Fan Coil Units (FCU):
Fan Coil Unit consists mainly of a coil where chilled or hot water flows and a fan to circulate the room air across the coil. Some fan coil units contain two coils, one for cooling supplied with chilled water and the other is for heat supplied with hot water. This allows for the simultaneous cooling and heating in different spaces of the building.
- Header
- Removable filter
- Adjustable outlet grills
- Fan scroll
- Air foiled bladed fan
- Fan motor
- Main drain pan
- Coil
- Three speed switch
Basic principles
These units must be properly controlled by thermostats for heating and cooling temperature control, by humidistats for humidity control, by blower control or other means for regulating air quantity, and by a method for adding ventilation air into the system.
Basic elements of fan-coil units are a finned-tube coil, filter, and fan section. The fan recirculates air continuously from the space through the coil, which contains either hot or chilled water. The unit may contain an additional electric resistance, steam, or hot water heating coil. The electric heater is often sized for fall and spring to avoid changeover problems in two-pipe systems.
A cleanable or replaceable 35% efficiency filter, located upstream of the fan, prevents clogging the coil with dirt or lint entrained in the recirculated air. It also protects the motor and fan, and reduces the level of airborne contaminants within the conditioned space. The fan-coil unit is equipped with an insulated drain pan. The fan and motor assembly is arranged for quick removal for servicing.
Prototypes of the units should be tested and labeled by Underwriters’ Laboratories (UL), or Engineering Testing Laboratories (ETL), as required by some codes.
Air-conditioning units, with a damper controlled opening for connection to apertures in the outside wall, are optional. These units are not recommended because wind pressure allows no control over the amount of outside air that is admitted, and caution must be exercised for freeze protection in cold climates. Room fan-coil units for the domestic market are generally available in nominal sizes of 200, 300, 400, 600, 800, and 1200 cfm, often with multispeed, high-efficiency fan motors. Where units do not have individual outside air intakes, means must be provided to introduce pretreated outside air through a duct system that engages each room or space.
Types and Location
Room fan-coil units are available in many configurations.
- Low vertical units are available for use under windows with low sills.
- Floor-to-ceiling, chase-enclosed units are available in which the water and condensate drain risers are part of the factory-furnished unit. Supply and return air systems must be isolated from each other to prevent air and sound interchange between rooms.
- Horizontal overhead units may be fitted with ductwork on the discharge to supply several outlets.
A single unit may serve several rooms (e.g., in an apartment house where individual room control is not essential and a common air return is feasible). High static pressure units with larger fan motors handle the higher pressure drops of units with ductwork.
Central ventilation air may be connected directly to the inlet plenums of horizontal units or introduced directly into the space.
If this is done, provisions should be made to ensure that this air is pretreated and held at a temperature equal to the room temperature so as not to cause occupant discomfort when the unit is off.
One way to prevent air leakage is to provide a spring-loaded motorized damper, which closes off the ventilation air whenever the unit’s fan is off.
Coil selection must be based on the temperature of the entering mixture of primary and recirculated air, and the air leaving the coil must satisfy the room sensible cooling and heating requirements.
Horizontal models conserve floor space and usually cost less, but when located overhead in furred ceilings, they create problems such as condensate collection and disposal, mixing of return air from other rooms, leakage of pans causing damage to ceilings, difficulty of access for filter and component removal.
Vertical models give better results in climates or buildings with high heating requirements.
Heating is enhanced by under-window or exterior wall locations. Vertical units can be operated as convectors with the fans turned off during night setback.
Selection
Some designers size fan-coil units for nominal cooling at the medium speed setting when a three-speed control switch is provided. This method ensures quieter operation within the space and adds a safety factor, in that capacity can be increased by operating at high speed. Sound power ratings are available from many manufacturers.
Only the internal space heating and cooling loads need to be handled by the terminal fan-coil units when outside air is pretreated by a central system to a neutral air temperature of about 21.11°C.
This pretreatment should reduce the size and cost of the terminal units.
