The Influence on Surface Coverings on the Performance of Radiant Floors for both Heating and Cooling.
This paper will outline the influence of floor coverings on radiant cooled floors. The performance of the radiant cooled floor subjected to different solar radiation intensities with each alternative will also be discussed.
Radiant systems are dimensioned in accordance with the ensuing radiant heat exchange in the space. The radiant system can be designed to provide both individual and zone control.
Previous papers by Simmonds (1994) have shown that radiant cooled floors are capable of removing between 35-40 W/m2 (11-12.5 Btu/ft2h) from spaces. Borressen (1994) and Simmonds, Gaw, Holst, and Reuss (1996) have shown that radiant cooled floors are capable of removing up to 85 W/m2 (27.0 Btu/ft2h) of energy from a space: 35 W/m2 (11.1 Btu/ft2h) by convection and 50 W/m2 (16 Btu/ft2h) by solar absorption. This paper uses a simple steady-state equation to explain the performance of a radiant cooled floor when performing at its maximum capacity of both reducing the space air temperature and absorbing solar radiation that is reaching the floor. Many papers have been written on the performance of radiant floor for heating; MacCluer, Athienitis, Simmonds, Olesen, and Meirhans have reported on the performance of active concrete systems. Simmonds, Mehlomakulu, and Eibert (2006) described that radiant cooled floors can be a feasible option for cooling by absorbing short wave radiation and preventing it to escape into the air while cooling the floor with chilled water.
There has been advancement in simulation programs that have permitted a more detailed analysis of the indoor environment. With these advanced simulation tools the individual elements necessary for the creation of a comfortable indoor climate using the predicted mean vote (PMV), as determined by Fanger (1972), and radiant heat exchange can be studied. Because each individual surface temperature and its relationship (i.e. position to the other surfaces) can be determined, a solution to the comfort equation can be found. The PMV/PPD comfort equation derived by Fanger and shown in ASHRAE Standard 55-2010 can be influenced by the control or balance of the radiant heat exchange in a space. When operating a radiant cooled floor correctly, the surface temperature of the floor can be regulated. Absorbing a major portion of the solar radiation entering the space also prevents the floor from emitting thermal energy, in the form of long wave radiation, back into the space and onto other surfaces.
When incorporating a radiant system and a constant volume ventilating system, the ventilation system need only be dimensioned to supply outdoor air for each person and to remove the latent and sensible loads from people, equipment, and lighting. The radiant system is then selected to remove the remaining cooling loads. Simmonds (1994) has reported on some of these designs and how effective they are in providing comfort climate control.
Since cooling surfaces make no contribution to air renewal, the surfaces usually operate in conjunction with a ventilating or air-conditioning plant, which also ensures the necessary dehumidification is provided. The combination of a radiant cooled system with a natural ventilation system and operable windows may also be possible in certain climate conditions.
Floor Surface Temperature
According to ANSI/ASHRAE 55-2010, Thermal Environmental Conditions for Human Occupancy, the surface temperature of the floor for people wearing typical indoor footwear shall be between 18 C (65 F) and 29 C (84 F) to minimize foot discomfort. For floors that people occupy with bare feet, the floor temperature will depend on the type of floor material (see ASHRAE Handbook-Fundamentals).
According to work from ASHRAE 55-2010, it is recommended that a minimum floor surface temperature of 19 C (67 F) is used because of temperature differences between the floor surface and occupants.
Performance of a Radiant Floor with Different Surface Coverings for Cooling System
The floor covering, as well as the floor construction, has a significant influence on the radiant floor system, especially for floor cooling. With increasing thermal resistance of the floor covering (carpet instead of tile), as well as the floor construction above the tubes, the cooling capacity decreases if the Entering Water Temperature (EWT) is kept constant. If carpet is used, the resistance of the carpet should be as low as possible and should be specified in advance. A higher resistance than calculated may cause a lower cooling capacity of the radiant floor system.
Entering Water Temperature (EWT)
The EWT is a function of many aspects and decreases with increasing thermal resistance of the floor construction. Increasing the water temperature difference between the EWT and Leaving Water Temperature (LWT) increases the cooling capacity of the floor and increases the acceptable distance between the water tubes.
It is recommended, as a first iteration, to select an EWT equal to the dew point temperature of the space air, however usually not below 13 C (55.4 F). The water temperature difference between the EWT and LWT may be selected as 5[degrees]K. With these fixed values and the given floor construction, it is possible to calculate the maximum cooling capacity of the radiant floor system.
If necessary, the EWT, the temperature difference between the EWT and LWT, as well as the tube distance can be changed to achieve a maximum performance of the system.
Recommended temperature levels of the hydronic floor cooling system:
Table 3 presents the recommended temperature for industrial purpose, but in this research, due to the high thermal resistance of carpet and wood, water temperature at 6 C was used in the simulation to achieve maintaining surface temperature at 19 C.
