Heat Exchanger အေၾကာင္း ၃

APPLICATION
primary energy source တစ္ခုတည္းမွ မတူညီေသာ အပူခ်ိန္မ်ား။ မတူညီေသာအသံုးၿပဳမွဳမ်ားနွင့္ မတူညီေသာ စြမ္းအင္ပမာဏမ်ား အတြက္ လုိအပ္သည့္ေနရာတြင္ လုိအပ္သေလာက္ရွိရန္ Heat exchanger မ်ား ကိုအသံုးၿပဳၾကသည္။

ASHRAE Handbook တြင္ ေဖာ္ၿပထားေသာ Heat exchangers အသံုးၿပဳပံုမ်ားမွာ

က) boiler မွ ထြက္လာသည့္ steam ကို central water systems အတြက္ hot water အၿဖစ္ condense လုပ္ရန္
ခ) ဟုိတယ္ႏွင့္ေဆးရုံမ်ားတြင္ လုိအပ္ေသာ Hot water system မ်ားတြင္အသံုးၿပဳရန္

ဂ) special temperature requirements လိုအပ္သည့္အခါ ႏွင့္ Hot fluid ႏွင့္ Cold Fluid တို့ သီးၿခား မေရာေႏွာ လုိအသည့္အခါ (isolation)
ဃ) energy-saving applications မ်ား အတြက္
င) refrigeration applications မ်ား အတြက္ (evaporators, condensers, and liquid coolers)

SELECTION CRITERIA
A heat exchanger ကို ကြန္ၿပဴတာ ပရုိဂရမ္မ်ား အသံုးၿပဳ၍ ေရြးခ်ယ္ၾကသည္။ ထုပ္လုပ္သူမ်ားႏွင့္ အၾကံေပးတုိင္ပင္ကာေရႊးခ်ယ္ေလ့ရွိသည္။

Thermal/Mechanical Design
Shell-and-tube heat exchangers မ်ားကို ဖိအားလိုအပ္ခ်က္ အရ ေရြးခ်ယ္ၾကရသည္။ ဒုတိယေနၿဖင့္ အပူ စီးကူးရန္ ေရြးခ်ယ္ၾကသည္။

Plate heat exchangers မ်ားကို ဖိအားလိုအပ္ခ်က္ မရွိသည့္ အခါမ်ိဳးတြင္ အေကာင္းဆံုးေသာ အပူ စီးကူးရန္ ၿဖစ္ရန္အတြက္ ေရြးခ်ယ္ၾကသည္။

Thermal Performance.
Heat exchanger ၏ thermal performance သည္ အရြယ္အစား(size) ႏွင့္ geometry of the heat transfer surface area တုိ့အေပၚတြင္မူတည္သည္။

Heat transfer surface ၏ materials အမ်ိဳးအစားသည္လည္း performance ကို ေၿပာင္းလဲေစသည္။ ေၾကးနီသည္ stainless steel ထက္ အပူစီးကူးမွဳပိုေကာင္းသည္။(higher coefficient of heat transfer)

Fluid စီးႏွဳန္း Flow rates (velocity), ေစးၿပစ္မွဳ(viscosity), ႏွင့္ အပူေလွ်ာက္ကူးမွဳ (thermal conductivity) တုိ့သည္  overall heat transfer coefficient U ၏ တန္ဘိုးကို ဆံုးၿဖစ္ေပးသည္။

shell-and-tube အမ်ိဳးအစား heat exchangers cold fluid သည္ tube side အတြင္းတြင္ရွိရသည္။ အကယ္၍ cold fluid ကို shell side ဘက္တြင္ စီးဆင္းပါက  overall U ၏ တန္ဘုိးမွာက်ဆင္းသြားသည္။

Thermal Stress.

အပူခ်ိန္ကြာဟခ်က္ (large temperature differences) မ်ားသည့္ Heat exchanger မ်ားသည္ ၾကီးမားေသာ thermal
stresses ကို ခံရသည္။

fixed tubesheets မ်ားၿဖင့္ၿပဳလုပ္ထားသည့္ Heat exchanger မ်ားသည္ အပူခ်ိန္ကြာဟခ်က္ (large temperature differences) မ်ားသည့္ ၾကီးမားေသာ thermal
stresses ကို ခံနုိင္ရည္မရွိေပ။ ထုိအတူပင္ Gasketed plate units မ်ားသည္ ၾကီးမားေသာ thermal
stresses ကို ခံနုိင္ရည္မရွိေပ။

Pressure Drop. Fluid velocity and normal limitations on tube
length tend to result in relatively low pressure drops in shell-andtube
heat exchangers. Plate units tend to have larger pressure drops
unless the velocity is limited. Often a pressure drop limitation rather
than a thermal performance requirement determines the surface area
in a plate unit.

Fouling. Often, excess surface area is specified to allow for scale
accumulation on heat transfer surfaces without a significant reduction
of performance. This fouling factor or allowance is applied
when sizing the unit. Fouling allowance is better specified as a percentage
of excess area rather than as a resistance to heat transfer.
Shell-and-tube exchangers with properly sized tubes can handle
suspended solids better than plate units with narrow flow channels.
The high fluid velocity and turbulence in plate exchangers make
them less susceptible to fouling.
The addition of surface area (tube length) to a shell-and-tube
exchanger does not affect fluid velocity, and, therefore, has little
effect on thermal performance. This characteristic makes a fouling
allowance practical. This is not the case in plate units, for which the
number of parallel flow channels determines velocity. This means
that as plate pairs are added to meet a load (heat transfer surface
area) requirement, the number of channels increases and results in
decreased fluid velocity. This lower velocity reduces performance
and requires additional plate pairs, which further reduces performance.