Wiring
Fan-coil conditioner fans are driven by small motors generally of the shaded pole or capacitor type, with inherent overload protection. Operating wattage of even the largest sizes rarely exceeds 300W at the high speed setting. Running current rarely exceeds 2.5A.
In planning the wiring circuit, local and national electrical codes must be followed. Wiring methods generally provide separate electrical circuits for fan-coil units and do not connect them into the lighting circuit.
Separate electrical circuits connected to a central panel allow the building operator to turn off unit fans from a central point during unoccupied hours. While this panel costs more initially, it can lower operating costs in buildings that do not have 24 hour occupancy. Use of separate electrical circuits is advantageous in applying a single remote thermostat mounted in a well-exposed perimeter space to operate unit fans.
Piping
Even when outside air is pretreated, a condensate removal system should be installed on the terminal units. This precaution ensures that moisture condensed from air from an unexpected open window that bypasses the ventilation system is carried away. Drain pans are an integral feature of units. Condensate drain lines should be oversized to avoid clogging with dirt and other materials, and provision should be made for periodic cleaning of the condensate drain system. Condensation may occur on the outside of the drain piping, which requires that these pipes be insulated. Many building codes have outlawed systems without condensate drain piping due to the damage they could cause.
Capacity Control
Fan-coil unit capacity can be controlled by coil water flow, air bypass, fan speed, or a combination of these. Water flow can be thermostatically controlled by either return air or wall thermostats.
Fan speed control may be automatic or manual. Automatic control is usually on-off with manual speed selection. Units are available with variable-speed motors for modulated speed control. Room thermostats are preferred where fan speed control is used. Return air thermostats do not give a reliable index of room temperature when the fan is off.
On-off speed control is poor because:
- Alternating shifts in fan noise level are more obvious than the sound of a constantly running fan.
- Air circulation patterns within the room are noticeably affected.
Maintenance
Room fan-coil units are equipped with either cleanable or disposable filters that should be cleaned or replaced when dirty. Good filter maintenance improves sanitation and provides full airflow, ensuring full capacity. The frequency of cleaning varies with the application.
Fan-coil unit motors require periodic lubrication. Motor failures are not common, but when they occur, it is possible to replace the entire fan quickly with minimal interruption in the conditioned space. The defective motor can be repaired or replaced. The condensate drain pan and drain system should be cleaned or flushed periodically to prevent overflow and microbiological buildup. Drain pans should be trapped to prevent any gaseous backup.
Water Distribution
The piping arrangement determines the quality of performance, ease of operation, and initial cost of the system.
Two-Pipe Distribution
Two-Pipe Changeover
This method has low initial cost and supplies either chilled water or hot water through the same piping system. The fan-coil unit has a single coil, and room temperature controls reverse their action, depending on whether hot or cold water is available at the unit coil. This system works well in warm weather when all rooms need cooling and in cold weather when all rooms need heat. The two pipe system does not have the simultaneous heating or cooling capability that is required for most projects during intermediate seasons when some rooms need cooling and others need heat. This problem can be especially troublesome if a single piping zone supplies the entire building. This difficulty may be partly overcome by dividing the piping into zones based on solar exposure. Then each zone may be operated to heat or cool, independent of the others.
4.1.8.1.2 Two-pipe changeover with partial electric strip heat
This arrangement provides simultaneous heating and cooling in intermediate seasons by using a small electric strip heater in the fan-coil unit. The unit can handle heating requirements in mild weather, typically down to 4.5°C, while continuing to circulate chilled water to handle any cooling requirements. When the outdoor temperature drops sufficiently to require heating in excess of the electric strip heater capacity, the water system must be changed over to hot water.
4.1.8.1.3 Two-pipe non-changeover with full electric strip heat
This system is not recommended for energy conservation, but it may be practical in areas with a small heating requirement.
4.1.8.2 Four-Pipe Distribution
The four-pipe system generally has the highest initial cost, but it has the best fan-coil system performance.
