Perspective : GEORGE BERBARI (CEO)
Member : ASHRAE

Introducing fresh air into air-conditioned buildings in hot and humid climates requires careful analysis. Bringing in fresh air can constitute a substantial portion of the total energy consumed by the building. For example, if this makeup air is improperly treated and controlled, it can cause elevated humidity levels.

A primary/secondary system design wherein fresh air is introduced and treated via a dedicated fresh air-handling unit (AHU) and then delivered to a secondary system offers system flexibility and improved humidity control. This type of system can also help save energy. In this article, the application of runaround coils configured in this type of a system is described. This arrangement can save energy while improving humidity control.

The dominant application of runaround coils is to exchange heat between fresh air and exhaust air. In hot and humid climates more than 50% of the load is latent. Therefore, cooling coils have to over-cool the air to achieve a sufficiently low sensible heat factor. This usually forces the use of reheat to avoid high indoor humidity levels. For many of the buildings for which analyses were conducted, the primary cause of high indoor relative humidity was oversizing. A secondary, but still important factor, was improper treatment of introduced outside air.

Figure 1 illustrates an application of the runaround coil, wherein one of the coil is placed before the cooling coil and the second after the cooling coil. The recovery of energy from the hot outdoor air is used in reheating the overcooled air, which achieves dual energy savings. This scheme simultaneously reduces the required cooling and reheat energy as shown in Figure 2.

Figure 1 : A schematic of the fresh air AHU with runaround recovery coils.

Runaround Coil Energy Savings
A simplified analysis is used to illustrate the possible energy savings. The analysis is based on the following assumptions:

• Constant volume of AHU operating at 10,000 cfm (4719L/s).
• Supply fresh air properties at standard conditions (70°F [21°C] and 50% RH are used).

An equation is used to express the runaround coil’s capacity Q at various
conditions for the previously specified AHU:

                Q = F (Tal – T wi)                                  (1)

Where Q is in Btu/h, F is the coil proportionality factor, Tal is the temperature of the air entering the coil, and Twi is the temperature of the water entering the coil.

A manufacturer product selection program was used to evaluate the coils at three different ambient conditions:

                        DB/WB temp. = 115/85°F (46/29°C)
                        DB/WB temp. = 95/80°F (35/27°C)
                        DB/WB temp. = 75/70°F (24/21°C)

A polynomial was then utilized to interpolate the equation variables F and T. The basic solution is to equalize the log mean temperature differential (LMTD) of the two runaround coils:

Estimated circulating pump electric energy is: 5,312 Hrs x 1.03 Kw = 5,471 kW-Hr/year. Estimated circulating pump power is: 0.746 Hp/kW gpm x (11.5 ft + 7 ft) / (3,960 x 70% eff.) = 1.03 kW.

Table 1: Energy analysis of a 10,000 cfm fresh air AHU with runaround heat recovery coils in Augusta, Ga.

Table 2 : Energy analysis of a 10,000 cfm fresh air AHU summer cooling requirement in augusta, Ga.

                Twi = (Tal + Ta3)/2 + 1.5303 x Tal – 0.029625 x Tal + 0.0000625

                x Tal² F = 7277 + 52.78 x (Tal – Twi) x (Tal – Twi)²                (2)

Where Ta3 is the leaving air condition of the cooling coil controlled at 56°F (13°C).

A simplified control system is assumed. The main cooling coil should dehumidify the air to a level of 66.5 grains (.0095 Kg/Kg of dry air), which corresponds to that of the comfort conditions of 76°F (24°C) and 50% RH. This can be attained via a dew point temperature controller or a simpler DB temperature controller set at 56°F (13°C).

In this system the runaround coil is operated all summer with an on-off control. A diverting valve can be used to control the leaving air temperature of the second downstream runaround coil, which should not exceed 70°F (21°C). An optional supplementary reheat coil, if used, can control the final leaving air temperature which should not drop below 70°F (21°C).

Two hot and humid climates were chosen for the analysis: Augusta, Ga. in the United States and Abu Dhabi in United Arab Emirates. The Bin Hour method was used estimate the annual energy required.

Heat pipes are widely used now in place of runaround water coils. Heat pipes are slightly more effective as they do not require circulating pumps. They are refrigerant phase change as a heat transfer media between hotter and colder deck. Runaround coils use recirculating water for the same purpose.

Heat pipe manufacturer’s software was used to evaluate the heat pipe’s performance using the same weather data for Augusta, Ga. The heat transfer results and annual recovered energy for the heat pipes were in close accordance with those of the runaround coils.

Benefits of Runaround Coils
The runaround coils reduced the cooling coils total annual cooling energy by 19% for Augusta (from 2056.4 x 106 Btu to 1666.8 x 106 Btu [2170 GJ to 1758 GJ] and by 21% for Abu Dhabi (from 4137 x 106 Btu to 3280 x 106 Btu [4364 GJ to 3461GJ]). The supplementary reheat coil total energy was reduced by 49% for Augusta (from 786 x 106 Btu to 398.1 x 106 Btu [829 GJ to 420 GJ]) and by 62% for Abu Dhabi (from 1303.9 x 106 Btu to 495.7 x 106 Btu [1375.6 GJ to 523 GJ]. The Augusta results are displayed in Tables 1 and 2.

A three-way diverting valve and a third recovery coil can be used for winter heat recovery between exhaust air and fresh air. Because water piping is much easier to install than air ducts, exhaust and fresh air duct distance is not an issue. Supplementary reheat is generally not required of indoor relative humidity is in the range of 60% to 65% at off-peak conditions.

At relatively low ambient conditions (below 70°F or 21°) the coils can be switched off and the system will operate in the economizer cycle mode, further reducing the cooling energy and eliminating supplementary reheat. In milder climates, the system can be used with reduced effectiveness. In this application, a lower supply air duct relative humidity is achieved, thus reducing the possibility of mold or mildew build-up inside the duct.

The system has proven its reliability with hundreds of installations in the UAE. As an example, one 5200 cfm unit (without supplementary reheat) was delivering a leaving air temperature of DB/WB = 71.5/64°F (21.9/17.8°C) when ambient temperature was 93°F (34°C) and 46% RH. After the runaround coil circulating pump was switched off and the system was left to stabilize, the leaving air condition was determined to be 60.5/60°F (15.8/15.6°C)

Figure 2: The psychometric process of the fresh air AHU with runaround recovery coils.

Cost Effectiveness of Coils
Runaround coils can achieve substantial energy savings in hot and humid climates. The hotter and more humid the climate, the more effective the system. The system’s design can be flexible. The runaround coils recovery system is relatively inexpensive. The added cost for installing a 10,000-cfm (4719 L/s) AHU is estimated at approximately US$8,000. This is offset by the reduction of cooling plant capacity by about 15 tons (53kW) for Augusta and 20 tons (70kW) for Abu Dhabi, with an installed cost of around US$800 per ton. Substantial energy savings are gained mostly during peak summer demand.

The runaround recovery system described in this article provides a real opportunity for capital cost savings, as well as operating cost savings. It is reliable and flexible system that can be used with other system components to further save on energy.

Bibliography
1982. Stoeker, W.F. and J.W. Jones. Refrigeration & Air Conditioning, Second Edition. New York: McGraw-Hill.

ASHRAE Standard 90.1 – 1989, Energy Efficient Design of New Building Except Low-Rise Residential Buildings.

1992. ASHRAE Handbook, HVAC Systems & Equipment.

1995. Desiccant Technology Transfer Workshop Manual, Technology Transfer Workshop: Desiccant Cooling Systems