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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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Water Side Pressure Drop in Shell and Tube Heat Exchangers

Posted August 26, 2014 1:00 AM by larhere

In previous posts I have presented the methods and equations used to design flooded evaporator and water cooled condenser tube bundles for specific heat transfer conditions. These water chiller heat exchangers operate with water on the tube side and refrigerant on the shell side. They are designed for water velocities in the range of 3 to 10 FPS. The water flow rate is usually one of the design inputs.

Both heat transfer performance and water pressure drop increase with increased water velocity. It is important to design for the highest water velocity that will have an acceptable pressure drop in the application. The method for calculation of pressure drop is shown below.

For the tube bundle, determine the water flow area.

Af=Nt*(π*Di2/4)/Np

Where

Nt=number of tubes

Di=inside tube diameter, ft

L=length of tube, ft

Np=number of passes

For internally enhanced tubes, a good approximation of L is the length of the enhanced surface, as that will be the majority of the friction.

Water side pressure drop (ref ASHRAE Handbook-HVAC Systems & Equipment, Chapter 35):

∆P=Np*(KH+f*L/Di)*ρ*V2/2*g

And

V=GPM/(448.8*Af)

Where

∆P=pressure drop, psi

Np=number of tube passes

KH=entrance & exit flow resistance & flow reversal coefficient, number of velocity heads (V2/2*g)

f =friction factor

ρ =fluid density, lb/ft3

V=fluid velocity, fps

g=gravitational constant=32.17 lbm*ft/(lbf*s2)

For smooth tubes the friction factor, f, can be obtained from the Moody Diagram.

For internally enhanced tube and Reynolds Numbers greater than 20,000 the friction factor is usually in the form

f=C* ReD

where

Re= V* Di* ρ/µ

and

µ=viscosity, lbf*sec/ft2

The coefficients C and D can be provided by the enhanced tube manufacturer.

Flooded evaporators and water cooled condensers are major components of water chillers. As such they represent a large part of the chiller cost. The tube bundle design determines the length of the chiller and is a major factor in its width.

Using the heat transfer and water side pressure drop calculation methods, designers can perform iterative analyses to vary the tube bundle lengths, number of tubes and number of passes. This will result in the heat exchangers with the desired performance, lowest cost and chiller dimensions that best meet the requirements of the market.

Want to learn more about HVAC Technologies?

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

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U.S. EPA Moves to Phase-out Familiar HFC'S

Posted August 20, 2014 8:00 AM by larhere

The U.S. Environmental Protection Agency has now formally proposed to limit certain potent greenhouse gases from use in air conditioners, refrigerators, aerosols and foams in favor of safer, more climate-friendly alternatives. The movement from high to low GWP refrigerants we outlined in a recent post (Refrigerants Update: Clarity or Confusion?) took a giant step forward with the new proposal which introduces limits on refrigerants and blends containing R-134a, 143a, 125, HCFCs and others. Among the proposed changes:

  • Bans the use of R-134a in motor vehicle air-conditioning systems starting with model year 2021. R-134a had been listed as approved on the SNAP list of acceptable alternatives. The recent approval of HFO-1234yf for mvac use provided a more environmentally acceptable alternative.
  • For new, stand-alone retail food refrigeration and new vending machines, as of January1, 2016 the use of HFC-134a and certain other HFC refrigerant blends is banned.
  • HFC blends R-507A and R-404A banned for new and retrofit retail food refrigeration (including stand-alone equipment, condensing units, direct supermarket systems, and indirect supermarket systems) and for new and retrofit vending machines, as of January 1, 2016.

This proposed rule is part of the Significant New Alternatives Policy (SNAP) program, under which the EPA continuously reviews alternatives to ODS to find substitutes that pose less overall risk to human health and the environment. As safer alternatives become available for particular applications, the list of acceptable substitutes is evaluated and revised.

The question being asked now is just how low a GWP does a refrigerant need to be?

Download this fact sheet for specific refrigerants, applications and dates.

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

36 comments; last comment on 08/25/2014
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Refrigerants Update: Clarity or Confusion?

Posted August 06, 2014 12:40 PM by larhere

The "Refrigerant Issue" has been a key factor for the HVAC/R industry since the 1987 Montreal Protocol entered into force. Environmental awareness increased almost over night. One would think the issue would have been resolved in the 27 years of intense R & D and many millions of dollars of investments in new plants and processes to produce more environmentally friendly refrigerants.

Not the case. The clarity we hoped for can be illustrated by the following slide from a recent presentation by Emerson. Click on the slide to see it full size.

Familiar refrigerant types like R-22, 404A, 410A and 407C have a dizzying array of proposed alternatives with strange names like DR7, L40, ARM70, N40 and XP10 among others.

One can see the current trend from HCFCs and high GWP HFCs to lower GWP refrigerants forcing the industry to develop a new classification of "Mildly Flammable" refrigerants that are more environmentally friendly. Use of these A2L refrigerants is still under review by industry and government organizations.

How low of a GWP value is required to be adequately safe for the environment?

The EU has proposed banning the sale of domestic refrigerators and freezers containing HFCs with a global warming potential (GWP) of 150 or more as of January 1, 2015. European countries such as Denmark, Austria, and Switzerland have already banned most uses of HFC's. A US/Mexico/Canada proposal to phase down HFC use by 70% by the year 2029 is being evaluated. Proponents of natural refrigerants will tell you the near zero GWP of ammonia and propane are the way to go.

The above current "picture" of the refrigerant issue does not display the most important issue of energy efficiency which makes up 95 to 98% of the global warming gas emissions for the majority of the stationary air conditioning products. Small improvements in energy efficiency have a far greater impact than moving to a lower GWP alternative in such "low leak" installations. Hopefully, the overemphasis on refrigerant GWP will be brought back into balance with the more important Energy Efficiency criteria.

Additional Reading

Europe Moves Forward on F-Gas Controls

Recent Developments In Refrigerants For Air Refrigerants For Air-Conditioning Conditioning And Refrigeration Systems

Refrigerants for residential and commercial air conditioning applications

AHRI Low Global Warming Potential Alternative Refrigerants Evaluation Program
(Low-GWP AREP) - Summary of Phase I Testing Results PDF

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

10 comments; last comment on 08/14/2014
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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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