full-load HHV energy input requirements for simple-cycle reciprocating engine-driven chillers, under standard ARI conditions, range from 10,000 Btuh/ton (0.83 kWh/kWr) or 1.2 COP for a small reciprocating compressor-based unit to 5,500 Btuh/ton (0.46 kWh/kWr) or 2.2 COP for a large-capacity, high-efficiency

Waukesha Engine Division
Fig. 37-37 Schematic Diagram of High-Temperature Forced Circulation/Steam Cooling Heat Recovery System for Reciprocating Engine-Driven Chiller. Source: Waukesha Engine Division and The American Gas Cooling Center

centrifugal compressor-based unit. When full heat recovery is employed, the net energy input requirements, inclusive of displaced boiler efficiency, range from about 5,000 Btuh/ton (0.41 kWh/kWr), or 2.4 COPnet, to 2,900Btuh/ ton (0.24 kWh/kWr), or 4.1 COPnet.

Figure 37-38 provides performance detail on a 125 ton (440 kW^) automotive derivative reciprocating engine-driven chiller featuring a screw compressor. The general data table shows the impact of varying evaporator and condenser water temperatures on system performance. At 44°F (7°C) leaving chilled water temperature and 85°F (29°C) condenser water temperature, the design full-load capacity rating is 125 tons (440 kW^). The fuel rate is 1,120 scf of natural gas at an assumed HHV of 1,020 Btu/scf. Under the highlighted conditions, this indicates a full load fuel rate of 9,142 Btu/ton-h (0.762 kWhh/kWhr). Recoverable heat (HR) from the engine cooling system is listed in MBtu/h. In this case, the recoverable heat of 328 MBtu/h (96 kWh) at a hot water temperature of 201°F (94°C) corresponds to 2,624 Btu/ton-h (0.219 kWhh/kWhr).

When combined with heat recovered from engine exhaust, the total system can produce about 4,500 Btu/ton-h (0.38 kWhh/kWhr) at a hot water temperature of 217°F (103°C). The ARI-550 table and COP versus output curve shows part-load performance with varying condenser water temperature. The ARI-based IPLV COP rating of 1.7, which is greatly enhanced by variable-speed operation and integral microprocessor controls, is significantly greater than the full load rating of1.3. Acoustic levels are 96 dba at 3 ft (1 m), which can be reduced to 89 dba with an acoustical enclosure. This system also requires an electric input of 3 kW for parasitics.

Figure 37-39 through 37-41 are three types of performance curves for a 950 ton (3,340 kWr) gas-fired reciprocating engine-driven chiller operating under standard ARI conditions. Figure 37-39 shows fuel input in Btu/ton-h (HHV basis) versus cooling load. Notice that with this particular configuration, performance is optimized at 75% of full load and begins to fall off rapidly when operating below 50% of full load. Figure 37-40 plots COP for this system. Also included is a curve indicating total COP for a piggyback unit featuring a recovered heat-powered absorption chiller. In this case, COP is calculated by comparing the

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