Four-Pipe Distribution provides:
- All-season availability of heating and cooling at each unit,
- No summer/winter changeover requirement,
- Simpler operation,
- Use of any heating fuel, heat recovery, or solar heat.
In addition, it can be controlled to maintain a “dead band” between heating and cooling so that there is no possibility of simultaneous heating and cooling.
4.1.9 APPLICATIONS
Fan-coil systems are best applied to individual space temperature control. Fan-coil systems also prevent cross-contamination from one room to another. Suitable applications are hotels, motels, apartment buildings, and office buildings. Fan-coil systems are used in a number of hospitals, but are less desirable for critical care facilities because of the low-efficiency filtration and difficulty in maintaining adequate cleanliness in the unit and enclosure.
4.1.10 Advantages
- The system has all the benefits of a central water chilling and heating plant, while retaining the ability to shut off local terminals in unused areas.
- It gives individual room control with little cross contamination of recirculated air from one space to another.
4.1.11 Disadvantages
- Requires a lot of maintenance that must be done in occupied areas.
- Units that operate at low dew points require condensate pans and a drain system that must be cleaned and flushed periodically.
- Condensate disposal can be difficult and costly.
- It is also difficult to clean the coil.
- Filters are small, low in efficiency, and require frequent changing to maintain air volume.
4.2 Fan Coil Unit Component Description:
4.2.1 Casing:
Casing is made of galvanized steel, lined on the inside with 10mm thick closed cells insulation, with collar for supply duct connection. Plenum section and cleanable filter can be installed, the plenum has rear or bottom air return and is insulated internally by 10mm thick insulation.
4.2.2 Coils:
The standard cooling coil is made of copper tubes mechanically expanded into high efficiency corrugated Aluminum fins. In addition, each coil has a manual air vent.
Cooling coils may be used as heating coils in a 2 pipe system. The inclined coil design reduces the width of FCU and maintenance space required.
4.2.3 Fans:
Direct driven double width fan wheels have forward curved blades made of Aluminum alloy for quiet running operation. They are statically and dynamically balanced. Fan casing and inlet cones are made of galvanized steel for long life stable operation.
4.2.4 Drain Pan:
Drain pan is constructed of galvanized steel (electro static painted on both sides) and externally covered by closed cells insulation. The drain pan is sized to properly collect and discharge all condensate coming from the coil. It also covers the headers and returns bends area. The discharge connection is on the same side of coil headers.
4.2.5 Water connections:
Water connections are standardized on the right hand side when facing the air flow with 3/4” female brass header.
The coil can be supplied on either sides according to customer requirements and could also be turned on site for more flexibility.
4.2.6 Motor(s):
Fan motor(s) shall have multi standard selectable speeds. Three speeds are pre-wired as standard. All wiring is made at 220V, single – phase, 50 HZ. Installed capacitor is permanent split type. All motors have long life self – lubricated bearings, some types may be equipped with integral automatic temperature reset for protection.
Accessories:
Units may be supplied including the following accessories:
-2 Or 4 rows chilled/hot water coil.
-4 pipe system for cooling and heating.
– Direct expansion cooling coil.
– Copper tubes copper fins coil.
– U-bended rectangular finned electric heaters.
– Vertical units.
– Stainless steel drain pan.
– Discharge plenum.
– Dampers.
– Grilles.
– Wall mounted 3- speed switch.
Patient Room Load Estimation
General Zone Data
Typical patient rooms is about 5m by 4m giving a 20 m2 area with a ceiling height of 3 m. the south east wall in addition to the roof are assumed to be exposed to solar heat gain from outside.
