1. Chapter 17B: HEAT EXCHANGER EQUIPMENT
Agami Reddy (rev- May 2017)
Classification: indirect and direct types of HX
Evaporator designs used in refrigeration
- Direct expansion
- Flooded type
Condenser designs: open and closed
Finned tubing (used for conditioning air streams)
Heating and cooling coils
Cooling towers
HCB 3- Chap 17B: HX Equipment

2. Manner of Classification
indirect where the two fluid streams are physically separated from each other by a solid surface. Examples:
-heating or cooling coils inside air ducts,
-baseboard heaters in rooms,
-evaporators and condensers in refrig. systems,
- HX in boilers, furnaces and hot water heaters.
direct heat exchangers where the two streams come in contact with each other. Examples:
- evaporative coolers
-cooling towers to cool condenser water
HCB 3- Chap 17B: HX Equipment

3. HCB 3- Chap 17B: HX Equipment
Condensers and Evaporators
Both are essentially HX: fluid flow and
heat exchange processes have some common features
- In condenser, refrigerant cools and condenses (may be sub-cooled)
- In evaporator, refrigerant boils to saturated vapor (may be superheated)
HX designs greatly differ depending on
Whether refrigerant is inside tubes or outside on shell side
- Whether fluid used in condenser and evaporator is air or water
Most common HX configurations:
Shell and tube
Finned coil
HCB 3- Chap 17B: HX Equipment

4. HCB 3- Chap 17B: HX Equipment
Evaporators
An evaporator is any HX in which a volatile liquid is vaporized
to remove heat from a coolant or from a space
They are manufactured in a wide variety of types, shapes, sizes and designs: (i) Bare tube, (ii) Plate surfaces, (iii) Finned
DX or direct expansion:
- Refrigerant boils in the tubes and cools the fluid that passes over
the outside of the tubes – need an expansion valve to regulate flow
Shell-and-tube:
Medium sized systems
Refrigerant flows inside tubes
Flooded:
- Adopted in most larger systems (centrifugal),
- Refrigerant is on shell side
HCB 3- Chap 17B: HX Equipment

5. HCB 3- Chap 17B: HX Equipment
Direct expansion (DX)
Air cooled-forced convection
Fig Photo of a typical forced convection fan coil unit with a DX evaporator supplying cool air to a space
3-row cooling coil with expansion valve showing
the intricate distribution circuits
HCB 3- Chap 17B: HX Equipment

6. HCB 3- Chap 17B: HX Equipment
Direct expansion (DX)
Air cooled-natural convection
Fig Typical serpentine plate evaporator used in freezer section of a household refrigerator
HCB 3- Chap 17B: HX Equipment

7. HCB 3- Chap 17B: HX Equipment
The local inside heat transfer coefficient in a DX evaporator
coil varies greatly as the refrigerant boils
R-22
HCB 3- Chap 17B: HX Equipment

8. HCB 3- Chap 17B: HX Equipment
Shell and tube water cooled evaporators
For medium sized chillers-Refrigerant boils inside tubes
HCB 3- Chap 17B: HX Equipment

9. HCB 3- Chap 17B: HX Equipment
Flooded type evaporators
used for large chillers-
Refrigerant boils on shell side
Fig Flooded evap
HCB 3- Chap 17B: HX Equipment

10. Fins used to enhance heat transfer in air-cooled evaporators
A large variety of designs and configurations
Fig
Two types of fins on tubes to enhance heat transfer: (a) circular plate fins and (b) bar fins.
HCB 3- Chap 17B: HX Equipment

11. HCB 3- Chap 17B: HX Equipment
Condensers
Dry air-cooled
- less than 200 Tons (or 700 kW)
- cross-flow HX with fins
- always forced air flow
Water-cooled
shell and coil- refrigerant inside tubes
shell and tube (from s of Tons capacity)
- shell diameter: 4 – 60 inches
- tube lengths: 3 – 20 ft
- tube diameters: 5/8th to 2 inches (common)
- number of tubes: up to 1000 or more
Evaporative cooling towers (direct and indirect)
HCB 3- Chap 17B: HX Equipment

12. HCB 3- Chap 17B: HX Equipment
Typically three distinct regions in condenser
Fig
Refrigerant and air temperature profiles within an air-cooled condenser.
HCB 3- Chap 17B: HX Equipment

13. HCB 3- Chap 17B: HX Equipment
Design of air-cooled condensers requires consideration of:
Heat rejection rate - determined at peak heat discharge rate.
Airflow rate - a balance between excessive pressure drop for high flow rates and high initial cost for large heat transfer surface.
- Usual value: ft3/min/ton ton (80 to 160 L/s/ kW) Fan power: typically hp/ton (20 to 40 W/kW).
Temperature difference (refrigerant to entering air)
Typical values: ° F (8 - 22° C).
Noise : large air-cooled condensers can be noisy.
- Airflow around unit : Avoid obstructions and short-circuiting
HCB 3- Chap 17B: HX Equipment

