SECTION H. Engineering Guide Radiant Products. Please refer to the Price Engineer s HVAC Handbook for more information on Radiant Heating and Cooling

Transcription

1 SECTION H Radiant Products Please refer to the Price Engineer s HVAC Handbook for more information on Radiant Heating and Cooling

2 Introduction To Radiant Heating and Cooling Radiant heating and cooling systems offer an energy efficient alternative to all-air systems. In most cases, the supply air volume of the air handling system is limited in size to satisfy only the ventilation and latent loads, with the radiant system making up the balance of the heating and cooling loads. This comfortable method of heating and cooling may save energy, space and building maintenance costs. The following pages offer an introduction to the products, systems and design methodology, as well as the advantages and limitations of radiant heating and cooling. Management of heat loads can generally be classified into two different types: allair systems or hybrid systems. All-air systems have been the most prominent in North America during the 20th century and have been in use since the advent of air conditioning. These systems use air to service both the ventilation requirement as well as the building cooling load. In general, these systems have a central air handling unit (or rooftop unit) that delivers enough cool or warm air to satisfy the building load. Diffusers mounted in the zone deliver this air in such a way as to promote comfort and evenly distribute the air. In many cases, the amount of air required to cool or warm the space or the fluctuations of loads make designing in accordance to these principles difficult. Draft is not uncommon, and some ceiling diffusers have been known to dump at low capacities. Hybrid systems have two components: an air-side ventilation system and a hydronic (or water-side) radiant system. The air-side is designed to meet all of the ventilation requirements for the building, as well as satisfy all latent loads. It is a 100% outside air system and because the primary function of the supply air system is ventilation as opposed to cooling, it can be supplied at higher supply air temperatures than is typical of overhead air distribution systems. The water-side is designed to meet the balance of the sensible cooling and heating loads. These loads may be handled by water based products, such as radiant panels, which transfer heat mainly by thermal radiation, and chilled sails, which transfer heat using a combination of thermal radiation and natural convection. Radiant panels have been used for sensible heating and cooling in North American buildings for over half a century, and are a widely recognized and well-established technology. Chilled sails were originally developed in Europe in the late 1990s, and are a relatively new technology in North America. H-2 All Metric dimensions ( ) are soft conversion. Copyright Price Industries Limited 2011.

3 Concepts and Benefits Radiant heating and cooling systems provide an effective method for satisfying the heating and/or cooling loads of a space while promoting a high level of occupant comfort and energy efficiency. Hydronic systems have been successfully used in several applications having dramatically different characteristics. Some examples of areas where radiant systems have been applied include: Green Buildings Hospitals Burn Centers Isolation Rooms Schools Data Centers Office Buildings Airports Cafeterias Television Studios Theaters Casinos Benefits of Air-Water Systems There are many benefits to heating and cooling using radiant panels and chilled sails. Advantages of these water based heating and cooling systems over other mechanical systems include: Energy and system efficiency Reduced system horse power Indoor environmental quality Improved indoor air quality Increased thermal comfort Reduced mechanical footprint Lower maintenance costs Improved system hygiene Radiant systems are a good choice where: Thermal comfort is a major design consideration Areas have high sensible loads Areas require a high indoor air quality (100% outdoor air system) Energy conservation is desired Energy Efficiency The heat transfer capacity of water allows for a reduction in the energy used to transport an equivalent amount of heat as an all-air system (Stetiu, 1998). These reductions can be found primarily through reduced fan energy. The higher chilled water supply (CHWS) temperatures used with active and passive beam systems, typically around 58 F [14.5 C], provide many opportunities for a reduction in energy use, including increased water-side economizer use. This increased CHWS temperature also allows for more water-side economizer hours than would be possible with other systems where CHWS temperatures are typically ~45 F [7 C]. Figure 1: Examples of radiant heating and cooling Indoor Air Quality Depending on the application, under maximum load, only ~15 to 40% of the cooling air flow in a typical space is outdoor air and is required by code to satisfy the ventilation requirements. The balance of the supply air flow is recirculated air which, when not treated, can transport pollutants through the building. Radiant systems transfer heat directly to/from the zone and are often used with a 100% outdoor air system which exhausts polluted air directly to the outside, reducing the opportunity for VOCs and illness to travel between air distribution zones. Noise Radiant systems do not usually have fan powered devices near the zone. This typically results in lower zone noise levels than what is achieved with all-air systems. In situations where passive beams are used in conjunction with a quiet air system, such as displacement ventilation, the opportunities for noise reduction increase further. Reduced Mechanical Footprint The increased cooling capacity of water allows the transport system to be reduced in size. It is generally not unusual to be able to replace ~60 ft² [6 m²] of air shaft with a 6 in. [150 mm] water riser, increasing the amount of floor space available for use or lease. Due to the simplicity of the systems (i.e. reduction in the number of moving parts and the elimination of zone filters, drain pans, condensate pumps, and mechanical components), there tends to be less space required in the interstitial space to support the HVAC system. Lower Maintenance Costs With no terminal unit or fan coil filters or motors to replace, a simple cleaning is all that is required in order to maintain the product. Copyright Price Industries Limited All Metric dimensions ( ) are soft conversion. H-3

