Heat exchanger rating assessment

When an evaluation is performed to assess the suitability of an existing heat exchanger for given process conditions or for new conditions, this exercise is called 

The criteria can be established for the suitability of an existing exchanger for given or new services as two necessary and sufficient conditions  

When these two conditions are fulfilled, an existing exchanger is suitable for the process conditions for which it was rated. When the process conditions undergo significant changes, a rating should be performed to make sure the exchanger can perform the task satisfactorily under the new conditions. 

1 Assess the Suitability of an Existing Exchanger for Changing Conditions 

When it is considered to use an existing exchanger for changing conditions or new services, rating assessment must be conducted well in advance for the suitability of existing exchangers for such services. 

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One of the most useful methods of evaluating the performance of an existing heat exchanger or to assess a proposed design is to determine its effectiveness rj, which is defined as the ratio of the actual rate of heat transfer Q to the maximum rate Qmm that is thermodynamically possible or ... 

In order to assess the design of both the reactor and the heat exchanger required to control T, it is necessary to use the material balance and the energy balance, together with information on rate of reaction and rate of heat transfer, since there is an interaction between T and /A. In this section, we consider two cases of nonisothermal operation adiabatic (Q = 0) and nonadiabatic (Q = 0). 

The first strategy appears to be very effective in fact, as shown by (2.30) and (2.31), the direct (linear) effect of increasing the flow rate is augmented by the increase of the jacket-side heat exchange coefficient. This control action can be realized without a noticeable time delay by a simple control valve, the only drawback for its quantitative assessment being the nonlinear relationship between the overall heat exchange coefficient and the flow rate of the heat exchange fluid. 

In order to reduce capital cost many plants are installed with the minimum of instrumentation. Under these circumstances it may be difficult or even impossible, to make a proper assessment of the fouling encountered in heat exchangers under operating conffitions. Temperatures and flow rates may have to be inferred with the problems of accuracy that this raises. 

An improvement on the assessment of individual heat exchanger performance is to pass a sidestream through some kind of monitor. In general monitors resemble the test equipment used in the laboratory. The use of monitors allows a more careful control of the fouling conditions to be exercised particularly in terms of temperature, temperature changes and flow rates, using actual plant streams. The data obtained are more reliable than those obtained directly from the operation of full scale plant. 

From time to time, a process plant wishes to increase feed rate and/or make different product yields due to economic drivers. In feasibility evaluation, it is essential to assess the suitability of existing heat exchangers for new process conditions and find the most economical ways to handle significant changes. 

The rating assessment was conducted for heat exchangers in the preheat train and the assessment results indicated that fouling occurred, to a varying degree, to most of the... 

Pump-Around Many fractionation towers have pump-arounds to remove excess heat in the key sections of the tower. The effect of increasing pump-around rate is reduced internal reflux rate in the trays above the pump-around, but increased internal reflux rate below the pump-around. Thus, change in pumparound duty affects fractionation. On the other hand, pump-around rates and return temperature have effects on heat recovery via the heat exchanger network. It is not straightforward in optimizing pump-around duties and temperamres since the effects on both fractionation and heat recovery can only be assessed in a simulation model. An APC application incorporated with process simulation should be able to handle this optimization. 

Ageing effects may be detected by a change in measurable parameters. For example, increase in temperature or pressure may be an indication of die accumulation of corrosion products in the tube of a heat-exchanger and instrument drift may be an indication of electronic component degradation. Parameters should be measured periodically in a consistent manner and the readings should be compared and assessed. Physical parameters, such as temperature, pressure, flow rate, control rod drop times, radiation level (e.g. neutron and gamma), water quality, are indicators of the state of a system, structure or component. 

It can happen that our information is limited and the detailed chemical components balance not needed. Then only heat and mass balances are set up see Section 5.4. A typical example is a heat exchanger network. In the equations (5.7.11), the quantities (5.7.9) are approximated as functions of temperature only, say h (T ) in stream j. Formally, we can consider in addition certain source terms s n) in some nodes n e T , for example due to heats of reactions, a priori assessed or regarded as unknown p2U ameters to be computed from the set of constraints (given measured values of mass flowrates and temperatures). For example in a heat exchanger network, P ) = (P-Tq) is the sensible heat of stream y, with temperature P, specific heat cj, and reference temperature. We introduce the vectors hs of components hi, j e, and h of components for j 6 J (5.4.8), then the vectors (5.4.9) of components hi (j e J ) and h (5.4.10) of components for j e J the quantity /ij = is the heat flowrate in material stream j, with heat content factor hi. Finally s is the vector of components s(n), n g T . Then the heat and mass balance is represented by the equations (5.4.6). Again, the heat transfer rates through dividing walls can be eliminated by summation of the two scalar equations in (5.7.11), viz. the n]-th... 

The tiiermal behaviour of a chemical reactor depends on the thermodynamics of the process, on the reaction rate, vtdiich has already been mentioned above, but also on the mode of exchange with the environment. This is completely described in the overall heat balance of the system. The following sections will present the main balance equations, which are required for the subsequent safety assessment, as well as definitions and interpretation of characteristic numbers used in their presentatioiL... 

Catalyst studies have promoted attention with description of the use of iron salts to prevent ether formation during ester exchange polymerization. Model compounds have been employed to elucidate the meehanisms of metal ion catalysis in both transesterification and polycondensation reactions. A differential microcalorimeter has been used to assess the relative reactivities of catalyst systems for the poly-transesterification of bis-(2-hydroxyethyl tere-phthalate) and the relationship between the viscosity of the polymerizate and the temperature of the maximum rate of heat production has been investigated. Studies on antimony(v) compounds have indicated that their activity increases during the course of 2GT synthesis. This observation has been ascribed to the reduction of the antimony(v) compounds by acetaldehyde produced by 2GT decomposition. 

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