A basic introduction to fin and tube designs – Part 1

By Marius La Grange, general manager, Thermocoil

“Heat exchangers (HEs) transfer heat from one fluid to another without the fluids coming in direct contact” – (ASHRAE, 2008, Chapter47). An exchange of heat energy taking place between at least two fluids. A fluid of course being a vapour or a liquid or a combination of both.

A refrigeration system would absorb heat energy from an enclosed environment and reject it in another environment. A compressor is used to drive this process drawing and compressing low pressure vapour from the low-pressure portion of the system and discharging it into the high-pressure portion of the system.

On the high pressure (HP) side of a vapour compression system, the flow of heat energy is from within the system at the air-cooled condenser into the ambient air – a phase change from vapour to liquid taking place. The ambient air drawn across the finned area gains heat as it passed towards the exit side. Hence the exchange of heat energy from within the system is rejected into the ambient air.

In the low-pressure portion of any system, the heat energy from an enclosed environment is absorbed by the refrigerant on the inside of the evaporator. The refrigerant changing phase, in this case, from a HP liquid to a vapour within the enclosed HP side of the system.

Coil with no finned surface area. Photo by Thermocoil

Another important rule to keep in mind is that heat energy is transferred or flows from one fluid to another when there is a difference in temperature between the two. Heat energy will flow from warm to cold and not the other way around and that is very much applies in each heat exchanger as well.

In the case of an evaporator the HE is commonly used to cool an airflow on the outside with a refrigerant expanding and absorbing heat resulting in a phase change from liquid to vapour. The transfer of heat energy of course from the warmer air on the outside driving the phase change of the liquid refrigerant on the inside.

The ideal exchange of heat takes place when the two fluids are perfectly counter flow. This applies to any heat exchanger type.

With two fluids flowing through a heat exchanger, the rate of heat transfer (duty) could be calculated as:

Discrepancies between the theoretical duty and the actual duty is often due the two fluids not being perfectly counter flow. A perfect counter flow is not always possible with a fin and tube coil.

This flow of heat could be in the form off conduction, convection or radiation within any head exchanger. The material separating the two mediums might conduct the flow of heat energy from the surface on the one side to the surface on the other side through the material.

The type of materials used is, for the greater part, dependant on the environment that the HE will be functioning in. How warm or how aggressive might the mediums experience that form part of the system.

<heading2> Fin and tube heat exchanger in the HVAC&R industry

Early heat exchangers were commonly made up of tubes alone. This limited the surface area and potential heat transfer. Heat Exchangers (HE) are essential components used in every cooling, heating, air conditioning and refrigeration system.                                   

Since each of these systems absorbs heat in one area and rejects it in another, we would find at least two heat exchangers of some kind in any of these systems.

Heat transfer was “extended” with the use/addition of fins. The transfer rate (W) in DX applications could be defined as follows. (Stoeker, 1976, p163):

he overall heat transfer coefficient, being a product of the materials used and their respective wall thicknesses. With reduction in the difference in temperature between tf and tr the surface area (A) would need to be bigger in order to achieve the same potential heat transfer.

There is a wide variety of HE types on offer with new developments contributing to the options to consider in specific applications. A very commonly used type of HEs would be the fin and tube type. There are various reasons for this trend. One of the most obvious would be the fact that volumes of air are drawn across the HE, in most cases, to offer the cooling effect or to reject the heat from a functioning system (in the case of a air cooled condenser).

The air across the fin and tube coil is being cooled or provide cooling.

“Why make condensers air cooled?” This is a question I have received a number of times and I believe the best answer is simply that the ambient air volumes you have to make use of are in abundant supply and “free of charge”. In the case of an air-cooled condenser the ambient air drawn into the unit would gain some of the heat energy on the inside of the coil tubes as it comes into contact with the coil tubes and fins.

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Types of fin and tube heat exchangers commonly used

Due to the relative ease of manufacturing fin and tube designs, these are used in a variety of HVACandR applications. Not limited to these of course, but these would be the most commonly found in operation.

• Air cooled condensers – DX systems
  An air-cooled condenser is commonly used as part of any direct expansion (DX) type system to reject heat energy from the system. The heat energy resulting from the compressor’s function also known as the input power (IP), plus heat energy required by the system to absorb (evaporative duty) is known as the system’s total heat rejection (THR). The IP being a by-product of a DX system.