4 When To Use Radiant Systems Hygienic System With the elimination of the majority of filters and drain pans, there is a reduced risk of mold or bacteria growth in the entire mechanical system. Radiant systems such as radiant panels and chilled sails are well-suited to some applications and less so to others. As a result, each application must be reviewed for potential benefits as well as the suitability of these types of systems. One consideration which can assist in the decision to employ hydronic systems as opposed to an all-air system, is the air-side load fraction or the percentage of the total air supply that must be delivered to the zone to satisfy code and dehumidification requirements. Table 1 shows the load fraction for several spaces. In the table the best applications for hydronic systems are those with the lowest air-side load fraction as they are the ones that will benefit the most from the efficiencies of hydronic systems. Another factor which should be examined is the sensible heat ratio or the percentage of the cooling load that is sensible as opposed to latent. The latent loads must be satisfied with an air system and offer some sensible cooling at the same time because of the temperature of dehumidified air. If the total sensible cooling load is significantly higher than the capacity of the air supplied to satisfy the latent loads, a radiant system might be a good choice. Commercial Office Buildings In an office building hydronic heating and cooling systems provide several benefits. The lower supply air volume of the air handling system provides significant energy savings. In addition, the smaller infrastructure required to move this lower air flow allows for small plenum spaces, translating into shorter floor-to-floor construction or higher ceilings. The lower supply air volume and elimination of fans at or near the space offers a significant reduction in generated noise. Often the lower air flow translates to reheat requirements being reduced. In the case of 100% outside air systems, the lighting load captured in the return plenum is exhausted from the building, lowering the overall cooling load. Schools Schools are another application that can benefit greatly from radiant panels and chilled sails systems. Similar to office buildings, the benefits of a lower supply air volume to the space are lower fan power, shorter plenum height, reduced reheat requirement, and lower noise levels (often a critical design parameter of schools). Application Total Air Volume (Typ.) Hospital Patient Rooms Hospitals are unique applications in that the supply air volume required by local codes for each space is often greater than the requirement of the cooling and heating load. In some cases the standard or code requires these higher air-change rates for all-air systems only. In these cases the total air-change rate required is reduced if supplemental heating or cooling is used. This allows for a significant reduction in system air volume and yields energy savings and other benefits. Furthermore, because these systems are generally constant air volume with the potential to reduce the primary air-change rates, reheat and the cooling energy discarded as part of the reheat process is a significant energy savings opportunity. Depending on the application, a 100% outside air system may be used. These systems utilize no return air and no mixing of return between patient rooms, potentially lowering the risk of hospital associated infections. Hotels / Dorms Hotels, motels, dormitories, and similar type buildings can also benefit from hydronic systems. Fan power savings often come from the elimination of fan coil units located in the occupied space. The energy savings associated with these local fans is similar in magnitude to that of larger air handling systems. It also allows for the elimination of the electrical service required for the installation of fan coil units as well as a reduction in the maintenance of the drain and filter systems. The removal of these fans from the occupied space also provides lower noise levels, which can be a significant benefit in the sleep areas. Ventilation Requirement (Typ.) Office 1 cfm/ft 2 [5 L/s m 2 ] 0.15 cfm/ft 2 [0.75 L/s m 2 ] 0.15 School 1.5 cfm/ft 2 [7.5 L/s m 2 ] 0.5 cfm/ft 2 [2.5 L/s m 2 ] 0.33 Lobby 2 cfm/ft 2 [10 L/s m 2 ] 1 cfm/ft 2 [5 L/s m 2 ] 0.5 Patient Room 6 ach 2 ach 0.33 Load-driven Lab 20 ach 6 ach 0.3 Table 1: Typical load fractions for several spaces in the United States Air-Side Load Fraction Limitations There are several areas in a building where humidity can be difficult to control, such as lobby areas and locations of egress. These areas may see a significant short term humidity load if the entrances are not isolated in some way (revolving doors or vestibules). In these areas, a choice of a complimentary technology such as fan coil units or displacement ventilation is ideal. Other applications may have high air flow/ ventilation requirements, such as an exhaust driven lab. The majority of the benefit provided by the hydronic system is linked to the reduction in supply air flow. As such, these applications may not see sufficient benefit to justify the addition of the hydronic circulation system, making them not likely to be a good candidate for this technology. H-4 All Metric dimensions ( ) are soft conversion. Copyright Price Industries Limited 2011.

5 Products - Radiant Panels Operation Radiant panels mainly use thermal radiation to handle the heating or cooling loads of a space. Thermal radiation heat exchange is based on differences in surface temperatures as discussed in Chapter 3 Introduction to Heat Transfer from the Price Engineer's Handbook. Radiant panels add energy to or remove it from a room mainly using radiation with surfaces in the room, but also directly to occupants (Figure 2). To a lesser extent, the panels also heat or cool a room through convection of the room air as it is heated or cooled by the panel surface. Because radiant panels can handle the sensible portion of a building load they must be paired with an air system for ventilation and latent load removal. In heating, for example, heat from warm water is transferred to the panel surface via conduction. The heat passes through the tubing, the mounting extrusion (the fin ), and the panel itself, to the panel surface. At the surface, heat is both radiated to other surfaces in the room and transferred to room air via natural convection. The heat transfer through a radiant panel can easily be modeled with a thermal resistance circuit, as in Figure 3. The resistance circuit represents the actual components of a radiant panel. The nodes represent various temperatures of the panel component surfaces, and the resistors represent the heat conduction through the panel components and to the surrounding room. The t w node represents the mean water temperature that transfers through the copper tubing to the actual panel components. To achieve the maximum possible surface temperature of the panel, Tsurf, the conduction from the pipe to the fin to the panel surface must be maximized, or, inversely, the resistance must be minimized. This can be achieved by using materials that are highly conductive such as copper tubing and aluminum for the fin and panel. Even surface contact between the water tubing and the fins decreases resistance, along with thermal paste which can be applied between the fin and the panel surface to help spread heat evenly to the panel surface (Figure 4) Figure 2: Radiation pathways T air, ceiling R conv,s T panel, outer insulation R conv,room T air, room R insulation R fin AUST, ceiling R rad,s T fin, ave R panel surface T surf, panel R rad,room AUST, room RADIATION AUST = Area-weighted temperature of all indoor surfaces of walls, ceiling, floor, windows, doors, etc. Towards slab Towards room and occupants Copper tubing with Fin Surface Figure 3: Thermal resistance circuit diagram of a modular radiant panel Without Thermal Paste With Thermal Paste Figure 4: Surface temperature distribution of a radiant panel Copyright Price Industries Limited All Metric dimensions ( ) are soft conversion. H-5

