**Thermal Design of Cooling and Dehumidifying Coils**

M. Khamis Mansour and M. Hassab

*Mechanical Engineering Department, Faculty of Engineering, Beirut Arab University, Lebanon* 

#### **1. Introduction**

366 Heat Exchangers – Basics Design Applications

*T correl* \_ –difference in air temperature between inlet and outlet section calculated from

*T* \_ model –difference in air temperature between inlet and outlet section received from

– temperature difference between a point on a fin surface and the surroundings

– ratio between the heat removed from the tube/fin component to the tube/fin

Ansys 12 Product Documentation, Available from ANSYS Customer Portal,

Hewitt G. H., Shires G. L., Bott T. R. (1994). *Process Heat Transfer*, CRC Press Inc., ISBN 0–

Incropera F. P., Dewitt D. P., Bergman T. L., Lavine A. S. (2006), *Fundamentals of Heat and* 

Kraus A., Aziz A., Welty J. (2001) *Extended surface heat transfer*, A Willey-Interscience

Wais P. (2010), *Fluid flow consideration in fin-tube heat exchanger optimization*, Archives of Thermodynamics, Vol. 31, No. 3, (September 2010), pp. 87-104, ISSN 1231–0956

Mills A. F. (1995), *Heat and Mass Transfer*, Richard D. Irwin Inc., ISBN 0–256–11443–9, USA McQuiston F. C., Tree D. R. (1972), *Optimum space envelopes of the finned tube heat transfer surface*, ASHRAE Transactions, Vol. 78, Part 2, pp. 144-152, ISSN: 0001–2505 Shah R. K., Sekulic D. P. (2003), *Fundamentals of Heat Exchanger Design*, John Wiley & Sons,

*Mass Transfer*, John Wiley & Sons, ISBN 978–0–471–45728–2, USA

*T TT Fluid IN OUT* - difference in fluid temperature between outlet and inlet section

*X <sup>t</sup>* – transverse tube pitch (perpendicular to the flow) tube pitch

*T* – effective mean temperature difference

*<sup>s</sup>* – material density of solid (tube and fin)

https://www1.ansys.com/customer/default.asp

Publication, ISBN 0–471–39550–1, USA

ISBN 0–471–32171–0, USA

Greek symbols

correlation

weight 

*t* 

numerical computation

– fin thickness

*<sup>f</sup>* – fin efficiency

– fluid density

**7. References** 

– tube thickness

– optimization function

– fluid dynamic viscosity,

8493–9918–1, USA

The cooling and dehumidifying coil is a critical component of air conditioning. Its performance has a strong bearing on the ultimate indoor environmental conditions, which in turn, has a significant impact on the indoor air quality. Decisions made to select a cooling coil influence the initial investment as well as the costs of installing, providing, and maintaining thermal comfort. The efficient thermal design of the cooling coil leads to a crucial reduction in the coil surface heat transfer area and of course, its capital cost and its weight. On the other hand, the enhancement in the coil thermal performance will usually be established at expense of the hydraulic performance of the cooling coil and in turn, its running cost. Because the cooling coil is an integral part of the air distribution system, its geometry — size, number of rows, fin spacing, and fin profile — contributes to the airside pressure drop and affects the sound power level of the fans. (Fan power needed to circulate air through the duct system may warrant extra sound attenuation at the air handler.) Cooling coils are an integral part of the chilled water system or the refrigeration unit, too. The extent to which coils raise the chilled water temperature or the evaporation temperature dramatically affects both capital investment in the cooling coil or the pumping power. Coil performance can even influence the efficiency of the chiller or Dx-unit. The focus of this chapter is on the description of the methodology should be used in thermal design of the cooling coil either chilled water coil or Dx-coil.

Methods to design the cooling and dehumidifying coil either chilled water coil or Dx evaporator coil are usually based on log mean enthalpy or log equivalent dry-bulb temperature difference [1]. In both methods, the cooling coil is treated as a single zone/region and hence the required surface area is determined [2]. This manner of the cooling coil design could lead to an imprecise design particularly when the cooling coil is partially wet. In this chapter, the numerical calculation using a discrete technique "row-byrow method" will be presented to calculate the detailed design of the cooling coil in order to enhance the calculation accuracy and trace the air and coil surface temperature locally.

#### **2. Types of cooling coils**

Cooling coils are classified to direct-expansion (DX) coils and chilled water coils as shown in Figure 1. Some coil manufacturers fabricate coils from 5/8 inch OD copper tubes, others

Thermal Design of Cooling and Dehumidifying Coils 369

ST

The average air velocity across the face area is called the coil face/frontal velocity and it is

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The number of rows of tubes in the direction of air flow is termed as depth of coil (rows deep, D). Coils with 3, 4, 6 or 8 rows are commonly used. Refrigerant or chilled water enters the first row and leaves the coil from the last row. A coil in which chilled water or refrigerant is supplied to all the tubes in the first row (also referred to as tubes high or tubes in face) is called a maximum or full circuit coil (see Figure 3). Thus a typical coil of 17.5 inch (0.44 m) height which has 10 tubes in face (based on 1.75 inch (0.044 m) pitch) will have a maximum of 10 circuits. If the supply is given to alternate tubes in face, we get a half-circuit coil with 5 circuits as against 10 circuits. The U-bends at the end of the tubes can be arranged, at the time of manufacturing, to obtain the number of circuits desired. See Figure

Face velocity is restricted to 500 fpm (2.5 m/s) to avoid carryover of condensate from the coil. The value of 500 fpm (2.5 m/s) is very commonly used for coil sizing and it works very well for cfm/ton in the range of 500 to 600 (2.5 to 3 m3/s per ton). If cfm/ton ratio falls below 500 (2.5 m3/s per ton), this generally happens when room sensible heat factor goes below 0.8 due to high room latent load, a 4-row coil at 500 fpm (2.5 m/s) becomes inadequate. A 5-row coil is not very common. Hence by lowering face velocity, a 4-row deep coil can be selected at 400 fpm (2 m/s), when cfm/ton is about 400 (2 m3/s per ton).. As cfm/ton ratio reduces further, 6-row or 8-row coils have to be selected. This situation is

encountered when the occupancy and/or fresh air components are high.

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Fig. 3. A 4-row coil with a 4-tube face.

4 for full and half circuit coils with 4 tube face.

calculated as follows [3]:

from 1/2 inch copper tube and still others use 3/8 inch tubes. Selection of the tube size is a matter of manufacturer's choice and market demand. Price, as always, plays a major part in the tube size selection.

Fig. 1. Description of the cooling coil for a)- Dx-cooling; b)- Chilled water coil (Aerofin heat transfer products).
