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GEA's Global HVAC Technology Blog

GEA's Global HVAC Technology Blog covers a range of topics including:

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Designing Water Cooled Condenser Tube Bundles

Posted July 23, 2014 1:00 AM by larhere

In a recent post titled Designing Flooded Evaporator Shell & Tube HXRs I outlined the method and corresponding equations for the heat transfer design of flooded evaporator S & T HXRS. This post will do similar for Water Cooled Condenser Tube Bundles.

In application a water cooled condenser must reject the heat removed from the chilled water circuit plus the compressor work at specified water flow and entering & leaving temperatures. Water-cooled condenser has water on the tube side & refrigerant on the shell side. The unit compressor capacity and power consumption are functions of the refrigerant saturation temperature, among other operating parameters.

For the tube bundle, determine the flow areas and heat transfer areas.

Af=Nt*(π*Di2/4)/Np, tube side flow area, ft2

Ao=Nt*π*Do*L, shell side heat transfer area, ft2

Ai=Nt*π*Di*L, tube side heat transfer area, ft2

Aia= Nt*ai*L, tube side heat transfer area for fouling, ft2

ai=actual enhanced tube side area/ft, ft2/ft

B= Ao/ Ai

WhereB2=Ao/ Aia

Nt=number of tubes

Np=number of tube passes

Di=inside tube diameter, ft

Do=outside tube diameter, ft

L=length of tube, ft

There are two equations that apply

Q=Uo*Ao*LMTD

and

Q=mw*cp*( Two -Twi)

Where

Q=Capacity, BTU/ Hr

Uo=Overall Heat Transfer Coefficient, BTU/ (Hr-Ft2-oF)

LMTD=Log Mean Temperature Difference, oF

LMTD=(Two-Twi)/ln((Ts-Twi)/ (Ts-Two))

mw=Mass Flow of water, Lb/Hr

cp=Specific heat of water, BTU/(Lb-oF)

Ts=Refrigerant saturation temperature, oF

Twi=Entering Water Temperature, oF

Two=Leaving Water Temperature, oF

The overall heat transfer coefficient is a function of the refrigerant side coefficient, the tube metal resistance, the water side coefficient and the fouling resistance.

Uo=1/((1/ho')+(B/hi)+(B2*Rf))

Where

ho'=refrigerant side heat transfer coefficient, BTU/(hr-ft2-oF)

Note: The metal wall resistance is commonly included with the refrigerant side heat transfer coefficient. No fouling resistance allowance is needed on the refrigerant side.

hi=water side heat transfer coefficient, BTU/(hr-ft2-oF)

Rf=water side fouling resistance, (hr-ft2-oF)/BTU

A common engineering problem is to determine the refrigerant saturation given the condenser heat rejection, the water flow rate and the entering water temperature. The solution for Ts is as follows:

Using the two equations that determine Q

Uo*Ao*LMTD=mw*cp*(Two-Twi)

Substituting the equation for LMTD

Uo*Ao*( Two-Twi)/ln((Ts-Twi)/ (Ts-Two))=mw*cp*(Two-Twi)

Uo*Ao/ln((Ts-Twi)/ (Ts-Two))=mw*cp

ln((Ts-Twi)/ (Ts-Two))= Uo*Ao/mw*cp

Define

C= Uo*Ao/mw*cp

Then

ln((Ts-Twi)/ (Ts-Two))=C

(Ts-Twi)/ (Ts-Two)=eC

Ts-Twi =eC* Ts - eC * Two

eC * Two- Twi= eC* Ts-Ts

Ts=(eC*Two- Twi)/( eC -1)

Designers can perform these calculations using spreadsheets or computer programs to determine the operating saturation temperature for a given tube bundle in a chiller application, or to size heat exchanger tube bundles to meet the required heat rejection, saturation temperatures and water temperatures.

