HCB 3- Chap 17B: HX Equipment1 Chapter 17B: HEAT EXCHANGER EQUIPMENT Agami Reddy (July 2016) 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

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 Equipment2

3 geration-and-Air-Conditioning/28 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 Equipment4 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:- Bare tube- Plate surfaces- 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 Equipment5 Direct expansion (DX ) Air cooled-forced convection Expansion valves have intricate distribution circuits Typical forced convection fan coil unit in with a DX evaporator supplying cool air to a space

HCB 3- Chap 17B: HX Equipment6 Typical serpentine plate evaporator used in freezer section of a household refrigerator Direct expansion (DX) Air cooled-natural convection

HCB 3- Chap 17B: HX Equipment7 The local inside heat exchange coefficient in a DX coil varies greatly R-22

HCB 3- Chap 17B: HX Equipment8 Shell and tube water cooled evapora tors For medium sized chillers- Refrigerant boils inside tubes

HCB 3- Chap 17B: HX Equipment9 Flooded type evaporators used for large chillers- Refrigerant boils on shell side

HCB 3- Chap 17B: HX Equipment10 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: 5/8 th to 2 inches (common) - number of tubes: up to 1000 or more -Evaporative cooling towers (direct and indirect)

HCB 3- Chap 17B: HX Equipment11 Typically three distinct regions in condenser

HCB 3- Chap 17B: HX Equipment12 The 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: ft 3 /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 Equipment13 Dry air condensers and indirect evaporative cooling towers on top of a high-rise apartment building

HCB 3- Chap 17B: HX Equipment14 Direct (open): Cooled fluid comes into direct contact with air, Water cooled chillers Indirect (closed): Cooled fluid isolated from air (similar to evaporative condenser), Process coolers, water loop heat pump systems Evaporative Condensers

HCB 3- Chap 17B: HX Equipment15 Indirect Evap Condensers - 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 Equipment16 Recall the concept of effectiveness of a heat exchanger. For, say the condenser, we have C cond,water = condenser water flow rate x specific heat T cond,water,out and T cond,water,in = condenser water outlet and inlet temperatures T cond,ref = refrigerant temperature in condenser

HCB 3- Chap 17B: HX Equipment17 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 Equipment18 Finned Tubing for Heating and Cooling Coils A large variety of designs and configurations: -Circular plate fins - Bar fins Cross-flow arrangements Examples of Finned tubing

Cooling and Heating Coil Design HCB 3- Chap 17B: HX Equipment19 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: 10 o – 20 o F (5 o – 10 o C) -Heating coil water temp: 120 o - 250°F (50°-120°C) -Heating coil steam temp: 2 – 10 psig (14 – 70 kPa) -Cooling coil water temp: 40 o - 60° F (4.5 – 15.5°C)

HCB 3- Chap 17B: HX Equipment20

HCB 3- Chap 17B: HX Equipment21

HCB 3- Chap 17B: HX Equipment22 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 Used in secondary systems: DX and air-side - More complicated since dehumidifcation is also involved

HCB 3- Chap 17B: HX Equipment23 Effectiveness Concept of HX can be used

HCB 3- Chap 17B: HX Equipment24 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 Equipment25

HCB 3- Chap 17B: HX Equipment26 Performance Data from Manufacturers

HCB 3- Chap 17B: HX Equipment27 1)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 Equipment28 From Mitchell and Braun Coil capacity as a function of air velocity

HCB 3- Chap 17B: HX Equipment29 -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 Conventional Air Cooled Condenser

HCB 3- Chap 17B: HX Equipment30 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 Equipment31 Cooling Towers Objective is to calculate: -TLW: leaving tower water temperature -Electrical consumption of tower fans

HCB 3- Chap 17B: HX Equipment32 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 Equipment33 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 Equipment34 Fill

HCB 3- Chap 17B: HX Equipment35 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 Equipment36 Range and Approach Temperatures Range: usually 10 – 20 o F –Difference in water temperature between inlet and outlet (T A – T B ) –Function of heat load Approach: can be as low a 7-9 o F –Difference between leaving water temperature and entering air wet- bulb (T B – T C ) –Function of tower characteristics

HCB 3- Chap 17B: HX Equipment37 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 Equipment38 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 Equipment39 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 Equipment40 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 Equipment41 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 Equipment42 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 Equipment43 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.