Friday, December 14, 2012

HVAC Absorption Chillers vs Electric Chillers

In any HVAC system,selection of chillers involve many factors to be considered judiciously.These factors simultaneously present many advantages and disadvantages to end users.For the selection of water chillers,HVAC engineers consider all these factors keeping in view all the resources they have at their facility.
HVAC chillers
Absorption chiller vs electric chiller

Comparison of Absorption Chillers and Electric (Compression) Chillers

Following is the comparison of absorption chiller and electric chiller is presented below with aim to help you to decide the type of chillers for HVAC  system.
    absorption vs compression
  1. Absorption chillers have COP (coefficient of performance )of only 0.54–1.1 and it competes poorly with electric chillers (rotary compressor chillers). Electric chillers on other hand have COP from 1.0 ~ 8.0. 
  2. Absorption HVAC chillers occupy approximately 50% more floor area than the equivalent  electric chillers (vapor compression chillers). 
  3. Additionally due to height of absorption chillers, mechanical equipment rooms must be 6–10 ft taller than rooms housing electric chillers. Finally, because the liquid solution is  contained in long, shallow trays within an absorption chiller, the floor must be as close to absolutely level as possible.
  4. In HVAC system,Absorption chiller will weight at least twice as much the equivalent electric chiller  (vapor compression chiller). 
  5. Due to greater  size, absorption chillers are sometimes shipped in several sections, requiring field welding for final assembly.This is not case with counterpart electric chillers or compression chillers. 
  6. Most of electric water chillers are shipped from the factory with their refrigerant charge  installed.While the refrigerant and absorbent (including additives) must be field installed in absorption HVAC chillers. 
  7. When we talk about noise and vibration, absorption chillers (unless direct-fired) are quiet and essentially vibration-free as compared to compression chillers (electric drive chillers). Noise and vibrations in HVAC system annoy more than anything else . 
  8. Due to the potential for crystallization of the lithium bromide in the chiller if it becomes too cool, the condenser water temperature must be kept above 75–808F.There is no problem of crystallization in electric chillers (vapor compression chillers). Forget about the crystallization and stay cooled if you have installed compression chillers . 
  9. Absorption water chillers require sometimes an emergency power source if lengthy power outages are more often expected. Without power and heat input, the chiller begins to cool and the lithium bromide solution may crystallize. However, as absorption chillers consume very little electric energy so a small dedicated back-up generator can serve this purpose well. 
  10. HVAC system also gives importance to the amount of heat to be rejected in condenser by water cooling or air-cooling media.The heat rejection rate from the condenser of lithium bromide chiller is 20–50% greater than for the equivalent electric chiller, requiring higher condenser water flow rates and larger cooling towers and condenser water pumps. 
  11. Finally, an indirect-fired absorption water chiller will be at least 50% more expensive to purchase than the equivalent electric-drive chiller. Direct-fired absorption chillers will cost almost twice as much as an electric machine, and have the added costs associated with providing combustion air and venting (stack). 
  12. Absorption water chiller or lithium bromide chiller uses natural refrigerants such water and eliminate the need of CFC or HCFC refrigerants which have high global warming potential. 
  13. If you have plenty of waste heat or direct fire available ,and want to reduce your electric load ,then probably absorption chillers are the best solution.But obviously all above factors are to be considered for the selection of HVAC water chillers.

Tuesday, December 11, 2012

Basic types of Maintenance Programs

maintenance engineer
Whether you own a car,house ,building  or professionally involved in operation and maintenance of  machines ,you need to know  some basics about maintenance.

Some form of maintenance you are practicing already but you always need to re-look at maintenance program so that you keep your asset(s) in correct form with minimal cost.

Maintenance is not only “the restoration of any asset to function after it has failed “instead spectrum of maintenance is broadened now  .

This post covers the basic maintenance types with special focus on following topics related to maintenance Programs.

  • Maintenance definition 
  • Objectives of maintenance
  • Nature of maintenance 
  • Types of maintenance programs 
  • Comparison of maintenance programs 
Maintenance definition

British Standard Glossary of terms (3811:1993) defined maintenance as:

"The combination of all technical and administrative actions, including supervision actions, intended to retain an item in, or restore it to, a state in which it can perform a required function"

"Maintenance is a set of organized activities that are carried out in order to keep an item in its best operational condition with minimum cost acquired."

In general, Maintenance means to hold, keep, sustain or preserve the building ,machine or structure to an acceptable standard.