Floor area:
20 m2
Building weight:
Medium
Lighting
Fixture type: Rec., Not vented
Total watts:
430.6
Watts
Wattage Multiplier:
1.25
Total wattage:
1000
Watts
People
Number of people:
4 people
Activity level:
seated at rest
Sensible gain:
67.4 W/Person
Latent gain:
35.2 W/Person
Miscellaneous loads
Sensible:
1000 Watts
Latent:
500 Watts
Required room conditions
Dry bulb temperature:
22°C
Relative humidity:
50 %
Weather data
The weather data collected by the meteorological authority in Egypt as shown in appendix A state that the extreme weather conditions occurs in July with
43.33 °C dry bulb temperature and a 27.66 % relative humidity with air enthalpy of 83.35 kJ/kg this condition is extreme in the sense of dry bulb temperature while if revising the July average weather condition it is shown that the air enthalpy is as high as 86.89 kJ/kg. This leads us to take the July average conditions to run the load calculation on the operating room.
But as stated in the AIAA paper (AIAA – 4199 – 2003), By Dr. E. E. Khalil,
the weather extreme conditions should be modified to 40 °C dry bulb temperature and 50 % relative humidity, in order to account for global warming as the weather conditions in Cairo deteriorates each year. Thus we have concluded the recommended weather conditions in Appendix B.
As such the weather conditions taken in the cooling coil design are;
Dry bulb temperature:
40 °C
Relative humidity:
50 %
Supply air temperature
13.9 °C
Refrigerant
Choosing chilled water system for cooling, the chilled water would be the refrigerant.
Refrigerant:
Chilled water
Inlet conditions:
6°C
Outlet conditions:
12°C
The water velocity inside tubes should be in the range of 0.5 to 1.5 m/s
4.3.2 Room sensible load:
Solar Gain through walls and glass Based on the assumption of a solar heat gain through the SE wall only in addition to the roof and glass the following results were obtained:
| Exposed surface | Orientation | (U) Overall heat transfer coefficient .(w/sqm/k) | Exposure area (sqm) |
| Wall | SE | 0.85 | 9.1 |
| Roof | SE | 1.7 | 20 |
| Glass | Horizontal | ——- | 2.9 |
Table 4
‑1
Solar Exposure
Solar heat gain through South east wall:
SHG wall = Area (m2)*Equivalent temperature difference (°C)*Transmission coefficient (W/m2 K)
SHG wall = 9.1 * 10 * 0.85 = 77.35 W
Solar heat gain through horizontal roof:
SHG roof = Area (m2)*Equivalent temperature difference (°C)*Transmission coefficient (W/m2 K)
SHG roof = 20 * 14.5 * 1.7 = 493 W
Solar heat gain through south east glass:
SHG glass = Peak solar heat gain (W/m2) * window Area(m2)* shade factor * Storage factor
SHG glass = 356.4* 2.9 * 0.9 * 0.69 = 641.8 W
Transmission gain except walls and roof
The adjacent room air conditions are so close to the inside room conditions and the variation are so small and could not be computed as the CLTD (cooling load temperature difference) factor is zero referring to the CARRIER HANDBOOK for load estimation.
Internal heat
People:
4 * 67.4 = 269.6 W
Electric equipment:
1000 Watts
Lights:
430.6 * 1.25 = 538.25 W
Miscellaneous load
1000 W
Total room sensible heat ( RSH):
RSH = 77.35 + 493 + 641.8 + 269.6 + 1000 + 538.25 + 1000 = 4020 W
4.3.3 Room latent load:
Infiltration
People
4 * 35.2 = 140.8 W
Steam and other appliances
There are no appliances that would generate steam or any source of latent load inside the patient room.
Miscellaneous load
500 W
Total room latent heat (RLH):
RLH = 140.8 + 500 = 640.8 W
4.3.4 Room sensible heat factor (RSHF)
RSHF = (4020) / (4020 + 640.8) = 0.8625
4.3.5 Air flow rate
But the supply air flow rate is constrained by the following equation:
Thus the supply air flow rate would be,
= 405.5 l/s
4.3.6 Psychrometric Processes
Figure 4
‑5
Psychrometric chart
Investigating the psychrometric chart, we have found the following:
Air mass flow rate 0.48 kg/s
On coil conditions: 23.8 °C dbt and 53.05 % RH.
Off coil conditions: 13.9 °C dbt and 78.16 % RH.
