Compressor efficiency control

Add insulation to walls Raise compressor efficiency Add insulation to ceiling Switch to demand defrost Improve controls Improve heat exchanger Improve door seals... 

To meet the 1993 Energy Standards, the industry undertook, at considerable cost, the optimization of the various refrigeration system components. The most significant improvement was the increase in compressor efficiency, from an EER of about 4 to about 5.5. Other system improvements included more efficient fan motors, more effective heat transfer by the evaporator and the condenser, and less defrost energy. In the early 1980s, both the Whirlpool Corporation and White Consolidate Industries introduced electronic defrost controls. Heretofore, an electric timer initiated the defrost cycle, typically every t A elve hours, whether the evaporator needed it or not. With the electronic control the defrost inteiwal is more a function of frost accumulation than of time, and thus referred to as a variable defrost control or as adaptive defrost. It saves energy by being activated only when needed. 

The load distribution can be computer-optimized by calculating compressor efficiencies (in units of flow per unit power) and loading the compressors in their order of efficiencies. The pressure controller (PC-22) directly sets the set points of SC-21 and SC-23, whereas the balancing controllers (FFC-22 and FFC-24) slowly bias those settings as they follow the total H2 generation of the electrolyzers (FT-4). The flow ratio controllers (FFCs) are also protected from reset windup, as was explained in Section 2.5.4. 

The Brayton unit compressor inlet temperature (CIT) is affected by varying the HRS flow rate (pump speed). Changes in the compressor inlet temperature influence reactor power and thus reactor outlet temperature and Brayton unit electric power output. This change in reactor power is caused by the change in reactor inlet temperature (Tcold) coming from the compressor and the resultant change in reactivity due to the reactivity temperature coefficient. It is desirable to operate at the lowest compressor inlet temperature for maximum plant efficiency. Control of the plant based on HRS flow rate thus prevents the system from operating at maximum efficiency. As such, this control scheme is not envisioned for use for normal operation. 

A preliminary, simple system state point evaluation was performed to estimate the system performance impacts for various leak sizes. This evaluation was non-conservative because the effects of reduced compressor efficiency and the increased pressure losses in the system were not considered in the evaluation. The effects of leak size on system performance are summarized in Figure 9-34. Included in this figure are the state point assumptions. Based on bypass control analysis performed in Reference 9- 64 an additional reduction in overall system performance is expected based on more detailed system models which capture the effects of system pressure, variations in component pressure drop and turbine/compressor efficiency variations. For a 10% bypass flow on the compressor side of the 100 kWe Brayton system in Reference 9- 64 a reduction in electrical output of approximately 37% was calculated compared to 24% predicted by the simpler state point model. 

Space needs to be provided for the auxiliaries, including the lube oil and seal systems, lube oil cooler, intercoolers, and pulsation dampeners. A control panel or console is usually provided as part of the local console. This panel contains instmments that provide the necessary information for start-up and shutdown, and should also include warning and trouble lights. Access must be provided for motor repair and ultimate replacement needs to be considered. If a steam turbine is used, a surface condenser is probably required with a vacuum system to increase the efficiency. AH these additional systems need to be considered in the layout and spacing. In addition, room for pulsation dampeners required between stages has to be included. Aftercoolers may also be required with knockout dmms. Reference 8 describes the requirements of compressor layouts and provides many useful piping hints. 

The steam balance in the plant shown in Figure 2 enables all pumps and blowers to be turbine-driven by high pressure steam from the boiler. The low pressure exhaust system is used in the reboiler of the recovery system and the condensate returns to the boiler. Although there is generally some excess power capacity in the high pressure steam for driving other equipment, eg, compressors in the carbon dioxide Hquefaction plant, all the steam produced by the boiler is condensed in the recovery system. This provides a weU-balanced plant ia which few external utiUties are required and combustion conditions can be controlled to maintain efficient operation. 

A compressor is typically a specially designed device, and comes with far less surplus capacity than other process components. As a result compressors merit great care in specification of flow, inlet pressure, and discharge pressure. Similarly, the control system and equipment need to be carefully matched to provide turndown with maximum efficiency. 

The first objective of the antisurge control system is to protect the compressor. This can be accomplished for some disturbances by using the PI algorithm with a large value of bj. However, it is also necessary to maximize the region in which the compressor can operate with the recycle valve closed. This increases the efficiency of the compressor at lower throughputs. Steady-state operation with recycle is extremely inefficient. Therefore, from this perspective, small values of bj are highly desirable. 