Cost
On applications with temperature crosses and close approaches,
plate heat exchangers usually have the lowest initial cost. Wide
temperature approaches often favor shell-and-tube units. If the
application requires stainless steel, the plate unit may be more
economical.           

Serviceability
Shell-and-tube heat exchangers have different degrees of serviceability.
The type of header used facilitates access to the inside of
the tubes. The heads illustrated in Figures 3, 6, and 7 can be easily
removed without special pipe arrangements. The tube bundles in all
of the shell-and-tube units illustrated, except the fixed-tubesheet
unit (Figure 6), can be replaced after the head is removed if they are
piped with proper clearance.
The diameter and configuration of the tubes are significant in
determining whether the inside of tubes of straight-tube units can be
mechanically cleaned. Figure 7 shows a type of head that allows
cleaning or inspection inside tubes after the channel cover is
removed.
Plate heat exchangers can be serviced by sliding the movable
pressure plate back along the carrying bars. Individual plates can be
removed for cleaning, regasketing, or replacement. Plate pairs can
be added for additional capacity. Complete replacement plate packs
can be installed.

Space Requirements
Cost-effective and efficient shell-and-tube heat exchangers
have small-diameter, long tubes. This configuration often challenges
the designer when allocating space required for service and
maintenance. For this reason, many shell-and-tube selections have
large diameters and short lengths. Although this selection performs
well, it often costs more than a smaller-diameter unit with
equal surface area. Be careful to provide adequate maintenance
clearance around heat exchangers. For shell-and-tube units, space
should be left clear so the tube bundle can be removed.
Plate heat exchangers tend to provide the most compact design in
terms of surface area for a given space.

Steam
Most HVAC applications using steam are designed with shell-and-
tube units. Plate heat exchangers are used in specialized industrial
and food processes with steam.

INSTALLATION
Control. Heat exchangers are usually controlled by a valve with
a temperature sensor. The sensor is placed in the flow stream of the
fluid to be heated or cooled. The valve regulates flow on the other
side of the heat exchanger to achieve the sensor set-point temperature.
Chapter 46 discusses control valves.
Piping. Heat exchangers should be piped such that air is easily
vented. Pipes must be able to be drained and accessible for service.
Pressure Relief. Safety pressure relief valves should be installed
on both sides between the heat exchanger and shutoff valves to
guard against damage from thermal expansion when the unit is not
in service, as well as to protect against overpressurization.
Flow Path. The intended flow path of each fluid on both sides of
a heat exchanger design should be followed. Failure to connect to
the correct inlet and outlet connections may reduce performance.
Condensate Removal. Heat exchangers that condense steam
require special installation. Proper removal of condensate is particularly
important. Inadequate drainage of condensate can result in
significant loss of capacity and even in mechanical failure.
Installing a vacuum breaker aids in draining condensate, particularly
when modulating steam control valves are used. Properly
sized and installed steam traps are critical. Chapter 10 discusses
steam traps and condensate removal.
Insulation. Heat exchangers are often insulated. Chapter 23 of
the 2005 ASHRAE Handbook—Fundamentals has further information
on insulation.

If you are intend to purchase a shell and tube heat exchangers, you must consider 7 critical factors before deciding to purchase one. Consider the following:
1) Heat exchanger tube diameter
The diameter of the tube can be manipulated by the provider. A key point to consider is the nature of the particular liquids used in the pipes. Smaller pipes warrant will clean faster, yet more pipes may be less effective and less compact with respect to space.
2) Thickness of the tube
The thickness of the pipe refers to several factors. Corrosion, flow resistance, axial force, pressure, and the availability of spare parts in connection with a heat exchanger tube thickness.
3) Heat exchanger shell diameter and tube length
A heat exchanger costs is directly influenced by the shell diameter and tube length. Customers who are concerned about the cost of heat exchangers questions which the longest length of pipe to provide without compromising its effectiveness. The possibility of long tubes may be limited because of the limited space, specific job specifications, capabilities and replacement.
4) Tube corrugation
The corrugation of tubes influences the performance of a shell and tube heat exchanger. Corrugated cardboard, the tube increased turbulence of fluids in turn deliver better results.
5) Tube Layout
“Tube layout ‘refers to how a heat exchanger tube is placed in the skull. To date, four major layouts to consider: triangular, twisted triangular, square and rotated square. Triangular tube facilitates a better heat transfer, while the square tubing provides a longer period of purity.
6) Tube pitch
“Tube pitch” refers to the distance between the centers of separate but interconnected tubes. A general rule determines the pitch of a pipe shall not be less than 1.25 times the outside diameter tubes.
7) Heat exchanger baffles
“Baffles” are used in shell and tube heat exchangers for liquid flow in the direct beam. Baffles prevent tubes from sagging, and can also prevent them from vibrating. Baffle spacing is important in relation to pressure drop and heat transfer. Baffles closely shared a greater pressure drop causes, but still too far apart may cause cooler spots between them.