reducing shipping vibration of compressors in roof top air-conditioning units

Air-Conditi ning Units

Reducing Compressor Vibration during Shipping of Rooftop

© 2014, HCL Technologies. Reproduction Prohibited. This document is protected under Copyright by the Author, all rights reserved.

Abstract

Market Trends/Challenges

Experiments Conducted

The Solution

Tolerance Stack-up Study for the manufacturability of the part

Validation

Key Benefits to the Customer

Best Practices

Common Issues

Conclusion

Reference

Author Information

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Table of Contents


A rooftop unit is a packaged air-conditioning unit which is used for both residential and commercial purposes. This air-conditioning or heating system uses refrigerant as the heat exchange fluid and has an evaporator, compressor and condenser as separate components. In most modern commercial applications, the compressor and condenser are combined into a single piece of equipment called a condensing unit. Refrigerant piping is custom-designed to meet the physical requirements of each individual application which connect to various system components.

This unit is divided into 2 parts - the outdoor and the indoor unit. The outdoor unit mainly housesthe compressor, condenser, convenience outlet transformer, motor master etc. while the indoorthe compressor, condenser, convenience outlet transformer, motor master etc. while the indoorunit mainly houses the evaporator, re-heat coils, the blower section, etc. In the outdoor section, the basepan is the main component that supports the compressor and the condenser coil.While the roof top units are being shipped, the compressor vibrates which induces high stress on the base pan and also leads to refrigeration circuit failure. Roof top units vary in weight anywhere from 3 to 6 tons.

One of the major issues faced in the HVAC (heating, ventilating, and air conditioning) industry is the fluctuating response levels of these compressors at the time of transportation.The compressorcontributes to the second largest mass on the unit after the outdoor condenser coil. When the contributes to the second largest mass on the unit after the outdoor condenser coil. When the response level of the compressor mounted on the basepan is high, it induces high stress on the base pan. This stress is so high that it exceeds the ultimate strength of the material, thereby resulting in the failure of the base pan, which is one of the most critical structural members of the unit. This leads to a catastrophic failure of the unit before it even reaches the customer site. Subsequently, it would lead to water leaking into the building through the duct work.

In order to better understand the vibration levels on the base pan and compressor, Modal Analysis is In order to better understand the vibration levels on the base pan and compressor, Modal Analysis is performed, which predicts the mode shapes that are developed on the base pan and compressor at different frequencies. Animated results can also be obtained to understand the relative movement of the components, which is very helpful for design iterations. The natural frequency of the other components can also be determined and it can also check if any of the components are going into resonance.

This white paper is intended primarily for Designers or Engineers who are dealing with vibration This white paper is intended primarily for Designers or Engineers who are dealing with vibration related issues during product design and who wish to understand how vibrations on the base pan of a roof top unit can be minimized. This paper presents an overview of modal analysis, the response of components within a roof top unit, various techniques used to minimize response of the compressor on the basepan, the evaluation of design alternatives, the benefits of reducing the response level of a compressor, and a case study showing how the design was evaluated to reduce the compressors’ response.

Abstract

Reducing Compressor Vibration during Shipping of Roof Top Air-Conditioning Units | 3


It is a common phenomenon that compressors in roof top air-conditioning units tend to vibrate more violently during transportation. Such vibrations can lead to failure in the piping system or the compressor going into resonance when it is vibrating during shipping. Typical shipping frequencies are in the range of 10-18 Hz. The natural frequency of any of the other components mounted on the base pan should not lie in this range, as it might lead to the component going into resonance and thereby resulting in a failure of the design.

It is a common phenomenon that compressors in roof top air-conditioning units tend to vibrate more violently during transportation. Such vibrations can lead to failure in the piping system or the compressor going into resonance when it is vibrating during shipping. Typical shipping frequencies are in the range of 10-18 Hz. The natural frequency of any of the other components mounted on the base pan should not lie in this range, as it might lead to the component going into resonance and thereby resulting in a failure of the design.

Various methodologies were adopted to reduce the response of the compressor, which would lead to reducing the stress level in the base pan. The base pan on which the compressor is mounted is made of Draw-Quality material. The thickness is 16GA (1.50mm THK).