6 Products - Radiant Panels The amount of thermal energy that is transferred to the room surfaces via radiation is dependent on the view factors from the panel to the various room surfaces, along with the emissivity of the panel surface. A larger temperature gradient results in greater thermal radiation. Also, the view factors from the panel to the room, as well as the emissivity of the panel surface affect the temperature of the receiving surface. Refer to Chapter 3 Introduction to Heat Transfer of the Engineer's Handbook for further details on the theory of radiant heat transfer. Insulation on the back of the panel helps decrease the amount of heat that travels by radiation or convection to the ceiling. Applications Radiant panels can be applied to virtually any space, especially areas with high sensible loads, areas that require a high indoor air quality, or areas where thermal comfort and energy conservation are major design considerations. Typical applications of hydronic radiant panels are hospitals including patient rooms, isolation rooms, and burn centers schools, data centers, office buildings, and airports. 1. Linear Radiant Panels Linear radiant panels are constructed of a series of integrated aluminum heat sinks and copper tubing. Multiple heat sinks form the visible face of the panel and are joined via tongue-and-groove connections. Insulated backing helps keep the radiant exchange limited to the occupied space. The components of linear radiant panels can be seen in Figure Modular Radiant Panels Modular radiant panels are designed to be integrated into or alongside standard suspended ceiling systems or to suspend from the ceiling in an exposed application. The visible side of the modular radiant panel is a formed steel or aluminum sheet to which the aluminum heat sinks are attached. Copper tubing runs through the heat sinks, and insulated backing helps keep the radiant exchange limited to the occupied space. The components of modular radiant panels can be seen in Figure 6. Connecting Radiant Panels Both linear and modular radiant panels can be connected in series, as shown below. The panels are supplied with straight tubing, using 180 return connections for end panels and interconnects between panels. A typical series application of panels is a perimeter layout with the panels running from wall to wall where an even temperature distribution across several panels is desired. The loose connection pieces allow the panels to be trimmed in order to fit. In these applications, the final connections are done in the field (Figure 7). H-6 Figure 5: Components of a linear radiant panel without insulation Interconnect Figure 7: Series connection details for linear radiant panels Figure 6: Components of a modular radiant panel without insulation Return DESIGN TIP Linear and modular radiant panels can be connected in series in a cloud configuration, provided the panel surface temperatures do not vary significantly and water-side pressure drop is maintained at acceptable levels. A grouping of 4 to 6 modular panels at 2 ft [600 mm] wide and 4 ft [1200 mm] long is common as the panel surface temperature will typically be within 2 to 4 F [1 to 2 C] across the grouping in cooling or 10 to 20 F [6 to 12 C] in heating. All Metric dimensions ( ) are soft conversion. Copyright Price Industries Limited 2011.

7 Products - Chilled Sails Operation Chilled sails provide a functional and unique alternative to conventional radiant panels. Sails couple the radiant cooling effects of standard radiant panels with a convective component. In cooling mode, chilled sails create natural convection by cooling the surrounding air as it passes over the surface facing the plenum. As the air falls into the occupied zone, where warm air is pulled over the sail, the convective cooling capacity of the sail is coupled with the radiant capacity of the cool sail surface, resulting in a cooling capacity greater than that of standard radiant panels. In cooling, the approximate breakdown of heat mode transfer of chilled sails is 30% by thermal radiation and 70% by natural convection. A general air flow diagram of an exposed chilled sail in heating and cooling mode can be seen in Figure 8. In certain applications, sails can also be used for heating. In heating mode, the sails use radiation only to heat the zone below. Because sails have no insulation on their reverse side, heat is radiated not only towards the room, but also towards the building structure. As the slab warms, it in turn helps heat the room to a small extent by thermal radiation and natural convection. Like radiant panels, chilled sails can also be analyzed using a thermal resistance circuit diagram, as seen in Figure 9. The resistance circuit represents the actual components of a chilled sail. The nodes represent various temperatures of the sail component surfaces or the conditions of the room, and the resistors represent the heat conduction through the panel components or heat transfer between the sail and the room. The mean water temperature, t w, node represents the mean water temperature that transfers through the copper tubing to the actual sail. Most chilled sails are one single extrusion, which means that the fin and sail are one solid piece of aluminum. To maximize heat transfer through the sail, or, conversely, to minimize resistance, a material with high thermal conductivity, such as aluminum, is typically used. As seen in Figure 10, a chilled sail transfers heat to a room with a combination of radiation and natural convection. Because chilled sails have no insulation on their reverse sides, heat is transferred from the copper tubing/fin to the slab and plenum. The heat transfer from the sail to the room has three components: natural convection with the room air, thermal radiation with the room surfaces, and thermal radiation from the top of the sail with either the suspended ceiling or the fixed ceiling, depending on the design details. Cooling Sail Heating Sail Figure 8: Air flow pattern of an exposed chilled sail in cooling and heating mode Towards slab R fin/sail Towards room and occupants T air,ceiling R conv, ceiling R conv, room T air, room AUST, ceiling R rad, ceiling Copper tubing with T surface, fin/soil Sail/Fin R rad, room AUST, room Figure 9: Thermal resistance circuit diagram of a chilled sail Top Bottom Figure 10: Typical chilled sail Copyright Price Industries Limited All Metric dimensions ( ) are soft conversion. H-7