4.3.7 Results
Room effect
Supply air conditions:
13.9°C dbt and 78.16 % relative humidity.
Room air conditions:
22 °C dbt and 50% relative humidity.
Room sensible heat:
4. kW.
Room latent heat:
0.642 kW.
Air conditioning apparatus
Total cooling capacity:
7.336 kW.
Moisture removal:
1 g/s.
On coil air conditions:
23.8°C dbt and 53.05 % relative humidity.
Off coil air conditions:
13.9°C dbt and 78.16 % relative humidity.
4.3.8 Computer assisted load estimation programs:
Using Carrier load estimation program, the E20.II as shown in appendix D, the load would be about 8 kW. The leaving dry bulb, wet bulb would thus be 13.9/ 13.4 °C.
4.4 Cooling coil Design
The stated before in the precedent Chapter, the cooling coil design methods are the same, and the code generated under EES in slightly modified. The same design strategy is applied and the 5/8 Inch tubes are selected.
4.4.1 Developed code for coil design
{surface 2 (5/8″) properties }D_o = 17.1704/1000D_i = 15.875/1000S_T = 38.1/1000S_L = 44.45/1000S_f = 3.276/1000T_f = 0.4064/1000D_h = 3.864864/1000F_s = 22.86A_oi = 19.31A_nff = 0.497{A_fin / A_o = 0.905}{log J_colburn factor = -0.3559192 * log (Re E-3) – 2.06083}J_c = 10^((-0.3559192 * Log10(Re_a/1000)) – 2.06083){input data}Vdot_air = 0.46T_o=23.8P_=101.325R_o = 0.578T_r = 22R_r = 0.5T_1 =13.7R_1 = 0.926{T_o=23.8P_=101.325R_o = 0.5305T_R = 22R_R = 0.5T_1 =13.9R_1 = 0.7816}{assumed data}t_rin = 7t_rout = 13vel_fair = 2.35v_water =0.7884{air properties}v_R=volume(AirH2O,T=T_R ,P=P_,R=R_R )miu_o=VISCOSITY(AirH2O,T=T_o,P=P_,R=R_o)miu_1 = VISCOSITY(AirH2O,T=T_1,P=P_,R=R_1 )miu_avg = (miu_o+miu_1)/2Cp_o = CP(AirH2O,T=T_o,P=P_,R=R_o)Cp_1 = CP(AirH2O,T=T_1,P=P_,R=R_1)Cp_avg = (Cp_o+Cp_1)/2k_o = CONDUCTIVITY(AirH2O,T=T_o,P=P_,R=R_o)k_1 = CONDUCTIVITY(AirH2O,T=T_1,P=P_,R=R_1 )k_avg = (k_o+k_1)/2h_o = ENTHALPY(AirH2O,T=T_o,P=P_,R=R_o)h_1 = ENTHALPY(AirH2O,T=T_1,P=P_,R=R_1)mdot_air =Vdot_air /v_RA_face=Vdot_air/vel_fair Q_cc = mdot_air *(h_o-h_1){G = mass velocity }G =mdot_air /A_nff/A_faceRe_a=D_h*G/miu_avgPr = (miu_avg*Cp_avg*1000/K_avg)St = J_c/(Pr^(2/3)){h_c = convection heat transfer coefficient}h_c = St*G*Cp_avg*1000{water properties}t_ravg = (t_rin+t_rout)/2roh_ravg = DENSITY(Water,T=t_ravg,P=200)miu_water = VISCOSITY(Water,T=t_ravg,P=200)Re_w = roh_ravg*v_water*D_i/miu_waterPr_w = PRANDTL(Water,T=t_ravg,P=200)k_f = CONDUCTIVITY(Water,T=t_ravg,P=200)Nu_D = 0.023*(Re_w^(4/5))*(Pr_w^0.4)h_r = Nu_D*K_f/D_i mdot_water = Q_cc /4.18/(t_rout-t_rin){R_cf = Coil factor }R_cf = h_c*(A_oi)/h_r/Cp_avg{according to ASHRAE, to