Objectives of Maintenance Program

  • Maximising production or increasing facilities availability at the lowest cost and at the highest quality and safety standards.
  • Reducing breakdowns and emergency shutdowns.
  • Optimizing resources utilization.
  • Reducing downtime.
  • Improving spares stock control.
  • Improving equipment efficiency and reducing scrap rate.
  • Minimizing energy usage.
  • Optimizing the useful life of equipment.
  • Providing reliable cost and budgetary control.
  • Identifying and implementing cost reductions.
Types of Maintenance Programs

Different types of maintenance are being practiced in various parts of world.Adoption of particular Maintenance type greatly depends upon the resources of organization or company.One maintenance approach may cost less than other so each professional has to see the resources and then decide for "which maintenance type is best for his plant".

types of maintenance
types of maintenance programs

1. Reactive or Corrective Maintenance 

Reactive maintenance is basically the "run it till it breaks" maintenance mode. No actions or efforts are taken to maintain the equipment as the designer originally intended to ensure design life is reached.
Advantages to reactive maintenance can be viewed as a double-edged sword. If we are dealing with new equipment,we can expect minimal incidents of failure. If our maintenance program is purely reactive, we will not expend manpower dollars or incur capitol cost until something breaks.

Advantages of reactive maintenance
disadvantages of reactive maintenance
  • Low cost.
  • Less staff.
Disadvantages of reactive maintenance
  • Increased cost due to unplanned downtime of equipment.
  • Increased labor cost, especially if overtime is needed.
  • Cost involved with repair or replacement of equipment.
  • Possible secondary equipment or process damage from equipment failure.
  • Inefficient use of staff resources
2. Preventive Maintenance

Preventive maintenance can be defined as follows: 
"Actions performed on a time- or machine-run-based schedule that detect, preclude, or mitigate degradation of a component or system with the aim of sustaining or extending its useful life through controlling degradation to an acceptable level."

Preventive maintenance frequency may be machine operating hours based or after fixed time interval as decided by maintenance engineer or recommended by vendor of machine.

Advantages of Preventive maintenance
  • Cost effective in many capital intensive processes.
  • Flexibility allows for the adjustment of maintenance periodicity.
  • Increased component life cycle.
  • Energy savings.
  • Reduced equipment or process failure.
  • Estimated 12% to 18% cost savings over reactive maintenance program.
Disadvantages of Preventive maintenance
  • Catastrophic failures still likely to occur. 
  •  Labor intensive.
  • Includes performance of unneeded maintenance.
  • Potential for incidental damage to components in conducting unneeded maintenance.
3. Predictive Maintenance

Predictive maintenance can be defined as follows: Measurements that detect the onset of a degradation mechanism, thereby allowing causal stressors to be eliminated or controlled prior to any significant deterioration in the component physical state. Results indicate current and future functional capability. Basically, predictive maintenance differs from preventive maintenance by basing maintenance need on the actual condition of the machine rather than on some preset schedule. You will recall that preventive maintenance is time-based. Activities such as changing lubricant are based on time, like calendar time or equipment run time. 

For example, most people change the oil in their vehicles every 3,000 to 5,000 miles traveled. This is effectively basing the oil change needs on equipment run time. No concern is given to the actual condition and performance capability of the oil. It is changed because it is time. 

This methodology would be analogous to a preventive maintenance task. If, on the other hand, the operator of the car discounted the vehicle run time and had the oil analyzed at some periodicity to determine its actual condition and lubrication properties, he/she may be able to extend the oil change until the vehicle had traveled 10,000 miles. This is the fundamental difference between predictive maintenance and preventive maintenance, whereby predictive maintenance is used to define needed maintenance task based on quantified material/equipment condition.The advantages of predictive maintenance are many.

A well-orchestrated predictive maintenance program will all but eliminate catastrophic equipment failures. We will be able to schedule maintenance activities to minimize or delete overtime cost. We will be able to minimize inventory and order parts, as required, well ahead of time to support the downstream maintenance needs. We can optimize the operation of the equipment, saving energy cost and increasing plant reliability.

Advantages of predictive maintenance
  • Increased component operational life/availability.
  • Allows for preemptive corrective actions.
  • Decrease in equipment or process downtime.
  • Decrease in costs for parts and labor.
  • Better product quality.
  • Improved worker and environmental safety.
  • Improved worker moral.
  • Energy savings.
  • Estimated 8% to 12% cost savings over preventive maintenance program.
Disadvantages of predictive maintenance
  • Increased investment in diagnostic equipment.
  • Increased investment in staff training.
  • Savings potential not readily seen by management.

4. Proactive Maintenance

The latest innovation in the field of predictive maintenance is so-called pro-active maintenance, which uses a variety of technologies to extend the operating lives of machines and to virtually eliminate reactive maintenance.

The major part of a pro-active program is root cause failure analysis, which is the determination of the mechanisms and causes of machine faults. The fundamental causes of machine failures can thus be corrected, and the failure mechanisms can be gradually engineered out of each machinery installation. It includes routine preventive and predictive maintenance activities and work tasks identified from them.

typical maintenance practices
typical maintenance practices

It has been known for a long time that imbalance and misalignment are the root causes of the majority of machine faults. Both of these conditions place undue forces on bearings, shortening their service life.Rather than continually replacing worn bearings in an offending machine, a far better policy is to perform precision balance and alignment on the machine, and then to verify the results by careful vibration signature analysis.

Among above maintenance types ,Proactive maintenance programs has privilege of having advantages which are sum of advantages of each of reactive maintenance ,preventive maintenance & predictive maintenance program.While it shares its disadvantages mostly with predictive maintenance program.