8 Products - Chilled Sails In cooling mode, the majority of the heat transfer occurs by natural convection as warm air rises due to natural buoyancy forces, passes over the chilled sails, cools, and then sinks down into the occupied zone. In heating mode, heat is transferred mainly through thermal radiation with room surfaces, where it increases the average unheated surface temperature of the room (AUST). As warm air rises past the heated sails, natural convection occurs, which results in warmer return air. Because sails are water-only systems, they can only handle the sensible portion of a building load and must be paired with a fresh air system for ventilation and latent load removal. Applications Their cooling capacity and unique design make chilled sails an excellent alternative to panel systems, particularly in applications that have an architectural focus. Typical applications of chilled sails include offices, meeting/conference rooms, theaters, studios, lobbies/foyers, waiting areas, or any areas were radiant panel use is appropriate. Chilled sails are designed for architectural appeal and are typically installed in T-bar ceiling grids or freely suspended. Figure 11: Exposed chilled sails Components Chilled sails are typically constructed from copper piping and aluminum extrusions designed to optimize capacity, as well as for architectural appeal (Figure 11). Exposed chilled sails are often installed as the finished ceiling by either installing as a cloud, as shown in Figure 11, or combining active and inactive sections for a continuous look, as seen in Figure 12. Chilled sails are designed to be installed either open to the room or below or behind Figure 12: Continuous chilled sail sections a perforated ceiling, and may be installed in large or discrete sections. In either case, the operation of the chilled sail requires that a portion of the ceiling is open to allow air circulation to the rear of the assembly. For installations behind a perforated ceiling or installed as a cloud in an open ceiling, this is generally not an issue. For installations where the sails are installed in a ceiling system, this is often accomplished by using non-active sections of sail to allow air to pass up to the area above the ceiling, as show in Figure 13. Passive Elements for Return Passive Elements for Return Figure 13: Active and inactive sections H-8 All Metric dimensions ( ) are soft conversion. Copyright Price Industries Limited 2011.

9 Products - Chilled Sails The amount of free area vs. active area of sail will affect the performance of the sail system according to Figure 14. In all cases, the amount of space between the back of the sail and the structural slab will affect the level of circulation, and thereby the convective cooling component. This capacity is affected according to Figure Clearance Between Chilled Sail and Slab, mm Capacity of Sail, % Free Area as a Percentage of Sail Area Figure 14: Free area vs. active area of sail Connecting Chilled Sails Depending on the width of the unit, the sails may have connection locations on opposite ends. Sails with an odd number of sections will have connections on opposite ends, and even number of sections will have connections on the same ends, as seen in Figures 16 and 17 below. Flex hose is generally used to connect the water flow between the units. Water Supply Effective Capacity of Sail, % Clearance Between Chilled Sail and Slab, in. Figure 15: Clearance between chilled sail and slab Sail 1 Flex Hose Sail 2 Flex Hose Sail 3 Water Return Figure 16: Typical piping of a sail with an odd number of passes Water Supply Flex Hose Figure 17: Typical even number of passes Copyright Price Industries Limited All Metric dimensions ( ) are soft conversion. H-9

10 Design Procedure Heating 1. Determine the ventilation requirement The ventilation requirement should be calculated to meet ventilatiown codes. For example, using ASHRAE Standard to determine the minimum fresh air flow rate: where Qoz = minimum fresh air flow rate, cfm [L/s] Rp = outdoor air flow rate per person, cfm/person [L/s(person)] P z = zone population or maximum number of occupants in zone R a = outdoor air flow rate per unit area, cfm/ft 2 [L/sm 2 ] A Z = zone floor area or net occupied area of the zone, ft 2 [m 2 ] 2. Determine required supply air dew-point temperature to remove the latent load L1 L2 where ql = latent load, Btu/h [W] Qs = supply air flow rate, cfm [L/s] ΔW = difference in humidity ratio between the supply air and the room condition, lbm,w/lbm,da or gr/lbm,da [kgw/kgda or gw/kgda] Typically, the moisture content of the ventilation air will be sufficiently low in the heating season to offset the internal gains. 3. Determine the occupied zone humidity ratio if there is excessive latent cooling From equation L2: where Woz = humidity ratio of the room condition, lbm,w/lbm,da or gr/lbm,da [kgw/kgda or gw/kgda] WSA = humidity ratio of the supply air, lbm,w/lbm,da or gr/lbm,da [kgw/kgda or gw/kgda] If Woz is determined to be too low for comfort, humidification of the ventilation air should be considered. L3 4. Determine the supply air volume The supply air volume is the maximum volume required by code for ventilation, and the volume required for controlling the latent load: L4 where QL = air flow rate required for controlling the latent load, cfm [L/s] 5. Determine the heating capacity of the supply air IP L5 SI L5 where qs,air = heating capacity of the supply air, Btu/h [W] ρ = fluid density, lbm/ft 3 [kg/m 3 ] cp = specific heat at constant pressure Btu/hlb F [kj/(kgk)] Qair = supply air flow rate, cfm [L/s] Δtair = air temperature change (treturn - tsupply), F [K] H-10 All Metric dimensions ( ) are soft conversion. Copyright Price Industries Limited 2011.

