**3. Coil construction and geometry**

In a coil, copper tubes are arranged parallel to one another, either in staggered pattern or non-staggered pattern, along the length L of the coil. A staggered pattern is more commonly used. For 5/8 inch tubes, the triangular pitch is 1.75 inch or 1.5 inch. For 1/2 inch tubes it is 1.25 inch. Plate or ripple fins are used to enhance the heat transfer area. Thus the primary surface area (outside area of bare copper tubes) is enhanced greatly by adding a secondary area of fins. The total area including fins is called outside surface area. The cross-section (L × H) which the air flows is called the face area or the finned area. Thus L is finned length and H is fin height (see Figure 2). Fins are arranged perpendicular to the tubes. Where, the fin spacing varies between 8 and 16 fins per inch of tube.

Fig. 2. Geometry configuration of the cooling coil (*Aerofin heat transfer products).*

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

a) b)

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

In a coil, copper tubes are arranged parallel to one another, either in staggered pattern or non-staggered pattern, along the length L of the coil. A staggered pattern is more commonly used. For 5/8 inch tubes, the triangular pitch is 1.75 inch or 1.5 inch. For 1/2 inch tubes it is 1.25 inch. Plate or ripple fins are used to enhance the heat transfer area. Thus the primary surface area (outside area of bare copper tubes) is enhanced greatly by adding a secondary area of fins. The total area including fins is called outside surface area. The cross-section (L × H) which the air flows is called the face area or the finned area. Thus L is finned length and H is fin height (see Figure 2). Fins are arranged perpendicular to the tubes. Where, the fin

Fig. 2. Geometry configuration of the cooling coil (*Aerofin heat transfer products).*

the tube size selection.

transfer products).

**3. Coil construction and geometry** 

spacing varies between 8 and 16 fins per inch of tube.

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

The average air velocity across the face area is called the coil face/frontal velocity and it is calculated as follows [3]:

$$\text{Face Velocity (} \text{ ${}^{m}$ /}\_{\text{S}}) = \frac{\text{Air flow rate (kg/s)}}{\text{Face area (m}^{2})}$$

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

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.

Thermal Design of Cooling and Dehumidifying Coils 371

and moisture transfer between them. The directions of heat and moisture transfer depend upon the temperature and vapor pressure differences between air and wetted surface. As a result, the direction of the total heat transfer rate, which is a sum of sensible heat transfer and latent heat transfers. The concept of enthalpy potential [4] is very useful in quantifying

QS = ho AS (ti – ta)

QL = hmass AS (Wi – Wa) hfg

QT = QS +QL = ho AS (ti – ta) + hmass As (Wi – Wa) hfg

Since the transport mechanism that controls the convective heat transfer between air and water also controls the moisture transfer between air and water, there exists a relation between heat and mass transfer coefficients, hC and hD as discussed in an earlier chapter. It

Hmass≈ho/cpm or ho/hmass.cpm = Lewis number ≈ 1.0 Where cpm is the humid air specific heat ≈ 1.0216 kJ/kg.K. Hence the total heat transfer is

QT = QS +QL = ho AS (ti – ta) + hmass AS (Wi – Wa) hfg = (ho AS/Cpm )[(ti – ta) +(Wi – Wa) hfg]

QT = QS +QL = (ho AS/cpm )[ (hi – ha)] The air heat transfer coefficient, ho has been computed from the experimental correlations derived in [3]. The heat transfer parameter is written as Stanton number, *St* times Prandtl number, *Pr* to the 2/3 power. It is given as a function of Reynolds number, *Re* where the function was established through curve-fitting of a set of the experimental data as follow:

�� × ��(���) = ������ × ��������

, and Re = (m� × d�)

(A��� x μ�)

 , Pr = �μ� × c��� k�

by manipulating the term in the parenthesis of RHS, it can be shown that:

Where these three dimensionless parameters are defined as:

St = (A��� x h�) �m� × c���

the total heat transfer in these processes and its direction.

the total heat transfer QT is given by:

t a = dry-bulb temperature of air, oC

hfg = latent heat of vaporization, J/kg

Wa = humidity ratio of air, kg/kg

t i= temperature of water/wetted surface, oC

Wi= humidity ratio of saturated air at ti, kg/kg ho = convective heat transfer coefficient, W/m2.oC hmass = convective mass transfer coefficient, kg/m2

has been shown that for air-water vapor mixtures,

Where:

given by:

The sensible (QS) and latent (QL) heat transfer rates are given by:

Fig. 4. Full circuit and half circuit four row coils with 4-tube face.

#### **3.1 Fin patterns**

There are three standard plate fin patterns that are usually used in the cooling coil: flatplate, wavy-plate, and star-plate fin patterns, as shown in Figure 5. They are made of Aluminum, copper, and stainless steel or carbon steel. The fins are permanently attached to the tubes by expansion of each tube. Full fin collars allow for both precise fin spacing and maximum fin-to-tube contact. The flat-plate fin type has no corrugation, which results in the lowest possible air friction drop and lowest fan horsepower demands while the wavy-plate fin corrugation across the fin provides the maximum heat transfer for a given surface area, and is the standard fin configuration used. The star-plate fin pattern corrugation around the tubes provides lower air friction. This pattern is used when lower air friction is desired without a large decrease in heat transfer capacity.

Fig. 5. (a) Wavy-plate fin; (b) Star-plate fin; (c) Flat-plate fin (*Aerofin heat transfer products).*