Editor's Note: CR4 would like to thank James Larson, GEA Consulting Associate, for contributing this blog entry, which originally appeared here

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Prefab Skyscrapers Reach for the Heavens Reshaping HVAC

Posted July 16, 2014 1:00 AM by larhere

Keeping an eye on the future is an important part of being successful in one's industry; in our case, the HVAC industry where the building industry is a primary driver of air-conditioning system design, technologies and materials.

Most recently we have been following the China Sky City project to build the world's tallest building, 220 stories with prefabricated modules built in advance in factories and assembled in record time and at significant cost savings. (See below for related articles).

The project has run into a number of delays including a quality issue with concrete, a "hold" put on the project due to concern over "ugliness" and re-evaluation of construction plans which have caused revision of the construction phase to go from 90 days to a current build cycle estimate of 4 months.

As Sky City struggles in China a new project in Dubai, the Da Vinci Tower, has been proposed that goes beyond in many ways.

First, it is not taller at "only" 78 floors, but it uses unique factory built modules and employs a fourth dimension of movement with time; each floor rotates independent from other floors (voice activated control on each floor, or controlled centrally according to a specific schedule, or the building can be stationary for a period of time then "redesigned" for a new look).

The Da Vinci Tower is part of the Dynamic Architecture movement which has been evolving over the last few decades. The earliest known high-rise was built in Brazil at the turn of the century.

The Tower's circular core will be site-poured concrete with the pie shaped factory built modules of offices, air-conditioning equipment, plumbing and domestic water, utilities, etc that will be lifted/guided into place as each floor is "built". A 30 percent reduction in building cost is projected by designers as well as shorter build cycles.

The Da Vinci Tower is totally energy self sustaining and will actually generate additional energy to power four additional buildings nearby using solar panels and a unique system of 77 turbines placed horizontally between each floor. Building controls will be using the latest SMART Technologies in the building industry.

How might this affect HVAC system design and operation (and operating costs?). How do you design a system when the conditioned space can have an infinite number of locations affecting solar load. What control freedom would there be between occupants on the same floor?... on different floors?...some of the questions being discussed right now.

From an OEM standpoint how will this affect the systems and components required to meet a wider range of loads and operating conditions? Will new requirements in sound, vibration, weight, size... require new technologies or cause a shift in the use of current technologies and materials? How will maintence need to change? Does system reliability need to move to a higher level?

Watch the four and a half minute Dynamic Architecture Video for more about the trend and the Da Vinci Tower project.

Recent Posts on Pre-Fabricated Buildings
Future Skyscrapers - Built By Drones?
World's Tallest Building, a Prefab, Stirs Controversy
World's Tallest Building (A Prefab) - Update From China

You might also enjoy
10 Predictions for US High Performance Building in 2014
IEA Calls on Buildings Industry to Save the 2 Degree CLCH Scenario
Beware the Shift - Energy Efficiency IS Important!
Why is Energy Efficiency Such a Hard Sell?

Editor's Note: CR4 would like to thank Larry Butz, GEA Consulting President, for contributing this blog entry, which originally appeared here.

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Designing Flooded Evaporator Shell & Tube HXR's

Posted June 25, 2014 8:00 AM by larhere

In application, a water chiller must provide a specified flow rate of chilled water at specified entering & leaving water temperatures. A flooded evaporator has water on the tube side & refrigerant on the shell side. The unit compressor capacity and power consumption are functions of the refrigerant saturation temperature, among other operating parameters.

For the tube bundle, determine the flow areas and heat transfer areas.

Af=Nt*(π*Di2/4)/Np, tube side flow area, ft2

Ao=Nt*π*Do*L, shell side heat transfer area, ft2

Ai=Nt*π*Di*L, tube side heat transfer area, ft2

Aia= Nt*ai*L, tube side heat transfer area for fouling, ft2

ai=actual enhanced tube side area/ft, ft2/ft

B= Ao/ Ai

B2=Ao/Aia

Where

Nt=number of tubes

Np=number of tube passes

Di=inside tube diameter, ft

Do=outside tube diameter, ft

L=length of tube, ft

There are two equations that apply

Q=Uo*Ao*LMTD

and

Q=mw*cp*(Twi-Two)