11 Design Procedure Heating 6. Determine the heating required from the water side L6 where qs, hydronic = heating capacity of the water side, Btu/h [W] qt = total sensible heating capacity, Btu/h [W] 7. Determine an appropriate temperature loss through the panels Specify a panel surface temperature, then find the related mean water temperature, t w. t panel - t room [K] tw - troom [R] t panel - t room [R] tw - troom [K] Figure 18: Connection between mean water temperature and panel surface temperature or, tpanel - troom = 0.74 (t w - troom) 8. Determine the heat transfer coefficients for the radiant panels The natural convection coefficient is: IP L7 SI L7 Where hc,natural = natural convection coefficient, Btu/hft 2 F [W/m 2 K] ta = room temperature, F [K] tpanel = panel temperature, F [K] Dh = hydraulic diameter, ft [m] Where Apanels = surface area of active panels, ft 2 [m 2 ] Ppanels = the pipe internal perimeter, ft [m] Dh = 4Apanels / Ppanels L8 Copyright Price Industries Limited All Metric dimensions ( ) are soft conversion. H-11

12 Design Procedure Heating The forced convection coefficient is: IP L9 SI L9 Where hc,forced = forced convection coefficient, Btu/hft 2 F [W/m 2 K] ach = air change rate, cfm/ft 2 [m 3 /hm 2 ] The total convection coefficient is: L10 Where hc,total = total convection coefficient, Btu/hft2 F [W/m2K] 9. Determine the specific capacity of the radiant panels The convective heat transfer per square foot to the panel is determined: where q c = convective heat flux or convective rate per cross sectional area, Btu/hft 2 [W/m 2 ] qc = convective heat transfer rate, Btu/h [W] A = surface area of the medium, ft 2 [m 2 ] Assuming that the wall temperature is equal to the air temperature, the radiant heat exchange with the panel is determined: IP L11 L12 SI L12 where q r = radiant heat flux, Btu/hft 2 [W/m 2 ] AUST = area-weighted temperature of all indoor surfaces of walls, ceiling, floor, windows, doors, etc. (excluding active panel surfaces), F [ C] The total heat transfer per unit of face area is L13 where q o = total heat flux, Btu/hft 2 [W/m 2 ] 10. Determine the area of panels required L14 where Apanels = area of panels, ft 2 [m 2 ] H-12 All Metric dimensions ( ) are soft conversion. Copyright Price Industries Limited 2011.

13 Example 1 - Patient Room (IP) Consider the patient room shown in the figure below. The patient room includes a television, monitoring equipment and overhead lighting. The temperature set-point is 75 F with a minimum relative humidity of 40%. The room is 10 ft wide and 20 ft long, with a 9 ft ceiling. The attached toilet room is 5 ft wide and 7 ft long, with an 8 ft ceiling. There is one exterior wall and window. The supply air temperature in heating mode is reset to 95 F, with the heating water temperature at 175 F. 20 ft 10 ft PATIENT ROOM Corridor 5 ft 7 ft Determine The water flow rate and pressure drop for the heating panels required to handle the heating load, assuming 15 F outdoor air temperature Overnight in winter, the envelope loss is 4800 Btu/h and the internal gains at that time are limited to the patient load: Design Considerations Patient 160 Btu/h Medical Staff/Visitors 0 Television 0 Medical Equipment 0 Overhead Lighting 0 Envelope Btu/h Total Btu/h Patient latent load 155 Btu/h Determine the Ventilation Requirement For this example, local code refers to ASHRAE Standard for the HVAC system. According to ASHRAE Standard , patient rooms with auxiliary heating require 4 ach of supply air, of which two are outdoor air. Determine the required supply air dew-point temperature to remove the latent load From equation L2: Using the ventilation rate: Copyright Price Industries Limited All Metric dimensions ( ) are soft conversion. H-13

14 Example 1 - Patient Room (IP) In this case, the supply air is a mix of the return air and the ventilation air. This mixture of outdoor air (at the outdoor conditions, assuming saturated air at 15 F with a humidity ratio of 12.5 gr/lb) and return air (assuming that it is at the design conditions of 75 F, 40% RH 52.5 gr/lb), will have more than enough capacity to handle the latent load. In applications where humidity is critical, further analysis may be done to determine the requirement of humidification. For more information refer to Chapter 5 Introduction to Psychrometrics of the Price Engineer's Handbook. Determine the heating capacity of the supply air Using equation L5: Determine the heating required from the water side Determine an appropriate temperature loss through the panels Using a mean water temperature of: Determine the heat transfer coefficients for the radiant panels Using equation L7 and the relation for Dh from equation L8, the natural convection coefficient is determined: Due to the configuration of the room, it can be assumed as a first estimation that the panels will be arranged at the perimeter where the load is, and run the width of the exposure (10 ft). Assuming also a 2 ft width of panel: Using equation L9, the forced convection coefficient is determined: Using equation L10, the total convection coefficient is determined: H-14 All Metric dimensions ( ) are soft conversion. Copyright Price Industries Limited 2011.