The MWT is defined as:
[t.sub.mwt] = [[t.sub.LWT] + [t.sub.EWT]/2] (1)
tmwt Mean water temperature ( C)
tEWT Entering water temperature ( C)
tLWT Leaving water temperature ( C)
Control Strategy of Radiant Cooled Floor for Different Flooring Types
A successful control strategy for maintaining the performance of a radiant cooled floor is critical to achieving comfort criteria and ensuring that surface temperatures do not drop below acceptable levels (19 C, 66 F).
The performance of the radiant floor at varying load conditions was determined under a control method by varying the short wave radiation load and keeping the convective and long wave radiation load constant.
It is acknowledged that there will be some time delay associated with control of the radiant floor, which is not directly addressed in this paper. However, the intent of this section is to show the overall effect of this control strategy, and additional work will be required to investigate the time responses of each system with a dynamic model.
The load conditions for each type of floor are listed in Table 4
Radiant Floor Model
The radiant floor being used for discussion purposes was in a room of 92.9 [m.sup.2] (1000 ft2), with an air space temperature of 23.33[degrees]C (74F) and a temperature below the floor covering of 18.89[degrees]C (66[degrees]F). The types of floor coverings used were bare concrete, oak wood, light carpet, and rubber tile. For the carpet, it is assumed that there are no air pockets between it and the insulation; same thing applies for oak wood and rubber tile. The short wave radiation load was varied from 0 W/m2 to 7 W/m2 (22 Btu/ft2h), and the specific space load was held constant at 12 W/m2 (37.5 Btu/ft2h). The water in the pipes under the floor coverings had an estimated floor cooling of 12.78[degrees]C (55[degrees]F), a minimal water temperature difference at 2.78[degrees]C (5[degrees]F), and the pipes had a distance of 15.24 centimeters (6 inches). The topping slab in each case was 7.62 centimeters (3 inches) thick, and the insulation and construction slabs were negligible. The thermal conductance for the topping slab was 1.4 W/mK (0.81 Btu/hFft). The pipes were 2.4 centimeters (0.94 inches) in diameter outside and 2 centimeters (0.79 inches) in diameter inside. The thermal conductivity of the pipes was 0.35 W/mK (0.2 Btu/hFft).
The steady-state relationship between the space air temperature, the floor surface temperature, the water supply and return temperatures was determined by apply the steady-state equations described at the beginning of this paper.
The simulation was run under 4 different conditions which include concrete, concreted covered with light carpet, oak wood and rubber tile. Table 5 shows the information of floor construction and tube used in the basic case for simulation. Properties of floor construction and tube were remained same for the other 3 cases except different thermal resistance of floor covering.
Based on different floor covering strategies, water supply temperature was supposed to be modified in order to realize that 4 cases could be simulated under same maximum cooling load from space which is around 65-66W/m2. And in all 4 cases, temperature difference between entering and returning water was kept 4[degrees]C. Then, with that information, thermal resistance in each layer was calculated and temperatures of bottom of floor covering and concrete were specified. Then detailed load in each case was testified to be the same and then maximum flow rate in each case was also worked out. Calculation summary is shown in Table. 6.
With the calculated flow rate in each case, entering temperature and returning temperature were calculated under different loads to maintain surface temperature at 19[degrees]C. On the other hand, calculated entering water temperature could also be used as parameter in control strategy. Entering temperature, flow rate, returning temperature and surface temperature in each case at varying loads are shown in Figure 1 to Figure 4.
The radiant cooling system performs roughly the same effects on the light carpet and oak wood, as well as concrete and rubber tile. Figures 1 and 4 show that concrete and rubber tile have similar characteristics for water temperatures at different cooling loads; this is also the case for Figures 2 and 3. Figure 1 to Figure 4 indicates a similar flow rate in each type. Also, by adjusting entering water temperature, surface temperature can be controlled at 19[degrees]C in all cases which is a good control strategy.
As for the other flooring options, light carpet is the most specific in terms of getting reasonable results. There are many options for a light carpet and the lighter the carpet is, the better the system will operate at cooling the floor. Heavier carpets tend to restrict the cooling to the entire floor, and that leads to lower water temperatures, and more energy, from the chiller to get the same results as carpets with lighter padding. For this paper, a light carpet with resistance to the floor surface was used for the data. With a light carpet, the cooling system had the same water mass flow rate as it did with rubber tile and oak wood.
Among all the floor covering strategies, the larger total thermal resistance of the construction is, the lower entering water temperature it needs to keep surface temperature at 19[degrees]C. Average water temperature in tube should stay the same in order to cover cooling load, while by changing temperature difference between entering and returning water temperature, a different water flow rate could be applied. But the best choice should be based on overall consideration of cost in producing supply water by chiller and cost in pumping at certain flow rate. In summary, the lower thermal resistance in the floor construction, the better performance we may get from radiant floor system.
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Peter Simmonds, PhD
Student Member, ASHRAE
John Gautrey, PE
Peter Simmonds is an associate mechanical engineer and John Gautrey is a partner and principal at IBE Consulting Engineers, now Stantec Inc. Nick Triana is a student at The Ohio State University. Rui Zhu is a Master's candidate at USC School of Architecture.