14. HCB 3- Chap 17B: HX Equipment
Fig (b)
Dry air condensers and indirect evaporative cooling towers
on top of a high-rise apartment building in a major city
HCB 3- Chap 17B: HX Equipment

15. HCB 3- Chap 17B: HX Equipment
Evaporative Condensers
(cool down hot water)
Direct:
Cooled fluid comes into direct contact with air,
water cooled chillers
Fig Indirect-contact:
Cooled fluid isolated from air (similar to evaporative condenser),
Process coolers, water loop heat pump systems
HCB 3- Chap 17B: HX Equipment

16. HCB 3- Chap 17B: HX Equipment
Indirect Evap Condensers
(condense refrigerant)
- Hybrid between dry-air condensers
and cooling towers.
- The air is evaporative cooled by
spray water which is then used to
cool the refrigerant inside the tubes.
- Can operate at lower condensing
temperatures than air cooled ones
- Can reduce water pumping and
chemical treatment
- Needs less coil surface than air cooled
- Compact in design
- Used for medium sized capacities
HCB 3- Chap 17B: HX Equipment

17. HCB 3- Chap 17B: HX Equipment
Recall the concept of effectiveness of a heat exchanger.
For, say the condenser, we have
Ccond,water = condenser water flow rate x specific heat
Tcond,water,out and Tcond,water,in = condenser water outlet and inlet temperatures
Tcond,ref = refrigerant temperature in condenser
HCB 3- Chap 17B: HX Equipment

18. HCB 3- Chap 17B: HX Equipment
Note: This equation is independent of HX configuration
applies to all types of HX where one of the fluids is at a cte temp.
HCB 3- Chap 17B: HX Equipment

19. Cooling and Heating Coil Design
Typical operating ranges:
- Air-side velocity: 3 to 25 ft/s (1 to 8 m/s)
Water velocities: 0.5 to 8 ft/s (0.2 to 2.5 m/s)
Water side temperature drops: 10o – 20o F (5o – 10oC)
Heating coil water temp: 120o - 250°F (50°-120°C)
Heating coil steam temp: 2 – 10 psig (14 – 70 kPa)
Cooling coil water temp: 40o -60° F (4.5 – 15.5°C)
HCB 3- Chap 17B: HX Equipment

20. Finned Tubing for Heating and Cooling Air Coils
Cross-flow arrangements
HCB 3- Chap 17B: HX Equipment

21. HCB 3- Chap 17B: HX Equipment
Heating Coil
HCB 3- Chap 17B: HX Equipment

22. HCB 3- Chap 17B: HX Equipment
Solution
HCB 3- Chap 17B: HX Equipment

23. HCB 3- Chap 17B: HX Equipment
- More complicated since dehumidifcation is also involved
Cooling Coils
Purpose:
Reduce temperature
and humidity of air
stream
Consist of series of
tubes with fins attached
to the outside of
the tubes to increase
area of heat transfer
Fig
Cross-flow air heating coil showing 10 tube passes per row. Heat exchangers with at least this number of tube passes can be well modeled as counterflow HXs.
Used in secondary systems: DX and air-side
HCB 3- Chap 17B: HX Equipment

24. HCB 3- Chap 17B: HX Equipment
Effectiveness
Concept of
HX can be
used
Fig
Diagram showing driving potentials for heat and mass transfer in a wet coil (Tw denotes water temperature).
HCB 3- Chap 17B: HX Equipment

25. HCB 3- Chap 17B: HX Equipment
Example – Calculate effectiveness of coil under design and compare it with the coil after it has been in operation for some time
HCB 3- Chap 17B: HX Equipment

26. HCB 3- Chap 17B: HX Equipment

27. HCB 3- Chap 17B: HX Equipment

28. Performance Data from Manufacturers
HCB 3- Chap 17B: HX Equipment

29. HCB 3- Chap 17B: HX Equipment
Each successive row removes less heat than previous one
2) Lower refrigerant temperature results in greater latent to sensible ratio
3) Increasing face velocity increases capacity but reduces DBT and WBT
differences
HCB 3- Chap 17B: HX Equipment

30. HCB 3- Chap 17B: HX Equipment
Coil capacity as a
function of air velocity
From
Mitchell and Braun
HCB 3- Chap 17B: HX Equipment

31. HCB 3- Chap 17B: HX Equipment
Conventional Air Cooled Condenser
Refrigerant heat is rejected directly to air
Air side heat transfer coefficients ate usually small compared to
refrigerant side heat transfer coefficients
Lowest temperature which refrigerant can reach is the air DBT
Condenser air flow is sensitive to changes in pressure across the condenser
HCB 3- Chap 17B: HX Equipment