Where

Q=Capacity, BTU/ Hr

Uo=Overall Heat Transfer Coefficient, BTU/ (Hr-Ft2-oF)

LMTD=Log Mean Temperature Difference, oF

LMTD=(Twi-Two)/ln((Twi-Ts)/ (Two-Ts))

mw=Mass Flow of water, Lb/Hr

cp=Specific heat of water, BTU/(Lb-oF)

Ts=Refrigerant saturation temperature, oF

Twi=Entering Water Temperature, oF

Two=Leaving Water Temperature, oF

The overall heat transfer coefficient is a function of the refrigerant side coefficient, the tube metal resistance, the water side coefficient and the fouling resistance.

Uo=1/((1/ho')+(B/hi)+(B2*Rf))

Where

ho'=refrigerant side heat transfer coefficient, BTU/(hr-ft2-oF) (determined by test)

Note: The metal wall resistance is commonly included with the refrigerant side heat transfer coefficient. No fouling resistance allowance is needed on the refrigerant side.

hi=water side heat transfer coefficient, BTU/(hr-ft2-oF) (determined by test)

Rf=water side fouling resistance, (hr-ft2-oF)/BTU (specified by customer or rating standard)

A frequent engineering calculation is to determine the refrigerant saturation given the required capacity, the water flow rate and the leaving water temperature. The solution for Ts is as follows:

Using the two equations that determine Q

Uo*Ao*LMTD=mw*cp*(Twi-Two)

Substituting the equation for LMTD

Uo*Ao*(Twi-Two)/ln((Twi-Ts)/ (Two-Ts))=mw*cp*(Twi-Two)

Uo*Ao/ln((Twi-Ts)/ (Two-Ts))=mw*cp

ln((Twi-Ts)/ (Two-Ts))= Uo*Ao/mw*cp

Define

C= Uo*Ao/mw*cp

Then

ln((Twi-Ts)/ (Two-Ts))=C

(Twi-Ts)/ (Two-Ts)=eC

Twi-Ts=eC*Two- eC *Ts

eC *Ts -Ts=eC*Two- Twi

Ts=(eC*Two- Twi)/( eC -1)

Designers perform iterative analyses with the aid of spreadsheets or computer programs to design optimum heat exchangers to meet the required duty.

Editor's Note: CR4 would like to thank James Larson, GEA Consulting Associate, for contributing this blog entry, which originally appeared here

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NAR HVAC Market Expecting Strong Growth

Posted June 11, 2014 1:00 AM by larhere

According to a new study by Grand View Research Inc, the North American Region HVAC market grew to $28.8 Billion in 2013, and is expected to reach $52.6 Billion by 2020, growing at a CAGR of 9.0%. The market was estimated to be 34.8 million units in 2013 and is expected to exceed 50 million units by 2020.

Split air conditioning systems accounted for over 60% of the overall revenue share in 2013, and are expected to remain the largest product segment over the next six years. The commercial segment is expected to be influenced by individuals using portable systems for camping trips and other outdoor activities.

The residential sector accounted for over 40 percent of the market in 2013, and it is expected to remain the largest sector.

The United States contributed to over 83 percent of the revenue generation in North America in 2013.

Growing demand for energy efficient products is expected to be the key driving force for the North America air conditioning systems market over the next six years. Mexico is expected to be the fastest growing regional market, at an estimated compound annual growth rate (CAGR) of 10.3 percent.

Commercial applications overall are expected to grow faster than the global average over the next six years.

Pressure continues on refrigerants that are increasingly seen as toxic and a threat to the environment. The market is expected to be fuelled by replacement demand of these products.

Stiff competition from Chinese manufacturers which sell products at lower prices as compared to global well-established brands may pose a restraint to market growth in North American countries.