Figure1. Simulation of base pan flexes at various frequencies

Market Trends/Challenges

Experiments Conducted

Why does this problem arise?



Basepan Pre-stresses by5.75” x 13.5” x 0.375” plywoodin the compressor area.

Basepan Pre-stresses by5.75” x 13.5” x 0.375” plywoodin the compressor area.

4” x 4” x 0.375” pre-stressat the center of the Basepan.

Ultimate Tensile Strength – 51.2 KSI

Yield Strength – 42.6 KSI

% Elongation – 44%

The response level of the compressor was measured by placing tri-axial accelerometers on top of the compressor and the compressor foot. The experimental setup is such that the unit will be placed on concrete blocks and a shaker, that would induce a round 2 lb-force of energy into the unit,will be placed under the unit’s right, at the center of the basepan.

A hole was drilled to have the shaker installed. The shaft of the shaker passes through this hole that has been drilled and supported by fastening a nut onto it. Care was taken to see that the shaker was placed on a plywood sheet and not directly on the ground. The experiment illustrated that there was a tremendous amount of energy on the compressor and it was physically impossible to constrain the tremendous amount of energy on the compressor and it was physically impossible to constrain the compressor. This would cause the air-conditioning piping system to fail due to the stress developed in it by the movement caused due to vibrations of the compressor. Later, various methods were adopted for introducing a piece of plywood below the base pan to make it pre-stressed in the compressor area, to stop the violent motion of the compressor.

The material properties of this basepan are listed below.

The dimensions of the plywood are 5.75” X 13.5” X 0.375”.It was found that the response level increased at this size.This might be due to the variability of the structure; non-uniform or repeatable end conditions, etc. It is desirable to select the size of the plywood so that it can cover more than 90% of the surface below which the compressor is mounted.compressor is mounted.

The dimensions of the plywood are 5.75” X 13.5” X 0.375” at the compressor area, plus an additional 4” X 4” X 0.375” plywood pre-stressed at the center of the base pan. This was further enhanced by two pieces of 1” X 4” X 0.5” plywood placed at the transformer mounting. It was found that the response level increased. This might be due to the variability of the structure; non-uniform or repeatable end conditions, etc. repeatable end conditions, etc.

Figure 2. Plywood for experiment 1

1” x 4” x 0.5” pre-stress at thetransformer mtg

Figure 3. Plywood for experiment 2

Experiment-1

Experiment-2



Slight Shift of the modes when putting OSB or Plywood under basepan

Unit with shipping OSB plywood and a 5.75”x13.5” 0.375” plywood piece under compressor

unit with bare base pan

Unit with shipping OSB plywood and a 5.75”x13.5” 0.375” plywood piece under compressor version 2