Table 3: Recommended EWT and Temperature Differentials EWT 13 C to 15 C (55.4-59 F) [DELTA]T 3 C to 6 C (5-10.8 F) Table 4 Specific space Load 12 Btu/[ft.sup.2]h 12 Btu/[ft.sup.2]h (Convective + Long (38 W/[m.sup.2]) (38 W/[m.sup.2]) wave radiation load) Short Wave Radiation 7 Btu/[ft.sup.2]h 6 Btu/[ft.sup.2]h Load (22 W/[m.sup.2]) (19 W/[m.sup.2]) Specific space Load 12 Btu/[ft.sup.2]h 12 Btu/[ft.sup.2]h (Convective + Long (38 W/[m.sup.2]) (38 W/[m.sup.2]) wave radiation load) Short Wave Radiation 5 Btu/[ft.sup.2]h 4 Btu/[ft.sup.2]h Load (16 W/[m.sup.2]) (13 W/[m.sup.2]) Specific space Load 12 Btu/[ft.sup.2]h 12 Btu/[ft.sup.2]h (Convective + Long (38 W/[m.sup.2]) (38 W/[m.sup.2]) wave radiation load) Short Wave Radiation 3 Btu/[ft.sup.2]h 2 Btu/[ft.sup.2]h Load (10 W/[m.sup.2]) (6.3 W/[m.sup.2]) Specific space Load 12 Btu/[ft.sup.2]h 12 Btu/[ft.sup.2]h (Convective + Long (38 W/[m.sup.2]) (38 W/[m.sup.2]) wave radiation load) Short Wave Radiation 1 Btu/[ft.sup.2]h 0 Btu/[ft.sup.2]h Load (3.2 W/[m.sup.2]) (3.2 W/[m.sup.2]) Table 5. Properties Information of Construction and Tube Thermal Resistance Floor Covering 0.00 Floor Construction Topping Slab Height 7.62 (Concrete) Thermal Cond. 1.40 Insulation Height 0 R-Value 1.23 Construction Slab Height 0 Thermal Cond. 1.40 Inside Diameter 2 Tube Outside Diameter 2.4 Conductivity Tube Material 0.35 thickness 0.2 [m.sup.2]K/W Floor Construction cm (Concrete) W/mK cm [m.sup.2]K/W cm W/mK cm Tube cm W/mK cm Table 6. First Stage Calculation of 4 cases Parameters Concrete Light Carpet Thickness of Concrete (m) 7.62x[10.sup.-2] 1.06x[10.sup.-1] Thermal Resistance of Floor Covering [R.sub.f] ([m.sup.2]K/W) 0 7.62x[10.sup.-2] Thermal Resistance of Concrete [R.sub.c] ([m.sup.2]K/W) 5.44x[10.sup.-2] 5.44x[10.sup.-2] Thermal Resistance of Pipe [R.sub.p] ([m.sup.2]K/W) 5.78x[10.sup.-3] 5.78x[10.sup.-3] Total Thermal Resistance [R.sub.total] ([m.sup.2]K/W) 6.01x[10.sup.-2] 1.66x[10.sup.-1] Surface Temperature ([degrees]C) 19 19 Temperature-top of concrete ([degrees]C) 19 10.71 Temperature-top of pipe ([degrees]C) 13.58 6.45 Entering Water Temperature ([degrees]C) 13 6 Load (W/[m.sup.2]) 66.52 66.34 Returning Temperature (max) ([degrees]C) 17 10 Flow Rate (kg/s) 3.96x[10.sup.-3] 3.95x[10.sup.-3] Parameters Oak Wood Rubber Tile Thickness of Concrete (m) 1.00x[10.sup.-1] 8.81x[10.sup.-3] Thermal Resistance of Floor Covering [R.sub.f] ([m.sup.2]K/W) 7.62x[10.sup.-2] 7.62x[10.sup.-2] Thermal Resistance of Concrete [R.sub.c] ([m.sup.2]K/W) 5.44x[10.sup.-2] 5.44x[10.sup.-2] Thermal Resistance of Pipe [R.sub.p] ([m.sup.2]K/W) 5.78x[10.sup.-3] 5.78x[10.sup.-3] Total Thermal Resistance [R.sub.total] ([m.sup.2]K/W) 1.61x[10.sup.-1] 6.89x[10.sup.-2] Surface Temperature ([degrees]C) 19 19 Temperature-top of concrete ([degrees]C) 11.18 18.17 Temperature-top of pipe ([degrees]C) 6.95 13.04 Entering Water Temperature ([degrees]C) 6.5 12.5 Load (W/[m.sup.2]) 65.41 65.27 Returning Temperature (max) ([degrees]C) 10.5 16.5 Flow Rate (kg/s) 3.89x[10.sup.-3] 3.89x[10.sup.-3]
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|Author:||Simmonds, Peter; Zhu, Rui; Triana, Nick; Gautrey, John|
|Publication:||ASHRAE Conference Papers|
|Date:||Dec 22, 2014|
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