A Simple Method to Evaluate Heat Exchanger Performance

In this post, I want to share a simple method to evaluate heat exchanger performance. One of the most helpful ways to evaluate the performance of heat exchanger is to determine its effectiveness by comparing the actual heat transfer rate to the maximum rate that is thermodynamically feasible. The simple formula is expressed below.

Where:

η        = effectiveness

Q       = actual heat transfer rate

Qmax    = heat transfer rate which would be achieved if it were possible to bring the exit temperature of the stream with the lower heat capacity, to the inlet temperature of the other stream. Read More

How to Estimate Time Required for Heating or Cooling

In this post, I want to share how to estimate time required for heating and cooling.

The contents of a large batch reactor or storage tank frequently need to be heated or cooled. In this circumstance, the physical properties of the liquor may change throughout the process, as well as the overall transfer coefficient. When estimating the amount of time needed to heat or cool a batch of liquid, it is frequently possible to assume an average value for the transfer coefficient. Steam condensing, either in a coil or some type of hairpin tube heater, is a common method for heating the content of storage tank.

It is reasonable to assume that the overall transfer coefficient U is constant in the context of a storage tank filled with liquor having mass m and specific heat Cp and heated by steam condensing in a helical coil. The rate of heat transfer is given by: If T s is the temperature of the condensing steam, T1 and T2 are the initial and final temperatures of the liquor, A is the area of the heat transfer surface, and T is the temperature of the liquor at any time t, then:

The time t for heating from T1 to T2 can be determined using this equation. If the steam condenses in a reaction vessel’s jacket, the same analysis may be applied.

Heat losses during the heating or, for that matter, cooling operation are not considered in this analysis. The heat losses increase naturally as the temperature of the vessel’s contents rises, and at a certain point, the heat supplied to the vessel equals the heat losses, making further increases in the temperature of the vessel’s contents impossible.

By increasing the rate of heat transfer to the fluid, for example, by agitating the fluid, and by minimizing heat losses from the vessel by insulation, the heating-up time can be shortened.

The amount of agitation that can be achieved in a large vessel is constrained, thus one attractive alternative is to circulate the fluid through an external heat exchanger.

Let’s see example below on how to estimate time required for heating or cooling. Read More

Example How to Design Thin-Walled Vessel under Internal Pressure

In this post, I want to share a simple example on how to design thin-walled vessel under internal pressure. Please check my previous post about the method on how to estimate minimum thickness of vessel component (shell, flat end closures, domed head closures).

Example

Estimate the thickness required for the component parts of the vessel shown in the diagram.

The vessel is to operate at a pressure of 16 bar (absolute) and design temperature of 300oC. The material of construction will be plain carbon steel. Welds will be fully radiographed. A corrosion allowance of 2 mm should be used. Read More

How to Design Thin-Walled Vessel under Internal Pressure

In this post, I want to share how to design thin-walled vessel under internal pressure. I will also share a simple example about the application in the next post.

For information, I do not have any experience of the calculation in my whole career until posted this post. The design of thin-walled vessel under internal pressure is usually job of mechanical engineer. But it is good for process engineers to understand it in general.

The design of thin-walled vessel under internal pressure, in general, will be divided into two parts: design of cylinder and spherical shells and design of heads and closures. Read More

How to do Projects Economic Evaluation – Example on How to Calculate DCFRR

In this post, I want to share how to do projects economic evaluation by using example. In this example, we will learn relationship between investment, sales, raw material cost, operating cost, to get the following parameters that usually used in projects economic evaluation:

• Net cash flow
• Cumulative cash flow
• Discounted cash flow

Example on How to do Projects Economic Evaluation

It is proposed to build a plant to produce a new product. The estimated investment required is 12.5 million dollars and the timing of the investment will be:

• Year 1    1.0 million (design costs)
• Year 2    5.0 million (construction costs)
• Year 3    5.0 million (construction costs)
• Year 4    1.5 million (working capital)

How to Do Projects Economic Evaluation – A Simple Example

In this example, I want to show you a simple example on how to do projects economic evaluation. As mentioned previously, there are several parameters that can be used to do projects economic evaluation, such as:

• Net present worth
• Rate of return (ROR)
• Discounted cash-flow rate of return
• Pay-back time

In this example we will use ROR and pay-back time.

Example on How to Do Projects Economic Evaluation

A plant is producing 12,000 ton per year product. The overall yield is 70%, on a mass basis (kg of product per kg raw material). The raw material cost $12 per ton, and the product sells for$37 per ton. A process modification has been devised that will increase the yield to 75%. The additional investment required is \$35,000, and the additional operating costs are negligible. Read More

How To Do Projects Economic Evaluation

In this post, I want to share how to do projects economic evaluation. In my experience, doing project economic evaluation is not always daily activities of process engineers. However, when I involved in feasibility study project, we need to do projects economic evaluation. Also in my own experiences, as process engineers focused on creating several alternatives of process configuration, our team from industrial engineers did the projects economic evaluation. However, it will not hurt to understand general idea of project economic evaluations 😊

Since investing in chemical plants is done to make money, a system for evaluating project economic is required. For small projects, the decision-making process can typically be simplified by comparing the capital and operating expenses of various processing methods and pieces of equipment. When choosing between large, complicated projects, especially when the projects differ significantly in scope, time frame, and product type, more advanced evaluation procedures and economic factors are required. In this part, some of the more popular methods of economic analysis and the standards by which economic performance is evaluated are described. Read More

Preliminary Sizing of Cyclones

In this post, I want to share how to do preliminary sizing of cyclones.

Cyclone is the most common and effective gas-solids separator that uses centrifugal force. They can be constructed from a variety of materials, are generally simple structures, and can be designed to operate at high temperatures and pressures.

The reverse-flow cyclone is the most common design. In a reverse-flow cyclone, the gas enters the top chamber tangentially, spirals down to the conical section’s apex, and then moves up in a second spiral with a lower diameter before leaving through a central vertical pipe at the top. The solids travel radially to the walls, down the walls, and then are gathered at the bottom. Read More

Preliminary Sizing of Hydrocyclones

In this post, I want to share how to do preliminary sizing of hydrocyclones. In previous post, hydrocyclones are utilized for solid-liquid separations. The centrifugal force is produced by the motion of the liquid in this centrifugal device, which has a stationary wall. Like a gas cyclone, the gas cyclone operates on much the same principles. Hydrocyclones are inexpensive, reliable separators that work with particle sizes ranging from 4 to 500 micron. Figure below shows hydrocyclones typical proportion geometry.

Basic Sizing Equation for Sedimentation Centrifuges

In this post, I want to share basic sizing equation for sedimentation centrifuges. Based on previous post, sedimentation centrifuges used to separate materials based on the density difference between the solid and liquid phases. They are usually used to produce cleared liquid.

Basic sizing equation for sedimentation centrifuges use the term sigma (Σ). The sigma is used to describe a performance of a centrifuge regardless of the physical characteristics of solid-fluid system. The sigma value is equivalent to the cross-sectional area of a gravity settling tank with the same clearing capacity. The sigma value is often expressed in cm2.

The sigma theory is a description of how centrifuge performance is described. It offers a way to compare sedimentation centrifuge performance and to scale up from laboratory and pilot scale experiments. Read More