15 Example 1 - Patient Room (IP) Determine the specific capacity of the radiant panels Using equation L11, the convective heat transfer per square foot to the panel is determined: The outside air temperature has a significant impact on the inside surface temperatures of exterior walls. The exterior wall temperature is determined with an h value, convective heat transfer coefficient of a vertical wall, of 1.46 Btu/(hft 2 F) and a U value, overall heat transfer coefficient, of Btu/(hft 2 F): The average unheated surface temperature is: Calculating the radiant heat exchange: t From equation L13, the total heat transfer per unit of face area is: Determine the area of panels required Using equation L14: Copyright Price Industries Limited All Metric dimensions ( ) are soft conversion. H-15

16 Example 1 - Patient Room (IP) Therefore, the assumption of panel size (20 ft 2 ) used to calculate the hydraulic diameter is appropriate. The flow rate required to manage the load with a panel ΔT of 10 F is: For simplicity, a 2 ft 10 ft Price RPL linear radiant panel is selected. This panel with 0.41 gpm will have a pipe velocity of 0.55 fps, which corresponds to a Reynolds number of 1900, which is in the laminar range. For a better selection, the flow rate is increased to 1.3 gpm, which corresponds with a Reynolds number of 6400, which is in the turbulent region. From the performance chart, this also increases the pressure drop from 0.31 ft to 3.7 ft, which will allow better flow control of the panel. Recalculating the temperature loss in the panel as well as the capacity: This increase in capacity will result in only requiring 15.7 ft 2, though it is more practical to stay with the original size in order to maintain aesthetics (the panel will run the length of the perimeter) as well as a standard module size (24 in. wide). Panels can be designed to have both active and inactive sections to maintain aesthetics. When running the entire length of the room, the trim and series option will allow the panel to be trimmed on site if the room size varies slightly during construction. Panel PATIENT ROOM Corridor H-16 All Metric dimensions ( ) are soft conversion. Copyright Price Industries Limited 2011.

17 Example 1 - Patient Room (SI) Consider the patient room shown in the figure below. The patient room includes a television, monitoring equipment and overhead lighting. The temperature set-point is 24 C with a minimum relative humidity of 40%. The room is 3 m wide, 6 m long, and has a 3 m ceiling. There is one exterior wall and window. The supply air temperature in heating mode is reset to 35 C and the heating water temperature is 72 C. 6 m 3 m PATIENT ROOM Corridor 1.75 m 2.25 m Determine The water flow rate and pressure drop for the heating panels required to handle the heating load, assuming -10 C outdoor air temperature. Overnight in winter, the envelope loss is 1400 W and the internal gains at that time are limited to the patient load: Design Considerations Patient 50 W Medical Staff/Visitors 0 Television 0 Medical Equipment 0 Overhead Lighting 0 Envelope W Total W Patient latent load 45 W Determine the Ventilation Requirement For this example, local code refers to ASHRAE Standard for the HVAC system. According to ASHRAE Standard , patient rooms with auxiliary heating require 4 ach of supply air, of which two are outdoor air. Determine the required supply air dew-point temperature to remove the latent load From equation L2: Using the ventilation rate: Copyright Price Industries Limited All Metric dimensions ( ) are soft conversion. H-17

18 Example 1 - Patient Room (SI) In this case, the supply air is a mix of the return air and the ventilation air. This mixture of outdoor air (at the outdoor conditions, assuming saturated air at -10 C with a humidity ratio of 1.8 g/kg) and return air (assuming that it is at the design conditions of 24 C, 40% RH 7.4 g/kg), will have more than enough capacity to handle the latent load. In applications where humidity is critical, further analysis may be done to determine the requirement of humidification. For more information refer to Chapter 5 Introduction to Psychrometrics of the Price Engineer's Handbook. Determine the heating capacity of the supply air Using equation L5: Determine the heating required from the water side Determine an appropriate temperature loss through the panels Using a mean water temperature of: Determine the heat transfer coefficients for the radiant panels Using equation L.7 and the relation for Dh from equation L.8, the natural convection coefficient is determined: Due to the configuration of the room, it can be assumed as a first estimation that the panels will be arranged at the perimeter where the load is, and run the width of the exposure (3 m). Assuming also a 600 mm width of panel: Using equation L9, the forced convection coefficient is determined: Using equation L10, the total convection coefficient is determined: H-18 All Metric dimensions ( ) are soft conversion. Copyright Price Industries Limited 2011.

19 Example 1 - Patient Room (SI) Determine the specific capacity of the radiant panels Using equation L11, the convective heat transfer per square foot to the panel is determined: The outside air temperature has a significant impact on the inside surface temperatures of exterior walls. The exterior wall temperature is determined with an h value, convective heat transfer coefficient of a vertical wall, of 8.29 W/(m 2 K) and a U value, overall heat transfer coefficient, of W/(m 2 K): The average unheated surface temperature is: Calculating the radiant heat exchange: From equation L13, the total heat transfer per unit of face area is: Determine the area of panels required Using equation L14: Copyright Price Industries Limited All Metric dimensions ( ) are soft conversion. H-19

20 Example 1 - Patient Room (SI) Therefore, the assumption of panel size (1.8 m 2 ) used to calculate the hydraulic diameter is appropriate. The flow rate required to manage the load with a panel ΔT of 5 K is: For simplicity, a 600 mm 3000 mm RPL linear radiant panel is selected. This panel with kg/s will have a pipe velocity of 0.24 m/s, which corresponds to a Reynolds number of 4300 with a pressure drop of 1.2 kpa, which is a good selection. When running the entire length of the room, the trim and series option will allow the panel to be trimmed on site if the room size varies slightly during construction. PATIENT ROOM Panel Corridor H-20 All Metric dimensions ( ) are soft conversion. Copyright Price Industries Limited 2011.