32. HCB 3- Chap 17B: HX Equipment
Cooling
Towers-
Heat
exchanger
for evaporative
cooling of
water
Changes to system
Water to refrigerant HX (shell-in-tube)
Pump to circulate water
Cooling tower
Better heat transfer with water compared to air
Lowest temperature reached is WBT
Reduced power needs of compressor
HCB 3- Chap 17B: HX Equipment

33. HCB 3- Chap 17B: HX Equipment
Cooling Towers
Objective is to calculate:
TLW: leaving tower water temperature
Electrical consumption of tower fans
HCB 3- Chap 17B: HX Equipment

34. HCB 3- Chap 17B: HX Equipment
Typical Performance Conditions
HCB 3- Chap 17B: HX Equipment

35. Cooling Tower Features
Air/water flow arrangement
Counterflow
Crossflow
Fan type
Axial (propeller)
Centrifugal
Fan Control
Single or multiple speeds
Variable speed
Air flow
Induced draft
Forced draft
Natural draft
HCB 3- Chap 17B: HX Equipment

36. HCB 3- Chap 17B: HX Equipment
Fill (or packing)
Internal latticework in the cooling tower used to break the fall of the water.
Increases water-air contact area
Increases heat and mass transfer coefficients
Splash fill
Plastic or wood bars
Break up and distribute droplets of water
Film fill
Plastic sheet
More efficient than splash fill, but more prone to fouling
HCB 3- Chap 17B: HX Equipment

37. HCB 3- Chap 17B: HX Equipment
Fill
HCB 3- Chap 17B: HX Equipment

38. HCB 3- Chap 17B: HX Equipment
Water Use
- Drift: amount of water carried out with exhaust air
- Eliminators: used to prevent drift
- Evaporation: amount of water evaporated into
the air as it travels through the tower
(typically 1 – 3% depending on outdoor conditions)
HCB 3- Chap 17B: HX Equipment

39. Range and Approach Temperatures
Range: usually 10 – 20o F
Difference in water temperature between inlet and outlet (TA – TB)
Function of heat load
Approach: can be as low a 7-9o F
Difference between leaving water temperature and entering air wet-bulb (TB – TC)
Function of tower characteristics
HCB 3- Chap 17B: HX Equipment

40. Psychrometrics and Heat Transfer
Generally, air gains sensible and latent heat,
exit state  saturation
Water temperature approaches inlet air
wet-bulb
Enthalpy potential concept
allows accurate analysis-
Ttwo different conditions
(A and D of entering air
and B is leaving air)
HCB 3- Chap 17B: HX Equipment

41. HCB 3- Chap 17B: HX Equipment
Capacity Control
Why is control needed?
Prevent freezing under low load conditions with low ambient conditions
Chiller low limit on condenser water temp
Water-side free-cooling
Controlled variables:
leaving water temperature
condenser pressure
System power (optimal control)
HCB 3- Chap 17B: HX Equipment

42. HCB 3- Chap 17B: HX Equipment
Water-Side Control
Variable condenser water flow rate
Flow rate range bounded by tube velocity criteria
Rate of change of flow limited by chiller controls
Tower bypass
Send a fraction of water to sump rather than over fill
Does not save energy—use as a safety
HCB 3- Chap 17B: HX Equipment

43. HCB 3- Chap 17B: HX Equipment
Air-Side Control
Cycle fans or vary fan speed
Variable speed drive gives lowest energy use, higher first cost
Fan speed options:
On-Off
Full-1/2-Off
Full-2/3-Off
Two-speed fan/multiple cell towers have many steps of capacity
HCB 3- Chap 17B: HX Equipment

44. HCB 3- Chap 17B: HX Equipment
Modeling Approaches
Detailed physical models: finite differences
Lumped grey-box models: effectiveness- NTU model
Black-box models-
Characteristic curves: common way for manufacturers to represent operating data
Leaving water temperature vs. air wet-bulb
Fixed range (heat load), air flow, water flow
Several manufacturers have selection software
HCB 3- Chap 17B: HX Equipment

45. HCB 3- Chap 17B: HX Equipment
Black-Box Models
Based on standard characteristic curves
Polynomial regression model
Model coefficients as functions of range (R) and entering air wet-bulb (EWB)
Gives model of temp. of leaving water (TLW) for fixed fan speed and water flow, with variables R and EWB
From Stoecker (1989)
TLW
EWB
HCB 3- Chap 17B: HX Equipment

46. HCB 3- Chap 17B: HX Equipment
Outcomes
Understanding the differences in HX classification and designs variations
Knowledge of the different types of evaporators and their operation
Knowledge of different types of condensers and their operation
Be able to solve simple problems involving evaporators and condensers
Knowledge of the different types of cooling and heating coils and their thermal performance
Be able to solve simple problems involving coil design and performance
Knowledge of the operation of cooling towers and related components and terminology
Knowledge of ways to control the water side and the air-side of cooling towers
Knowledge of different ways to model cooling tower thermal performance.
HCB 3- Chap 17B: HX Equipment