Y AxisDOFS

Frequency

Foot 1Baseg/lbf

Foot 1Compg/lbf

Foot 2Baseg/lbf

Foot 2Compg/lbf

Foot 3Baseg/lbf

Foot 3Compg/lbf

Foot 4Baseg/lbf

Foot 4Compg/lbf

Comp Zg/lbf

Comp Y g/lbf

Comp Xg/lbf

8.0012.7537.75

0.00530.01970.0378

0.00580.02320.0433

0.02060.01910.0558

0.01720.016200.0331

0.03770.01590.0267

0.03410.01380.0904

0.00820.02270.0031

0.01380.03270.0558

0.01950.01750.0522

0.02550.07780.1168

0.06450.05690.0985

Y AxisDOFS

Frequency

Foot 1Baseg/lbf

Foot 1Compg/lbf

Foot 2Baseg/lbf

Foot 2Compg/lbf

Foot 3Baseg/lbf

Foot 3Compg/lbf

Foot 4Baseg/lbf

Foot 4Compg/lbf

Comp Zg/lbf

Comp Yg/lbf

Comp Xg/lbf

8.2512.5035.50

0.00270.01450.0453

0.00340.01840.0453

0.03210.02970.0203

0.01810.01910.0144

0.06150.01040.0980

0.05640.00890.0022

0.01240.01890.0197

0.01990.03000.0240

0.02710.01050.0249

0.06260.09080.0856

0.10790.12100.0648

Y AxisDOFS

Frequency

Foot 1Baseg/lbf

Foot 1Compg/lbf

Foot 2Baseg/lbf

Foot 2Compg/lbf

Foot 3Baseg/lbf

Foot 3Compg/lbf

Foot 4Baseg/lbf

Foot 4Compg/lbf

Comp Zg/lbf

Comp Yg/lbf

Comp Xg/lbf

8.2513.0036.25

0.00030.00210.0006

0.00450.03590.0279

0.03490.01420.0150

0.00210.00080.0011

0.06750.01830.0120

0.06100.01450.0213

0.01440.03070.0060

0.02280.04110.0277

0.02990.02560.0248

0.07250.00850.0096

0.11250.10170.0143

Table 1 shows the response level of the compressor at various frequencies when a 2 lb-force is exerted on the basepan. A compressor plate was designed so that we could achieve double thickness at the compressor foot locations.The compressor compressor foot locations.The compressor plate was double riveted and we checked if this lowered the response level of the compressor.

Table 1. Response level of the compressor

Figure 4. Measurement positions on the compressor foot

Figure 5. Newly designed compressor plate

The RED color partshown is the newly designedcompressor plate toprovide additional support to the compressor. compressor.

Compressor plate is rivetted to the basepan at 6 locations.

Basepan

RivetPart No: AT34AZ201

Newly Designed Compressor Plate (Red color part)



As we see from Figure 6, the response level of the compressor increased in the X, Y and Z directions between 8 - 40 Hertz. Hence, double riveting of the sheet metal parts together was not an ideal solution.

An idea was developed to check if the vibrations of the compressor could be grounded and an experimental set-up was developed. It was noted experimental set-up was developed. It was noted that the response of the compressor drastically reduced. This was mainly due to the fact that the vibrations were grounded through the 4” X 4” block via the jacks. The challenge was to bring this into engineering reality.

There were 3 challenges to overcome:

1. A decision had to be taken regarding the position of the 4” X 4” block between the OSB (oriented strand board) and the ground.

2. The 4” X 4” block + ¾” plywood had to be placed in such a location that while the units are stacked during shipping, the 4” X 4” block would lie on the top crate of the bottom unit and not on the top cover itself.

3. Tolerance stack-up study had to be done to see that the OSB sheet along with the block could be manufactured and the supplier’s acceptance received. Here the main challenge was the thickness of the OSB sheet that the supplier could deliver.

The proposed drawing in Figure 8 shows the position of the 4” X 4” block + ¾” plywood.

4” x 4” Block +1/2” plywood

Jacks to act asground level asthe unit is suspended overconcetrete blocks.

Horizontal X-Plane (top of compressor)

Frequency Hz Frequency Hz Frequency Hz

Horizontal Y-Plane (top of compressor) Vertical Z-Plane (top of compressor)

The Solution

Position of 4” X 4” Block + 3/4” Plywood

Figure 6. Graphs showing experimental results

Figure 7. Experimental set-up for grounding the vibrations



The position of the block was selected such that while forking the unit the fork pocket ends should be away from the block by at least 2 inches. Figure 9 shows the position and the offset distance of the block from the fork truck openings.

As seen in Figure 10, the block is supported on the top crate of the unit rather than the top cover. This is 6 inch pine wood.

Fork Truck Opening

Fork Truck Opening

4” X 4” Block + 3/4” Plywood

A tolerance stack-up study was performed to understand the manufacturability of the parts and find out the number of components that would be rejected/accepted using VisVSA software. 0.1683 - 0.3717 are the values which are falling out of specification limits at a 95% confidence interval.

Tolerance Stack-up study for manufacturability of the parts

Figure 8. Experimental set-up for grounding

the vibrations

Figure 10. Position of 4” X 4” block on the top crate in stacked-up condition

Figure 9. Positioning of the block



The response level of the compressor was compared with various configurations as explained above. The initial setup is the base pan with only the OSB. The modified base pan is the addition of the compressor plate to double the thickness.

Frequency Hz Frequency Hz

Frequency Hz

This experiment was then verified with FEA using ANSYS workbench software. Von Misses Stress was determined with and without wood. In Figure 13, we see that the stress was reduced by 58% with the introduction of the 4” X 4” block.