Predictive maintenance vs proactive maintenance

  • Predictive maintenance is the technique to determine the condition of machine by sensing, measuring tools etc.
  • Example- Suppose one fan having abnormal vibration that find out by sensing tools (vibrometer etc. ) and we attend the job and rectified the current problem in fan. but in proactive maintenance , we search-out the main cause of the problem, which may, design problem, weather problem, operation problem etc, that to solve the actual problem .

Wednesday, December 5, 2012

Gate Valve

What is a Gate Valve

A gate valve is a linear motion valve used to start or stop fluid flow. The name gate is derived from the appearance of the disk in the flow stream. 

Gate Valve animation
Gate valve Diagram

The disk of a gate valve is completely removed from the flow stream when the valve is fully open.When the valve is fully closed, a disk-to-seal ring contact surface exists for 360 degree.With the proper mating of a disk to the seal ring, very little or no leakage occurs across the disk when the gate valve is closed.

A gate valve can be used for a wide variety of fluids and provides a tight seal when closed.  

Major disadvantages  of a gate valve
  • It is not suitable for throttling applications.
  • It is prone to vibration in the partially open  state. 
  • It is more subject to seat and disk wear than a globe valve.
  • Repairs, such as lapping and grinding, are generally more difficult to accomplish.
Disk Design of Gate Valve

Gate valves are available with a variety of disks. Classification of gate valves is usually made by the type disk used: 
  • solid wedge
  • flexible wedge
  • split wedge
  • parallel disk
Solid wedges, flexible wedges, and split wedges are used in valves having inclined seats. Parallel disks are used in valves having parallel seats.
Gate Valve
Gate valve

Regardless of the style of wedge or disk used, the disk is usually replaceable. In services where solids or high velocity may cause rapid erosion of the seat or disk, these components should have a high surface hardness and should have replacement seats as well as disks. If the seats are not replaceable, seat damage requires removal of the valve from the line for refacing of the seat, or refacing of the seat in place. Valves being used in corrosion service should normally be specified with replaceable seats.

Gate Valve Stem Design

Gate valves are classified as either rising stem or non rising stem valves. For the non rising stem gate valve, the stem is threaded on the lower end into the gate. As the hand wheel on the stem is rotated, the gate travels up or down the stem on the threads while the stem remains vertically stationary. This type of valve will almost always have a pointer-type indicator threaded onto the upper end of the stem to indicate valve position. 

The non rising stem configuration places the stem threads within the boundary established by the valve packing out of contact with the environment. This configuration assures that the stem merely rotates in the packing without much danger of carrying dirt into the packing from outside to inside.

Rising stem gate valves are designed so that the stem is raised out of the flow path when the valve is open. Rising stem gate valves come in two basic designs. Some valves have a stem that rises through the hand wheel while others have a stem that is threaded to the bonnet.

Gate Valve Seat Design

Seats for gate valves are either provided integral with the valve body or in a seat ring type of construction. Seat ring construction provides seats which are either threaded into position or are pressed into position and seal welded to the valve body. The latter form of construction is recommended for higher temperature service.

Integral seats provide a seat of the same material of construction as the valve body while the pressed-in or threaded-in seats permit variation. Rings with hard facings may be supplied for the application where they are required.

Small, forged steel, gate valves may have hard faced seats pressed into the body.In some series, this type of valve in sizes from 1/2 to 2 inches is rated for 2500 psig steam service. In large gate valves, disks are often of the solid wedge type with seat rings threaded in, welded in, or pressed in. Screwed in seat rings are considered replaceable since they may be removed and replaced.

Tuesday, November 20, 2012

Types of Chillers

Chiller Types have been designed to cater to the diversified requirement of industrial chillers.Water chillers have evolved due to the variety in availability of type of energy at industry or small level.Types of chillers have made the use of the chillers flexible in terms of energy input.

Water chillers
Water chillers

Industrial research in the field of chiller has eased the selection of chillers in an economical and robust manner.There is no need of worry whatever type of energy you have available with you.

Each type of chiller targets the specific users classified on the basis of type of energy they have.

HVAC systems employ different types of chillers within the available resources of clients or users.

A chiller may be the best option for particular application while the same chiller may costly or less feasible for another application or plant.

Hvac water chillers are classified primarily on the basis of cycle on which they work.

  • Vapor compression chillers or compression chillers or electric chillers
  • Vapor absorption chillers or absorption chillers or ammonia chillers or lithium bromide chillers

Types of HVAC chillers
HVAC chiller types
Vapor compression chillers are further classified on following basis.

  • Type of compressor i.e reciprocating compressor,rotory compressor or screw compressor type vapor compressor chillers
  • Type of condenser i.e air cooled condenser or air cooled chillers OR water cooled condensers or water cooled chillers
While Vapor absorption chillers are further classified as following:
  • Direct fired absorption chillers (Natural gas, Diesel or Kerosene Oil Operated)
  • Indirect fired absorption chillers


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