The report "North America Air Conditioning Systems Market Analysis by Product (Portable, Window, Split, Single Packaged, Chillers, Airside), by Application (Residential, Commercial, Industrial) and Segment Forecasts to 2020," is available here

Editor's Note: CR4 would like to thank Larry Butz, GEA Consulting President, for contributing this blog entry, which originally appeared here.

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Fuel & Power Budgeting In World Class Factories

Posted June 04, 2014 1:00 AM by larhere

Designing and supervising operation of efficient manufacturing facilities is what I have done for most of my career. Here is a simple step-by-step procedure for Fuel & Power budgeting to bring your factory to the world-class level.

Step 1. Determine the facility operating Base & Process energy loads by audit. HVAC energy consumption is an example of a base load. The motor energy required to exhaust air from a cleaning booth including the energy to produce steam or hot water is an example of a process load. The heating or cooling of the make-up air to this process is part of the base load. Energy cost reduction savings become apparent with these audits.

Step 2. Secure the Climatological Data for the location from NOAA. Heating & cooling degree-days are listed including normal and variation to the current day/month/ years. This data is required to set a standard ratio of base load energy to heating/cooling degree-days. Budgets are set to the normal ratio. Departure from normal provide detail of the variation to current month/day & previous year.

Step 3. Define the operation's productivity factor. Tons of product shipped, direct labor hours to produce are examples of factors that affect the process load. The ratio of these factors to the process loads set the standard for the current day/month/year performance to budget. Variations to budget either favorable or unfavorable result from changes in productivity, however, only the process portion of the total Fuel & Power load is affected by the changes

Step 4. Set the current year Fuel & Power budget based on previous year's performance. The base load should be based on normal degree-days. Previous years base load adjusted to normal is the starting point with changes to plant size &/or HVAC energy cost reduction as examples of projected changes. The process load is based on equal performance to previous year's budget. Additions or reductions to the productivity factor including any energy-influenced process changes will influence the current years process load.

Step 5. Provide ongoing projections of Fuel & Power budget variations based on changes to productivity & weather. Establish a score sheet on performance to budget with the objective to maintain/reduce costs & provide updated reports to operations personnel.

The procedure outlined above takes the responsibility of Fuel & Power Budgeting out the traditional Factory Cost Accounting function and places it properly with the Facility Engineering group. Operational management will understand variations to the current Budget and take logical steps forecasting future energy costs.

Editor's Note: CR4 would like to thank John Ramsden, GEA Consulting Associate, for contributing this blog entry, which originally appeared here.

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Europe Moves Forward On F-Gas Controls

Posted May 21, 2014 9:04 AM by larhere

Fluorinated gases ("F-gases") are a family of man-made gases used in a range of applications. The three groups of F-gases are hydrofluorocarbons (HFCs), perfluorocarbons (PFCs) and sulphur hexafluoride (SF6).

F-gas emissions have risen by 60% since 1990 in contrast to all other F-gases which have been reduced. Fluorinated gases (F-gas) account for a small percentage of the overall greenhouse gas emissions; however, these gases remain for a long time in the atmosphere and equipment containing these gases have a long life span.

The European Union passed a legislation in 2006 to control the use and servicing of F-gases as part of its policy to combat climate change. The European Commission proposed to further strengthen the regulation in order to cut F-gas emissions by two-thirds by 2030 from 2012 levels.

The revision of the F-gas Regulation aims at discouraging the use of F-gases with a high impact on the climate in favor of energy-efficient and safe alternatives, and further improving the containment and end-of-life treatment of products and equipment that contain F-gases.

At its first phase the proposal bans the sale of domestic refrigerators and freezers containing HFCs with a global warming potential (GWP) of 150 or more as of January 1, 2015; and refrigerators and freezers for commercial use containing HFCs with a GWP of 2500 or more, as of January 1, 2020.

The proposed legislation is designed to enhance economic growth, stimulate innovation and develop environment friendly technologies by improving market opportunities for alternative technologies and gases with a low impact on the climate.

Editor's Note: CR4 would like to thank Bassam Elassaad, GEA Consulting Associate, for contributing this blog entry, which originally appeared here.

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