21 Design Procedure Cooling 1. Determine the ventilation requirement The ventilation requirement should be calculated to meet ventilation codes. For example, using ASHRAE Standard to determine the minimum fresh air flow rate: L1 2. Determine required supply air dew-point temperature to remove the latent load L2 If the required humidity ratio is not practical, recalculate the supply air volume required with the desired humidity ratio. 3. Determine the supply air volume The supply air volume is the maximum volume required by code for ventilation and the volume required for controlling the latent load: L4 4. Determine the sensible cooling capacity of the supply air IP SI 5. Determine the sensible cooling required from the water side 6. Determine an appropriate temperature rise through the panels A panel temperature correction is unnecessary because the temperature differential between the water and air is small in cooling mode. For panels and sails that are designed well, the surface temperature can be approximated to be the mean water temperature: L5 L5 L6 L15 where tw = mean water temperature, F [K] tchws = chilled water supply temperature, F [K] tout = chilled water return temperature, F [K] 7. Determine the heat transfer coefficients for the radiant panels The natural convection coefficient is: IP L16 SI L16 Copyright Price Industries Limited All Metric dimensions ( ) are soft conversion. H-21

22 Design Procedure Cooling The forced convection coefficient is: IP L9 SI L9 The total convection coefficient is: L10 8. Determine the specific capacity of the radiant panels The convective heat transfer per square foot to the panel is determined: L11 Assuming that the wall temperature is equal to the air temperature, the radiant heat exchange with the panel is determined: IP SI The total heat transfer per unit of face area is: 9. Determine the area of panels required L12 L12 L13 L14 H-22 All Metric dimensions ( ) are soft conversion. Copyright Price Industries Limited 2011.

23 Example 2 - Small Office (IP) Consider a small office with a southern exposure. The space is designed for two occupants, a computer with LCD monitor, T8 florescent lighting, and has a temperature set-point of 75 F. The room is 10 ft wide, 12 ft long, and 9 ft from floor to ceiling. The owner expressed interest in using radiant panels. Window 9 ft 10 ft SMALL OFFICE 12 ft Space Considerations One of the primary considerations when using a radiant heating and cooling system is humidity control. As previously discussed, it is important to consider both the ventilation requirements and the latent load when designing the air-side of the system. The assumptions made for the example are as follows: Load/person is 250 Btu/h sensible and 155 Btu/h latent Lighting load in the space is Btu/h/ft² Computer load is 300 Btu/h (CPU and LCD Monitor) Total skin load is 1450 Btu/h Specific heat and density of the air are 0.24 Btu/lb F and lb/ft³ respectively Design conditions are 75 F, with 50% relative humidity Design dew point is 55 F Design Considerations Occupants 2 Set-Point 75 F Floor Area 120 ft² Exterior Wall 108 ft² Volume 1080 ft³ qoz 800 Btu/h ql 825 Btu/h qex 1450 Btu/h qt 3075 Btu/h Determine a) The ventilation requirement. b) The suitable supply air and supply water temperatures. c) The total convective heat transfer coefficient for radiant panels. Copyright Price Industries Limited All Metric dimensions ( ) are soft conversion. H-23

24 Example 2 - Small Office (IP) Solution a) Determine the ventilation requirement The ventilation requirement should be calculated to meet ventilation codes. For example, using ASHRAE Standard to determine the minimum fresh air flow rate for a typical office space: b) Determine required supply air dew-point temperature to remove the latent load From equation L2: Using the ventilation rate: At the design conditions (75 F, 50% RH), the humidity ratio is 65 gr/lb, requiring a difference in humidity ratio between the supply and room air of: From the figure below, the dew point corresponding to the humidity ratio is 40 F, which is too cool for standard equipment. Evaluating the humidity ratio at several temperatures led to the selection of a dew point of 50 F in order to use less expensive common equipment while also minimizing the supply air volume required to control humidity. Humidity Ratio Dew Point lb/lb gr/lb Relative Humidity Volume - ft 3 /lb of Dry Air Saturation Temperature, ºF Enthalpy - Btu/lb of Dry Air % 80% 75 70% 80 60% 50% 40% % Humidity Ratio, lbw/lbda % 10% H Dry Bulb Temperature, ºF All Metric dimensions ( ) are soft conversion. Copyright Price Industries Limited

25 Example 2 - Small Office (IP) The required air volume to satisfy the latent load is: The supply air volume to the office is the maximum volume required by code for ventilation and the volume required for controlling the latent load: c) Determine the heat transfer coefficients for the radiant panels For panels and sails that are designed well, the surface temperature can be approximated to be the mean water temperature. Assuming a chilled water supply temperature 2 F above the dew point in order to minimize the potential for condensation and a temperature rise of 4 F through the panel leads to a mean water temperature of: Using equation L16, the natural convection coefficient is determined: Using equation L9, the forced convection coefficient is determined: Using equation L10, the total convection coefficient is determined: Copyright Price Industries Limited All Metric dimensions ( ) are soft conversion. H-25

26 Example 2 - Small Office (SI) Consider a small office with a southern exposure. The space is designed for two occupants, a computer with LCD monitor, T8 florescent lighting, and has a design temperature set-point of 24 C. The room is 3 m wide, 4 m long, and 3 m from floor to ceiling. The owner expressed interest in using radiant panels. Window 3 m 3 m SMALL OFFICE 4 m Space Considerations One of the primary considerations when using a radiant heating and cooling system is humidity control. As previously discussed, it is important to consider both the ventilation requirements and the latent load when designing the air-side of the system. The assumptions made for the example are as follows: Load/person is 65 W sensible and 55 W latent Lighting load in the space is 25 W/m² Computer load is 80 W (CPU and LCD Monitor) Total skin load is 425 W Specific heat and density of the air are kj/kgk and 1.3 kg/m³ respectively Design conditions are 24 C, with 50% relative humidity Design dew point is 13 C Design Considerations Occupants 2 Set-Point 24 C Floor Area 12 m² Exterior Wall 12 m² Volume 36 m³ qoz 210 W ql 300 W qex 425 W qt 935 W Determine a) The ventilation requirement. b) The suitable supply air and supply water temperatures. c) The total convective heat transfer coefficient for radiant panels. H-26 All Metric dimensions ( ) are soft conversion. Copyright Price Industries Limited 2011.