check the surface dryness the following should be checked y = (t_rout -t_rin)/(h_o-h_1) h_ab= (dpt_o-t_rout+y*h_o+R_cf * hi_dpto)/(R_cf+y)now if h_ab is greater than or equal to the entering air enthalpy (h_o) then the coil is totally wet. h_1<= h_ab<=h_o, the surface is partially wet. h_ab is less than or equal to the exit air enthalpy (h_i) then the coil is totally dry}y = (t_rout-t_rin)/(h_o-h_1)dpt_o = DEWPOINT(AirH2O,T=T_o,P=P_,R=R_o)hi_dpto =ENTHALPY(AirH2O,T=dpt_o,P=P_,R=1)h_ab=(dpt_o-t_rout+y*h_o+R_cf * hi_dpto)/(R_cf+y){n = no. of stations x = specific heat transfer through each element.}n=50x=(h_o-h_1)/(n-1)h_a[1]=h_oDUPLICATE j=2,n h_a[j]=h_a[j-1] – xENDt_r[1] = t_routDUPLICATE j=2,n t_r[j]=t_r[j-1] – (x/4.18/mdot_water *mdot_air)ENDDUPLICATE j=1,n h_i[j] = 9.3625+1.7861*(t_i[j])+0.01135*(t_i[j])^2+0.00098855*(t_i[j])^3 (t_i[j]/R_cf)-(t_r[j]/R_cf)-h_a[j]+h_i[j]=0ENDDUPLICATE j=2,n mdot_air*(h_a[j-1]-h_a[j])=h_c*A_[j]/(Cp_avg*1000)*((h_a[j-1]+h_a[j])/2-(h_i[j-1]+h_i[j])/2)ENDA_cum[2]=A_[2]DUPLICATE j=3,n A_cum[j] = A_[j]+A_cum[j-1]ENDA_tot=A_cum[n]dbt_[1] = T_oDUPLICATE j=2,n mdot_air*1000*Cp_avg*(dbt_[j-1]-dbt_[j])=h_c*A_[j]*((dbt_[j-1]+dbt_[j])/2-(t_i[j-1]+t_i[j])/2)ENDN_r=A_tot/F_s/A_faceside_T=A_face^0.5N_T=side_T/S_TN_Tn=round(N_T)-3side_Tn=N_Tn*S_TW=A_face/side_Tnside_L=round(N_r)*S_Ln_circuits=mdot_water/roh_ravg/v_water/(pi/4*D_i^2){n_c =corrected no. of circuits}n_c=round(n_circuits){ v_watern =corrected water velocity}v_watern=mdot_water/roh_ravg/n_c/(pi/4*D_i^2)
4.4.2 Results
A_face = 0.1957 [m2]
A_tot = 17.89 [m2]
h_1 = 36.63 [kJ/kg]
h_ab = 49.61 [kJ/kg]
h_c = 57.05 [W/m2 K]
h_o = 51.04 [kJ/kg]
h_r = 3158 [W/m2 K]
mdot_air=0.543 [kg/s]
mdot_water=0.312
[kg/s]
n_c = 2
N_r = 4
N_Tn = 9
Q_cc = 7.826 [kW]
Re_a = 1187
Re_w = 9578
side_L = 0.1778 [m]
side_Tn = 0.3429 [m]
vel_fair = 2.35 [m/s]
v_watern = 0.7884 [m/s]
W = 0.5709 [m]
4.5Fan Coil Units Control:
Room operating unit including sensor, set point potentiometer and three speed switch will measure actual room conditions and signal DDC controller to position cooling valve for cooling
or start the electric heater for heating
or reheat to satisfy constant temperature in the controlled zone.
Differential pressure switch mounted across filter section will issue an alarm in case of filter dirty.
Differential pressure switch mounted across fan section will interlock control system start. No flow (or low differential pressure) will issue an alarm and signal fan to stop immediately.
Status and trip alarm of fan will be monitored through BMS.