  Ambient air being the fluid that the heat energy is transferred towards, is drawn across the coil surfaces (in most cases). The air is forced across the coils surface area and the greater the mass flow of air, the greater the potential heat rejection.

  Cooling loads vary in many systems so the required THR varies as well. This could be done making use of multiple fans and staging them or by using a variable speed fan arrangement. The greater the difference in temperature (ΔT) between the targeted condensing temperature condition and the ambient air conditions the smaller the condenser’s surface area could be.

  With a rise in altitude the ambient air density decreases, so a similar volume of air would weigh less requiring a greater volume of air to achieve the same relative exchange of heat to take place. (A greater volume of air needed for the same relative mass flow at a hight altitude).

  Most domestic appliance rely on convection with the condenser coil mounted in such a way that will allow warm air to rise and draw cooler ambient air in from the bottom.

• Evaporators – DX systems
  An evaporator’s function within a system is to absorb and remove heat energy from and enclosed area. Within the evaporator the condensed liquid from the HP side would be exposed to the heat source that causes the liquid to boil off changing phase to vapour. The heat source in this case being the air within the enclosed area that needs to be cooled.

  This air is of course also forced across the coil surface area by means of fans. Should the air across the coiled surface area decrease, the rate at which the liquid refrigerant boils off would also decrease with the expansion device modulating the mass flow of the refrigerant fed into the evaporator. At exit of the evaporator the refrigerant would need to be fully evaporated with few K of superheat as controlled by the expansion device with no liquid refrigerant remaining a vapour at the coil exit.

  The temperature for the refrigerant on the inside of the evaporator will obviously always need to be lower than the required air temperature of the enclosed area in order for the flow off. This difference in temperature (ΔT) means that most cold rooms or freezer rooms have a suction temperature well below 0°C. In such cases the surface area of the coil would ice up constantly requiring regular defrosting. This is done with electrical heating elements in many cases.

  A ‘hot gas’ defrost makes the most sense from a thermodynamic perspective, but it is less common on smaller systems. It adds a bit of complexity to the system and the sizable change in temperatures from cooling/freezing to defrosting several times a day results in the welded copper joints leaking over time. Synthetic refrigerants leaking into the atmosphere uncontrollably of course never being a good thing.

  A fin and tube HE is always going to result in a drop in air humidity when cooling the air. The rate of de-hydration can be limited by design. A ΔT between suction temperature and air of temperature of 6-8K offers a good balance between cost and system operating costs or energy consumption. A lower ΔT would mean a greater surface would be required to achieve the same relative heat absorption capacity.

System diagram-DX type. Photo by Marius La Grange

• Flooded evaporators – ammonia systems
  Similar to DX evaporators flooded evaporators have a surface area based on the capacity required and the air volume is forced across the surface area. The main difference in the this case being that the internal volume of the coil tubes are filled with liquid refrigerant so the transfer of heat is from the air on the outside of the coil towards the colder liquid refrigerant on the inside.

  A flooded evaporator would be piped onto a surge vessel with the liquid refrigerant circulated/pumped to maintain a specific liquid level in the evaporator in order the achieve designed heat absorption. One benefit of a flooded R717 system being the very low, if any, suction superheat that the evaporators operate at resulting in a greater system efficiency.

  The tubing circuits in contact with the refrigerant needing to be steel (as special version) or stainless steel (no copper since ammonia corrodes copper). With R717 applications, stringent regulations (SANS10147) apply when making use of carbon steel as part of the pressurized system.

• Air coolers
  Air coolers are commonly used in air conditioning applications. Water used as a secondary coolant being circulated through the coil. The heat energy being transferred from the air volume (requiring cooling) and the secondary coolant. The secondary coolant is circulated with a pump between the air cooler and a heat exchanger of some kind to absorb the heat gain that the secondary coolant had. In such systems additional energy is needed to pump the secondary coolant.

  One advantage of such systems would be the lower ΔT possible between the required air off conditions and the secondary coolant supply temperature. This can potentially lower the rate of de-hydration than a comparable DX evaporator coil making it more suitable for specific applications.

  The secondary coolant being water or a water and glycol mixture (pre-mixed to the applications specific ratio by volume). In applications where the secondary coolant is required to be below 0°C a mixture is required to prevent the mixture from freezing at any stage. Note that the glycol has a lower viscosity than clean water, so a greater amount of pump energy is needed to circulate the mixture. 

Continued in the next issue of the RACA Journal.