Test Results

Validation

Horizontal Y-Plane (top of compressor)Horizontal X-Plane (top of compressor)

Vertical Z-Plane (top of compressor)

Figure 11. Results of Tolerance stack-up study

Figure 12. Test results with different configurations.

Results at 95% Confidence Interval

Out ofSpecification Limits

Out ofSpecification Limits



The number of units which failed the transportation tests were significantly reduced

The OSB sheet provides a support to the base pan to avoid damages caused by the fork-lifts

The response level of the compressor reduced, increasing the durability of the piping system and thus maintaining the life of the product

The stress induced on the base pan was reduced by 58%

Increase in the lifespan of the outdoor base pan

Reliability and durability of the unit due to the basepan has increasedReliability and durability of the unit due to the basepan has increased

Reduction of warranty issues

Increase in the confidence level at design stage

After having designed a particular component, it is very important to ensure that it reaches thecustomer site safely. Therefore the system’s design needs to account for all vibrations that the system will undergo while it is being shipped. There are many best practices that are currently being incorporated in the manufacturing industry. However we need to take into account the least costly one.

Key Benefits to a Customer

Figure 13. Stress graphs

Best Practices

Von Misses Stress (Mpa). without wood

Von Misses Stress (Mpa). with wood

Top View

Top View Bottom View

Bottom View



Mechanical springs

Elastomers, rubber, cork, etc.

Wire rope isolators

Thermocol

Components going into resonance, thereby emitting lots of sound

Failure of system while it is delivered to the customer

Excessive stress developed in parts which might lead to failure before the warranty period

Some of the common issues if the shipping vibrations are not overcome:

Some of the best practices would then include:

Common Issues

Roof top units can be transported either via road, rail or air. In most cases the units are mainlytransported via road to the distribution centers. It is also noted that units are mainly damaged while being transported via road. These units are on the trailer for hours or days together, and experience rugged road conditions.

For testing purposes, these long hours were constrained to 1 hour transportation tests as per ASTM For testing purposes, these long hours were constrained to 1 hour transportation tests as per ASTM (American Society for Testing and Materials) D4169. There are various assurance levels defined in this test. However the most widely used is Assurance Level 1. Here the unit is placed on a hydraulic shake table and shaken for an hour, the overall RMS value of ‘g’ experienced by the unit is 0.75. The unit is then inspected after an hour to check all the failures. While units are being transported via road, they are always stacked up depending on the load the trailer is carrying.

We mainly observe that the amplification factor on top of the compressor increases by 2.5 times the We mainly observe that the amplification factor on top of the compressor increases by 2.5 times the input in the un-stacked configuration. While the unit is in the stacked configuration the amplification factor on top of the compressor is double of what it is on the bottom unit. Since the compressor is one of the heaviest components in the outdoor section and it rests on the base pan, it induces very high stress which may lead to a catastrophic failure of the unit.

Various experimental methodologies were conducted to find out how the response level of thecompressor could be reduced. In the end, pre-stressing the base pan with the wooden block attached to compressor could be reduced. In the end, pre-stressing the base pan with the wooden block attached to the OSB sheet and grounding the vibrations seemed to be the ideal solution for this problem.

Conclusion


This whitepaper is published by HCL Engineering and R&D Services.

The views and opinions in this article are for informational purposes only and should not be considered as a substitute for professional business advice. The use herein of any trademarks is not an assertion of ownership of such trademarks by HCL nor intended to imply any association between HCL and lawful owners of such trademarks.

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Reference

Author InfoMithun Srinivas Prabhakar

HCL Engineering and R&D Services

Vasu RaghuramHCL Engineering and R&D Services

1. http://www.kineticsystems.com/

2. http://www.newport.com/Vibration-Isolator-Selection-Guide/168531/1033/content.aspx

3. http://www.instructables.com/id/Types-of--Mechanical-Springs/


reducing shipping vibration of compressors in roof top air-conditioning units

Business

airconditioning units

outdoor unit

packaged airconditioning

housesthe compressor

acondensing unit

indoor unit

rooftop units

base pan ofa roof