The accurate calculation and proper evaluation of the losses within the axial-flow compressor are as important as the calculation of the bladeloading parameter, since unless the proper parameters are controlled, the efficiency drops. The evaluation of the various losses is a combination of experimental results and theory. The losses are divided into two groups (1) losses encountered in the rotor, and (2) losses encountered in the stator. The losses are usually expressed as a loss of heat and enthalpy. 

Head coefficient, 156 Head equation, adiabatic, 3 t Head equation, poly tropic, I Head, centrifugal, 156 Head, reciprocating, 58 Heat run test (dry), 413 Helical compressor, 5, 7 adiabatic efficiency, Itil applicalion mnge, 7. ly asymmetric profile, 96 bearings, 116 capacity control, 95 casings, 114 circular profile, 95 cooling, I i 1 discharge temperature (dry), I 17... 

A speed controller can help extend the operating range and efficiency of the compressor. As the flow rate increases, the compressor speed can be increased to handle the additional gas. Compressor speed will stabilize when the actual flow rate to be compressed equals the required flow rate for the cylinder at the preset suction pressure. As the flow rate decreases, the compressor slows until the preset suction pressure is maintained. 

The compressor manufacturer can control items a-c, e, f, and h however, the control of clearance volume at high compression ratios for gases/vapors with low specific heat ratios is of great concern. Compression efficiency is controlled by the clearance volume, valves, and valve pocket design. A decrease in compression efficiency leads to increased power requirements. ... 

At any constant or steady speed of operation of a compressor, the head-capacity and efficiency curves are characteristic of the impeller and casing design only. These curves that are determined by test can be translated to other reasonable speeds and conditions of operation of the wheel-casing combination of the affinity laws. The operation of the compressor must meet or establish the desired point on the head-capacity-system curve, which requires a combination of controls. 

In air conditioning circles, the tower normally represents the final heat sink in a turnkey package which would include compressors/condensers, pipework, ducting, fans, pumps, control gear, etc. Where consultants and experienced contractors are concerned, the tower specification is well defined and the purchases based upon economics related to efficiency. 

A greater difficulty arises where the compressor may go down to 33% or 25% capacity and the thermostatic expansion valve is called upon to control a much reduced flow Under such conditions, the thermostatic expansion valve maybe unstable and hunt , with slight loss of evaporator efficiency Since the required duty is less, this is of no great importance. It is possible to fit two expansion valves in parallel, one for the minimum load and both for the full load, but this arrangement is not usually necessary... 

Assuming a theoretical efficiency of the fuel-cell system of around 60% and an electric-drive-train efficiency of 90%, the overall fuel-cell system efficiency is about 55%. The theoretical efficiencies for a fuel cell cannot be realised in practice. The efficiency of the system (including fuel treatment, air supply and others) is already lower than that of the pure fuel-cell stack on its own the overall efficiency of the FC drive train falls to less than 40% as a result of additional components, such as compressors, control electronics and others, see Fig. 13.6. 

The first application of a rhodium-ligand system was realized in the LPO-process (low pressure oxo Fig. 18). Huge stirred tank reactors are used, equipped with internal heat exchangers to control the heat of reaction. The solution of the catalyst recycle is simple but efficient. The catalyst remains in the reactor, products and unconverted propene are stripped by a huge excess of synthesis gas. Because of strong foaming, only a part of the reaction volume is used. After the gas has left the reactor, the products are removed by condensing, the big part of synthesis gas is separated from the liquid products and recycled via compressors. The liquid effluent of the gas-liquid separator... 

A fuel processor for PEFC application contains sulfur removal, an ATR-enhanced UOB reformer, advanced shift reactors, a steam generation system, a product gas cooler, a PROX system, a gas compressor, an air compressor, an anode-off gas oxidizer, and a control system. Goal efficiency (LHV H2 consumed by fuel cell/LHV fuel consumed by fuel processor) is 69 to 72%. H2 concentration is presently >50% (dry). 

Synchronous motors are made in speeds from 1800 (two-pole) to 150 rpm (48-pole). They operate at constant speed without slip, an important characteristic in some applications. Their efficiencies are 1-2.5% higher than that of induction motors, the higher value at the lower speeds. They are the obvious choice to drive large low speed reciprocating compressors requiring speeds below 600 rpm. They are not suitable when severe fluctuations in torque are encountered. Direct current excitation must be provided, and the costs of control equipment are higher than for the induction types. Consequently, synchronous motors are not used under 50 HP or so. 

---