27 Example 2 - Small Office (SI) Solution a) Determine the ventilation requirement The ventilation requirement should be calculated to meet ventilation codes. For example, using ASHRAE Standard to determine the minimum fresh air flow rate for a typical office space: b) Determine required supply air dew-point temperature to remove the latent load From equation L2: Using the ventilation rate: At the design conditions (24 C, 50% RH), the humidity ratio is 9.5 g/kg of dry air, requiring a difference in humidity ratio between the supply and room air of: From the figure below the dew point corresponding to the humidity ratio is 5 C, which is too cool for standard equipment. Evaluating the humidity ratio at several temperatures led to the selection of a dewpoint of 10 C in order to use less expensive equipment while also minimizing the supply air volume required to control humidity. Humidity Ratio Dew Point g/kg Relative Humidity Volume - m 3 /kg of Dry Air Saturation Temperature, ºC Enthalpy - kj/kg of Dry Air % 80% 25 70% 60% % 40% % % % Humidity Ratio, gw/kgda Dry-Bulb Temperature, ºC Copyright Price Industries Limited All Metric dimensions ( ) are soft conversion. H

28 Example 2 - Small Office (SI) The required air volume to satisfy the latent load is: The supply air volume to the office is the maximum volume required by code for ventilation and the volume required for controlling the latent load: c) Determine the heat transfer coefficients for the radiant panels For panels and sails that are designed well, the surface temperature can be approximated to be the mean water temperature. Assuming a chilled water supply temperature 1 K above the dew point in order to minimize the potential for condensation and a temperature rise of 2 K through the panel leads to a mean water temperature of: Using equation L16, the natural convection coefficient is determined: Using equation L9, the forced convection coefficient is determined: Using equation L10, the total convection coefficient is determined: H-28 All Metric dimensions ( ) are soft conversion. Copyright Price Industries Limited 2011.

29 Product Selection Performance Radiant panels performance depends on several factors: The difference in surface temperatures between the panel and the surrounding surfaces The mean water temperature and the panel thermal resistance View factor of the panel to the surfaces to be cooled/heated Water flow rate Emissivity and absorption of affected surfaces The water flow rate in the coil affects two performance factors. First, the heat transfer between the water and the panel is dependent on whether the flow is laminar (poor), transitional (inconsistent) or turbulent (good). Secondly, it also affects the mean water temperature. The higher the flow rate, the closer the discharge temperature will be to the inlet, thereby changing the average water temperature imposed on the panel. As the separation between the mean water temperature and the surrounding room temperature (ΔT) increases, so does the capacity. In heating, the ΔT is limited by thermal comfort. In cooling, the ΔT is also limited by two factors, thermal comfort and condensation prevention. Good practice for panel selection in cooling avoids condensation by limiting the entering water temperature to the room s dew point + 2 F [1 K]. The most common design condition for spaces in cooling is 75 F [24 C] at 50% RH, producing a dew point of 55 F [13 C] and limiting entering water temperature to a minimum of 57 F [14 C]. Figure 19 shows the effect on the flow rate, indicated by Reynolds number, on the capacity of a typical radiant panel. As indicated on the chart, increasing the flow rate into the transitional range (Re > 2300, shown in blue on the graph) increases the output of the panel. Capacity, % Re Figure 19: Radiant panel capacity vs. water flow The water flow rate is largely dependent on the pressure drop and return water temperatures acceptable to the designer. In most cases the water flow rate should be selected to be fully turbulent (Re > 4000) under design conditions. The difference between the mean water temperature is defined as: L17 and the room/surrounding surface temperatures are the primary driver of panel performance. The larger this difference is, the greater the radiant and convective transfer rates are. As noted in equation L12, the radiant energy exchange between two surfaces is based on the absolute temperature to the fourth power. Conversely, a lower temperature difference will reduce the amount of potential energy exchange, and thereby capacity. As a result, it is desirable from a capacity standpoint to select entry water temperatures as low as possible in cooling, while maintaining it above the dew point in the room to ensure sensible cooling only. The location of radiant panels relative to loads in the space influences their capacity and is greatly dependent on the view factor of the panel to the objects that are to be conditioned. When used in spaces with high solar gain, such as perimeter zones, the capacity increases as the surrounding surface temperature increases. As surface temperatures change throughout the day, panel capacity changes accordingly. Furthermore, as the distance between the panel and the affected surface increases, the view factor diminishes, thus reducing direct radiant exchange between the two surfaces. Panel placement is based on a combination of surface temperature and distance to the occupant in order to ensure an effective operative temperature is achieved. Locating panels along glass perimeters without low emissivity coatings may have a negative effect on energy use as some energy will be lost to the outdoors through the glass. Copyright Price Industries Limited All Metric dimensions ( ) are soft conversion. H-29